Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[kernel test robot]

BCC: lkp@intel.com
CC: oe-kbuild-all@lists.linux.dev
In-Reply-To: <87jzsemmsd.fsf_-_@email.froward.int.ebiederm.org>
References: <87jzsemmsd.fsf_-_@email.froward.int.ebiederm.org>
TO: "Eric W. Biederman" <ebiederm@xmission.com>
TO: Sebastian Ott <sebott@redhat.com>
CC: "Thomas Weißschuh" <linux@weissschuh.net>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Christian Brauner <brauner@kernel.org>
CC: Kees Cook <keescook@chromium.org>
CC: Mark Brown <broonie@kernel.org>
CC: Willy Tarreau <w@1wt.eu>
CC: linux-fsdevel@vger.kernel.org
CC: linux-mm@kvack.org
CC: linux-kernel@vger.kernel.org
CC: stable@vger.kernel.org

Hi Eric,

kernel test robot noticed the following build warnings:

[auto build test WARNING on kees/for-next/execve]
[also build test WARNING on linus/master v6.6-rc3 next-20230926]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Eric-W-Biederman/binfmt_elf-Support-segments-with-0-filesz-and-misaligned-starts/20230925-210022
base:   https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/execve
patch link:    https://lore.kernel.org/r/87jzsemmsd.fsf_-_%40email.froward.int.ebiederm.org
patch subject: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts
:::::: branch date: 22 hours ago
:::::: commit date: 22 hours ago
config: i386-randconfig-141-20230926 (https://download.01.org/0day-ci/archive/20230926/202309261925.QvgPAYL7-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce: (https://download.01.org/0day-ci/archive/20230926/202309261925.QvgPAYL7-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <error27@gmail.com>
| Closes: https://lore.kernel.org/r/202309261925.QvgPAYL7-lkp@intel.com/

smatch warnings:
fs/binfmt_elf.c:431 elf_load() error: uninitialized symbol 'map_addr'.

vim +/map_addr +431 fs/binfmt_elf.c

^1da177e4c3f41 Linus Torvalds    2005-04-16  389  
a6409120b31666 Eric W. Biederman 2023-09-25  390  static unsigned long elf_load(struct file *filep, unsigned long addr,
a6409120b31666 Eric W. Biederman 2023-09-25  391  		const struct elf_phdr *eppnt, int prot, int type,
a6409120b31666 Eric W. Biederman 2023-09-25  392  		unsigned long total_size)
a6409120b31666 Eric W. Biederman 2023-09-25  393  {
a6409120b31666 Eric W. Biederman 2023-09-25  394  	unsigned long zero_start, zero_end;
a6409120b31666 Eric W. Biederman 2023-09-25  395  	unsigned long map_addr;
a6409120b31666 Eric W. Biederman 2023-09-25  396  
a6409120b31666 Eric W. Biederman 2023-09-25  397  	if (eppnt->p_filesz) {
a6409120b31666 Eric W. Biederman 2023-09-25  398  		map_addr = elf_map(filep, addr, eppnt, prot, type, total_size);
a6409120b31666 Eric W. Biederman 2023-09-25  399  		if (BAD_ADDR(map_addr))
a6409120b31666 Eric W. Biederman 2023-09-25  400  			return map_addr;
a6409120b31666 Eric W. Biederman 2023-09-25  401  		if (eppnt->p_memsz > eppnt->p_filesz) {
a6409120b31666 Eric W. Biederman 2023-09-25  402  			zero_start = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
a6409120b31666 Eric W. Biederman 2023-09-25  403  				eppnt->p_filesz;
a6409120b31666 Eric W. Biederman 2023-09-25  404  			zero_end = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
a6409120b31666 Eric W. Biederman 2023-09-25  405  				eppnt->p_memsz;
a6409120b31666 Eric W. Biederman 2023-09-25  406  
a6409120b31666 Eric W. Biederman 2023-09-25  407  			/* Zero the end of the last mapped page */
a6409120b31666 Eric W. Biederman 2023-09-25  408  			padzero(zero_start);
a6409120b31666 Eric W. Biederman 2023-09-25  409  		}
a6409120b31666 Eric W. Biederman 2023-09-25  410  	} else {
a6409120b31666 Eric W. Biederman 2023-09-25  411  		zero_start = ELF_PAGESTART(addr);
a6409120b31666 Eric W. Biederman 2023-09-25  412  		zero_end = zero_start + ELF_PAGEOFFSET(eppnt->p_vaddr) +
a6409120b31666 Eric W. Biederman 2023-09-25  413  			eppnt->p_memsz;
a6409120b31666 Eric W. Biederman 2023-09-25  414  	}
a6409120b31666 Eric W. Biederman 2023-09-25  415  	if (eppnt->p_memsz > eppnt->p_filesz) {
a6409120b31666 Eric W. Biederman 2023-09-25  416  		/*
a6409120b31666 Eric W. Biederman 2023-09-25  417  		 * Map the last of the segment.
a6409120b31666 Eric W. Biederman 2023-09-25  418  		 * If the header is requesting these pages to be
a6409120b31666 Eric W. Biederman 2023-09-25  419  		 * executable, honour that (ppc32 needs this).
a6409120b31666 Eric W. Biederman 2023-09-25  420  		 */
a6409120b31666 Eric W. Biederman 2023-09-25  421  		int error;
a6409120b31666 Eric W. Biederman 2023-09-25  422  
a6409120b31666 Eric W. Biederman 2023-09-25  423  		zero_start = ELF_PAGEALIGN(zero_start);
a6409120b31666 Eric W. Biederman 2023-09-25  424  		zero_end = ELF_PAGEALIGN(zero_end);
a6409120b31666 Eric W. Biederman 2023-09-25  425  
a6409120b31666 Eric W. Biederman 2023-09-25  426  		error = vm_brk_flags(zero_start, zero_end - zero_start,
a6409120b31666 Eric W. Biederman 2023-09-25  427  				     prot & PROT_EXEC ? VM_EXEC : 0);
a6409120b31666 Eric W. Biederman 2023-09-25  428  		if (error)
a6409120b31666 Eric W. Biederman 2023-09-25  429  			map_addr = error;
a6409120b31666 Eric W. Biederman 2023-09-25  430  	}
a6409120b31666 Eric W. Biederman 2023-09-25 @431  	return map_addr;
a6409120b31666 Eric W. Biederman 2023-09-25  432  }
a6409120b31666 Eric W. Biederman 2023-09-25  433  

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

[PATCH RFC] binfmt_elf: fully allocate bss pages

[Thomas Weißschuh]

When allocating the pages for bss the start address needs to be rounded
down instead of up.
Otherwise the start of the bss segment may be unmapped.

The was reported to happen on Aarch64:

Memory allocated by set_brk():
Before: start=0x420000 end=0x420000
After:  start=0x41f000 end=0x420000

The triggering binary looks like this:

    Elf file type is EXEC (Executable file)
    Entry point 0x400144
    There are 4 program headers, starting at offset 64

    Program Headers:
      Type           Offset             VirtAddr           PhysAddr
                     FileSiz            MemSiz              Flags  Align
      LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                     0x0000000000000178 0x0000000000000178  R E    0x10000
      LOAD           0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8
                     0x0000000000000000 0x0000000000000008  RW     0x10000
      NOTE           0x0000000000000120 0x0000000000400120 0x0000000000400120
                     0x0000000000000024 0x0000000000000024  R      0x4
      GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                     0x0000000000000000 0x0000000000000000  RW     0x10

     Section to Segment mapping:
      Segment Sections...
       00     .note.gnu.build-id .text .eh_frame
       01     .bss
       02     .note.gnu.build-id
       03

Reported-by: Sebastian Ott <sebott@redhat.com>
Closes: https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@redhat.com/
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---

I'm not really familiar with the ELF loading process, so putting this
out as RFC.

A example binary compiled with aarch64-linux-gnu-gcc 13.2.0 is available
at https://test.t-8ch.de/binfmt-bss-repro.bin
---
 fs/binfmt_elf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7b3d2d491407..4008a57d388b 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -112,7 +112,7 @@ static struct linux_binfmt elf_format = {
 
 static int set_brk(unsigned long start, unsigned long end, int prot)
 {
-	start = ELF_PAGEALIGN(start);
+	start = ELF_PAGESTART(start);
 	end = ELF_PAGEALIGN(end);
 	if (end > start) {
 		/*

---
base-commit: aed8aee11130a954356200afa3f1b8753e8a9482
change-id: 20230914-bss-alloc-f523fa61718c

Best regards,
-- 
Thomas Weißschuh <linux@weissschuh.net>

Re: [PATCH RFC] binfmt_elf: fully allocate bss pages

[Sebastian Ott]

Hej,

since we figured that the proposed patch is not going to work I've spent a
couple more hours looking at this (some static binaries on arm64 segfault
during load [0]). The segfault happens because of a failed clear_user()
call in load_elf_binary(). The address we try to write zeros to is mapped with
correct permissions.

After some experiments I've noticed that writing to anonymous mappings work
fine and all the error cases happend on file backed VMAs. Debugging showed that
in elf_map() we call vm_mmap() with a file offset of 15 pages - for a binary
that's less than 1KiB in size.

Looking at the ELF headers again that 15 pages offset originates from the offset
of the 2nd segment - so, I guess the loader did as instructed and that binary is
just too nasty?

Program Headers:
   Type           Offset             VirtAddr           PhysAddr
                  FileSiz            MemSiz              Flags  Align
   LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                  0x0000000000000178 0x0000000000000178  R E    0x10000
   LOAD           0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8
                  0x0000000000000000 0x0000000000000008  RW     0x10000
   NOTE           0x0000000000000120 0x0000000000400120 0x0000000000400120
                  0x0000000000000024 0x0000000000000024  R      0x4
   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                  0x0000000000000000 0x0000000000000000  RW     0x10

As an additional test I've added a bunch of zeros at the end of that binary
so that the offset is within that file and it did load just fine.

On the other hand there is this section header:
   [ 4] .bss              NOBITS           000000000041ffe8  0000ffe8
        0000000000000008  0000000000000000  WA       0     0     1

"sh_offset
This member's value gives the byte offset from the beginning of the file to
the first byte in the section. One section type, SHT_NOBITS described
below, occupies no space in the file, and its sh_offset member locates
the conceptual placement in the file.
"

So, still not sure what to do here..

Sebastian

[0] https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@redhat.com/

Re: [PATCH RFC] binfmt_elf: fully allocate bss pages

[Eric W. Biederman]

Sebastian Ott <sebott@redhat.com> writes:

> Hej,
>
> since we figured that the proposed patch is not going to work I've spent a
> couple more hours looking at this (some static binaries on arm64 segfault
> during load [0]). The segfault happens because of a failed clear_user()
> call in load_elf_binary(). The address we try to write zeros to is mapped with
> correct permissions.
>
> After some experiments I've noticed that writing to anonymous mappings work
> fine and all the error cases happend on file backed VMAs. Debugging showed that
> in elf_map() we call vm_mmap() with a file offset of 15 pages - for a binary
> that's less than 1KiB in size.
>
> Looking at the ELF headers again that 15 pages offset originates from the offset
> of the 2nd segment - so, I guess the loader did as instructed and that binary is
> just too nasty?
>
> Program Headers:
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags  Align
>   LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
>                  0x0000000000000178 0x0000000000000178  R E    0x10000
>   LOAD           0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8
>                  0x0000000000000000 0x0000000000000008  RW     0x10000
>   NOTE           0x0000000000000120 0x0000000000400120 0x0000000000400120
>                  0x0000000000000024 0x0000000000000024  R      0x4
>   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>                  0x0000000000000000 0x0000000000000000  RW     0x10
>
> As an additional test I've added a bunch of zeros at the end of that binary
> so that the offset is within that file and it did load just fine.
>
> On the other hand there is this section header:
>   [ 4] .bss              NOBITS           000000000041ffe8  0000ffe8
>        0000000000000008  0000000000000000  WA       0     0     1
>
> "sh_offset
> This member's value gives the byte offset from the beginning of the file to
> the first byte in the section. One section type, SHT_NOBITS described
> below, occupies no space in the file, and its sh_offset member locates
> the conceptual placement in the file.
> "
>
> So, still not sure what to do here..
>
> Sebastian
>
> [0] https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@redhat.com/

I think that .bss section that is being generated is atrocious.

At the same time I looked at what the linux elf loader is trying to do,
and the elf loader's handling of program segments with memsz > filesz
has serious remnants a.out of programs allocating memory with the brk
syscall.

Lots of the structure looks like it started with the assumption that
there would only be a single program header with memsz > filesz the way
and that was the .bss.   The way things were in the a.out days and
handling of other cases has been debugged in later.

So I have modified elf_map to always return successfully when there is
a zero filesz in the program header for an elf segment.

Then I have factored out a function clear_tail that ensures the zero
padding for an entire elf segment is present.

Please test this and see if it causes your test case to work.

Please also dig into gcc or whichever code generates that horrendous
.bss section and see if that can be fixed so the code can work on older
kernels.  A section that only contains .bss has no business not being
properly aligned.  Even if the data in that section doesn't start at the
beginning of the page, there is no reason to feed nasty data to other
programs.  It just increases the odds of complications for no good
reason.  At a minimum that is going to be needed to run that code on
older kernels.

Eric


diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7b3d2d491407..f6608df75df6 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -110,43 +110,66 @@ static struct linux_binfmt elf_format = {
 
 #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
 
-static int set_brk(unsigned long start, unsigned long end, int prot)
-{
-	start = ELF_PAGEALIGN(start);
-	end = ELF_PAGEALIGN(end);
-	if (end > start) {
-		/*
-		 * Map the last of the bss segment.
-		 * If the header is requesting these pages to be
-		 * executable, honour that (ppc32 needs this).
-		 */
-		int error = vm_brk_flags(start, end - start,
-				prot & PROT_EXEC ? VM_EXEC : 0);
-		if (error)
-			return error;
-	}
-	current->mm->start_brk = current->mm->brk = end;
-	return 0;
-}
-
 /* We need to explicitly zero any fractional pages
    after the data section (i.e. bss).  This would
    contain the junk from the file that should not
    be in memory
  */
-static int padzero(unsigned long elf_bss)
+static int padzero(unsigned long elf_bss, unsigned long elf_brk)
 {
 	unsigned long nbyte;
 
 	nbyte = ELF_PAGEOFFSET(elf_bss);
 	if (nbyte) {
 		nbyte = ELF_MIN_ALIGN - nbyte;
+		if (nbyte > elf_brk - elf_bss)
+			nbyte = elf_brk - elf_bss;
 		if (clear_user((void __user *) elf_bss, nbyte))
 			return -EFAULT;
 	}
 	return 0;
 }
 
+static int clear_tail(struct elf_phdr *phdr, unsigned long load_bias, int prot)
+{
+	unsigned long start, end;
+
+	/* Is there a tail to clear? */
+	if (phdr->p_filesz >= phdr->p_memsz)
+		return 0;
+
+	/* Where does the tail start? */
+	if (phdr->p_filesz)
+		start = load_bias + phdr->p_vaddr + phdr->p_filesz;
+	else
+		start = ELF_PAGESTART(load_bias + phdr->p_vaddr);
+
+	/* Where does the tail end? */
+	end = load_bias + phdr->p_vaddr + phdr->p_memsz;
+
+	/*
+	 * This bss-zeroing can fail if the ELF
+	 * file specifies odd protections. So
+	 * we don't check the return value
+	 */
+	padzero(start, end);
+
+	start = ELF_PAGEALIGN(start);
+	end = ELF_PAGEALIGN(end);
+	if (end > start) {
+		/*
+		 * Map the last of the bss segment.
+		 * If the header is requesting these pages to be
+		 * executable, honour that (ppc32 needs this).
+		 */
+		int error = vm_brk_flags(start, end - start,
+				prot & PROT_EXEC ? VM_EXEC : 0);
+		if (error)
+			return error;
+	}
+	return 0;
+}
+
 /* Let's use some macros to make this stack manipulation a little clearer */
 #ifdef CONFIG_STACK_GROWSUP
 #define STACK_ADD(sp, items) ((elf_addr_t __user *)(sp) + (items))
@@ -379,7 +402,7 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
 
 	/* mmap() will return -EINVAL if given a zero size, but a
 	 * segment with zero filesize is perfectly valid */
-	if (!size)
+	if (!eppnt->p_filesz)
 		return addr;
 
 	/*
@@ -596,8 +619,6 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 	struct elf_phdr *eppnt;
 	unsigned long load_addr = 0;
 	int load_addr_set = 0;
-	unsigned long last_bss = 0, elf_bss = 0;
-	int bss_prot = 0;
 	unsigned long error = ~0UL;
 	unsigned long total_size;
 	int i;
@@ -661,50 +682,13 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 				goto out;
 			}
 
-			/*
-			 * Find the end of the file mapping for this phdr, and
-			 * keep track of the largest address we see for this.
-			 */
-			k = load_addr + eppnt->p_vaddr + eppnt->p_filesz;
-			if (k > elf_bss)
-				elf_bss = k;
-
-			/*
-			 * Do the same thing for the memory mapping - between
-			 * elf_bss and last_bss is the bss section.
-			 */
-			k = load_addr + eppnt->p_vaddr + eppnt->p_memsz;
-			if (k > last_bss) {
-				last_bss = k;
-				bss_prot = elf_prot;
-			}
+			/* Map anonymous pages and clear the tail if needed */
+			error = clear_tail(eppnt, load_addr, elf_prot);
+			if (error)
+				goto out;
 		}
 	}
 
-	/*
-	 * Now fill out the bss section: first pad the last page from
-	 * the file up to the page boundary, and zero it from elf_bss
-	 * up to the end of the page.
-	 */
-	if (padzero(elf_bss)) {
-		error = -EFAULT;
-		goto out;
-	}
-	/*
-	 * Next, align both the file and mem bss up to the page size,
-	 * since this is where elf_bss was just zeroed up to, and where
-	 * last_bss will end after the vm_brk_flags() below.
-	 */
-	elf_bss = ELF_PAGEALIGN(elf_bss);
-	last_bss = ELF_PAGEALIGN(last_bss);
-	/* Finally, if there is still more bss to allocate, do it. */
-	if (last_bss > elf_bss) {
-		error = vm_brk_flags(elf_bss, last_bss - elf_bss,
-				bss_prot & PROT_EXEC ? VM_EXEC : 0);
-		if (error)
-			goto out;
-	}
-
 	error = load_addr;
 out:
 	return error;
@@ -828,8 +812,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	unsigned long error;
 	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
 	struct elf_phdr *elf_property_phdata = NULL;
-	unsigned long elf_bss, elf_brk;
-	int bss_prot = 0;
+	unsigned long elf_brk;
 	int retval, i;
 	unsigned long elf_entry;
 	unsigned long e_entry;
@@ -1020,7 +1003,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	if (retval < 0)
 		goto out_free_dentry;
 
-	elf_bss = 0;
 	elf_brk = 0;
 
 	start_code = ~0UL;
@@ -1040,32 +1022,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		if (elf_ppnt->p_type != PT_LOAD)
 			continue;
 
-		if (unlikely (elf_brk > elf_bss)) {
-			unsigned long nbyte;
-
-			/* There was a PT_LOAD segment with p_memsz > p_filesz
-			   before this one. Map anonymous pages, if needed,
-			   and clear the area.  */
-			retval = set_brk(elf_bss + load_bias,
-					 elf_brk + load_bias,
-					 bss_prot);
-			if (retval)
-				goto out_free_dentry;
-			nbyte = ELF_PAGEOFFSET(elf_bss);
-			if (nbyte) {
-				nbyte = ELF_MIN_ALIGN - nbyte;
-				if (nbyte > elf_brk - elf_bss)
-					nbyte = elf_brk - elf_bss;
-				if (clear_user((void __user *)elf_bss +
-							load_bias, nbyte)) {
-					/*
-					 * This bss-zeroing can fail if the ELF
-					 * file specifies odd protections. So
-					 * we don't check the return value
-					 */
-				}
-			}
-		}
 
 		elf_prot = make_prot(elf_ppnt->p_flags, &arch_state,
 				     !!interpreter, false);
@@ -1208,42 +1164,31 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			goto out_free_dentry;
 		}
 
-		k = elf_ppnt->p_vaddr + elf_ppnt->p_filesz;
+		/* Map anonymous pages and clear the tail if needed */
+		retval = clear_tail(elf_ppnt, load_bias, elf_prot);
+		if (retval)
+			goto out_free_dentry;
 
-		if (k > elf_bss)
-			elf_bss = k;
+		k = elf_ppnt->p_vaddr + elf_ppnt->p_filesz;
 		if ((elf_ppnt->p_flags & PF_X) && end_code < k)
 			end_code = k;
 		if (end_data < k)
 			end_data = k;
 		k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
 		if (k > elf_brk) {
-			bss_prot = elf_prot;
 			elf_brk = k;
 		}
 	}
 
 	e_entry = elf_ex->e_entry + load_bias;
 	phdr_addr += load_bias;
-	elf_bss += load_bias;
 	elf_brk += load_bias;
 	start_code += load_bias;
 	end_code += load_bias;
 	start_data += load_bias;
 	end_data += load_bias;
 
-	/* Calling set_brk effectively mmaps the pages that we need
-	 * for the bss and break sections.  We must do this before
-	 * mapping in the interpreter, to make sure it doesn't wind
-	 * up getting placed where the bss needs to go.
-	 */
-	retval = set_brk(elf_bss, elf_brk, bss_prot);
-	if (retval)
-		goto out_free_dentry;
-	if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
-		retval = -EFAULT; /* Nobody gets to see this, but.. */
-		goto out_free_dentry;
-	}
+	current->mm->start_brk = current->mm->brk = ELF_PAGEALIGN(elf_brk);
 
 	if (interpreter) {
 		elf_entry = load_elf_interp(interp_elf_ex,
@@ -1369,7 +1314,6 @@ static int load_elf_library(struct file *file)
 {
 	struct elf_phdr *elf_phdata;
 	struct elf_phdr *eppnt;
-	unsigned long elf_bss, bss, len;
 	int retval, error, i, j;
 	struct elfhdr elf_ex;
 
@@ -1425,19 +1369,9 @@ static int load_elf_library(struct file *file)
 	if (error != ELF_PAGESTART(eppnt->p_vaddr))
 		goto out_free_ph;
 
-	elf_bss = eppnt->p_vaddr + eppnt->p_filesz;
-	if (padzero(elf_bss)) {
-		error = -EFAULT;
+	error = clear_tail(eppnt, 0, 0);
+	if (error)
 		goto out_free_ph;
-	}
-
-	len = ELF_PAGEALIGN(eppnt->p_filesz + eppnt->p_vaddr);
-	bss = ELF_PAGEALIGN(eppnt->p_memsz + eppnt->p_vaddr);
-	if (bss > len) {
-		error = vm_brk(len, bss - len);
-		if (error)
-			goto out_free_ph;
-	}
 	error = 0;
 
 out_free_ph:

Re: [PATCH RFC] binfmt_elf: fully allocate bss pages

[Sebastian Ott]

On Sun, 24 Sep 2023, Eric W. Biederman wrote:
> Sebastian Ott <sebott@redhat.com> writes:
>
>> Hej,
>>
>> since we figured that the proposed patch is not going to work I've spent a
>> couple more hours looking at this (some static binaries on arm64 segfault
>> during load [0]). The segfault happens because of a failed clear_user()
>> call in load_elf_binary(). The address we try to write zeros to is mapped with
>> correct permissions.
>>
>> After some experiments I've noticed that writing to anonymous mappings work
>> fine and all the error cases happend on file backed VMAs. Debugging showed that
>> in elf_map() we call vm_mmap() with a file offset of 15 pages - for a binary
>> that's less than 1KiB in size.
>>
>> Looking at the ELF headers again that 15 pages offset originates from the offset
>> of the 2nd segment - so, I guess the loader did as instructed and that binary is
>> just too nasty?
>>
>> Program Headers:
>>   Type           Offset             VirtAddr           PhysAddr
>>                  FileSiz            MemSiz              Flags  Align
>>   LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
>>                  0x0000000000000178 0x0000000000000178  R E    0x10000
>>   LOAD           0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8
>>                  0x0000000000000000 0x0000000000000008  RW     0x10000
>>   NOTE           0x0000000000000120 0x0000000000400120 0x0000000000400120
>>                  0x0000000000000024 0x0000000000000024  R      0x4
>>   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>>                  0x0000000000000000 0x0000000000000000  RW     0x10
>>
>> As an additional test I've added a bunch of zeros at the end of that binary
>> so that the offset is within that file and it did load just fine.
>>
>> On the other hand there is this section header:
>>   [ 4] .bss              NOBITS           000000000041ffe8  0000ffe8
>>        0000000000000008  0000000000000000  WA       0     0     1
>>
>> "sh_offset
>> This member's value gives the byte offset from the beginning of the file to
>> the first byte in the section. One section type, SHT_NOBITS described
>> below, occupies no space in the file, and its sh_offset member locates
>> the conceptual placement in the file.
>> "
>>
>> So, still not sure what to do here..
>>
>> Sebastian
>>
>> [0] https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@redhat.com/
>
> I think that .bss section that is being generated is atrocious.
>
> At the same time I looked at what the linux elf loader is trying to do,
> and the elf loader's handling of program segments with memsz > filesz
> has serious remnants a.out of programs allocating memory with the brk
> syscall.
>
> Lots of the structure looks like it started with the assumption that
> there would only be a single program header with memsz > filesz the way
> and that was the .bss.   The way things were in the a.out days and
> handling of other cases has been debugged in later.
>
> So I have modified elf_map to always return successfully when there is
> a zero filesz in the program header for an elf segment.
>
> Then I have factored out a function clear_tail that ensures the zero
> padding for an entire elf segment is present.
>
> Please test this and see if it causes your test case to work.

Sadly, that causes issues for other programs:

[   44.164596] Run /init as init process
[   44.168763] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
[   44.176409] CPU: 32 PID: 1 Comm: init Not tainted 6.6.0-rc2+ #89
[   44.182404] Hardware name: GIGABYTE R181-T92-00/MT91-FS4-00, BIOS F34 08/13/2020
[   44.189786] Call trace:
[   44.192220]  dump_backtrace+0xa4/0x130
[   44.195961]  show_stack+0x20/0x38
[   44.199264]  dump_stack_lvl+0x48/0x60
[   44.202917]  dump_stack+0x18/0x28
[   44.206219]  panic+0x2e0/0x350
[   44.209264]  do_exit+0x370/0x390
[   44.212481]  do_group_exit+0x3c/0xa0
[   44.216044]  get_signal+0x800/0x808
[   44.219521]  do_signal+0xfc/0x200
[   44.222824]  do_notify_resume+0xc8/0x418
[   44.226734]  el0_da+0x114/0x120
[   44.229866]  el0t_64_sync_handler+0xb8/0x130
[   44.234124]  el0t_64_sync+0x194/0x198
[   44.237776] SMP: stopping secondary CPUs
[   44.241740] Kernel Offset: disabled
[   44.245215] CPU features: 0x03000000,14028142,10004203
[   44.250342] Memory Limit: none
[   44.253383] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---

[PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Eric W. Biederman]

Implement a helper elf_load that wraps elf_map and performs all
of the necessary work to ensure that when "memsz > filesz"
the bytes described by "memsz > filesz" are zeroed.

Link: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
Reported-by: Sebastian Ott <sebott@redhat.com>
Reported-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
 1 file changed, 48 insertions(+), 63 deletions(-)

Can you please test this one?

With this patch I can boot a machine, and I like the structure much
better.  Overall this seems a more reviewable and safer patch although
it is almost as aggressive in the cleanups.

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7b3d2d491407..8bea9d974361 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -110,25 +110,6 @@ static struct linux_binfmt elf_format = {
 
 #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
 
-static int set_brk(unsigned long start, unsigned long end, int prot)
-{
-	start = ELF_PAGEALIGN(start);
-	end = ELF_PAGEALIGN(end);
-	if (end > start) {
-		/*
-		 * Map the last of the bss segment.
-		 * If the header is requesting these pages to be
-		 * executable, honour that (ppc32 needs this).
-		 */
-		int error = vm_brk_flags(start, end - start,
-				prot & PROT_EXEC ? VM_EXEC : 0);
-		if (error)
-			return error;
-	}
-	current->mm->start_brk = current->mm->brk = end;
-	return 0;
-}
-
 /* We need to explicitly zero any fractional pages
    after the data section (i.e. bss).  This would
    contain the junk from the file that should not
@@ -406,6 +387,51 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
 	return(map_addr);
 }
 
+static unsigned long elf_load(struct file *filep, unsigned long addr,
+		const struct elf_phdr *eppnt, int prot, int type,
+		unsigned long total_size)
+{
+	unsigned long zero_start, zero_end;
+	unsigned long map_addr;
+
+	if (eppnt->p_filesz) {
+		map_addr = elf_map(filep, addr, eppnt, prot, type, total_size);
+		if (BAD_ADDR(map_addr))
+			return map_addr;
+		if (eppnt->p_memsz > eppnt->p_filesz) {
+			zero_start = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
+				eppnt->p_filesz;
+			zero_end = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
+				eppnt->p_memsz;
+
+			/* Zero the end of the last mapped page */
+			padzero(zero_start);
+		}
+	} else {
+		zero_start = ELF_PAGESTART(addr);
+		zero_end = zero_start + ELF_PAGEOFFSET(eppnt->p_vaddr) +
+			eppnt->p_memsz;
+	}
+	if (eppnt->p_memsz > eppnt->p_filesz) {
+		/*
+		 * Map the last of the segment.
+		 * If the header is requesting these pages to be
+		 * executable, honour that (ppc32 needs this).
+		 */
+		int error;
+
+		zero_start = ELF_PAGEALIGN(zero_start);
+		zero_end = ELF_PAGEALIGN(zero_end);
+
+		error = vm_brk_flags(zero_start, zero_end - zero_start,
+				     prot & PROT_EXEC ? VM_EXEC : 0);
+		if (error)
+			map_addr = error;
+	}
+	return map_addr;
+}
+
+
 static unsigned long total_mapping_size(const struct elf_phdr *phdr, int nr)
 {
 	elf_addr_t min_addr = -1;
@@ -829,7 +855,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
 	struct elf_phdr *elf_property_phdata = NULL;
 	unsigned long elf_bss, elf_brk;
-	int bss_prot = 0;
 	int retval, i;
 	unsigned long elf_entry;
 	unsigned long e_entry;
@@ -1040,33 +1065,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		if (elf_ppnt->p_type != PT_LOAD)
 			continue;
 
-		if (unlikely (elf_brk > elf_bss)) {
-			unsigned long nbyte;
-
-			/* There was a PT_LOAD segment with p_memsz > p_filesz
-			   before this one. Map anonymous pages, if needed,
-			   and clear the area.  */
-			retval = set_brk(elf_bss + load_bias,
-					 elf_brk + load_bias,
-					 bss_prot);
-			if (retval)
-				goto out_free_dentry;
-			nbyte = ELF_PAGEOFFSET(elf_bss);
-			if (nbyte) {
-				nbyte = ELF_MIN_ALIGN - nbyte;
-				if (nbyte > elf_brk - elf_bss)
-					nbyte = elf_brk - elf_bss;
-				if (clear_user((void __user *)elf_bss +
-							load_bias, nbyte)) {
-					/*
-					 * This bss-zeroing can fail if the ELF
-					 * file specifies odd protections. So
-					 * we don't check the return value
-					 */
-				}
-			}
-		}
-
 		elf_prot = make_prot(elf_ppnt->p_flags, &arch_state,
 				     !!interpreter, false);
 
@@ -1162,7 +1160,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			}
 		}
 
-		error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
+		error = elf_load(bprm->file, load_bias + vaddr, elf_ppnt,
 				elf_prot, elf_flags, total_size);
 		if (BAD_ADDR(error)) {
 			retval = IS_ERR_VALUE(error) ?
@@ -1217,10 +1215,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		if (end_data < k)
 			end_data = k;
 		k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
-		if (k > elf_brk) {
-			bss_prot = elf_prot;
+		if (k > elf_brk)
 			elf_brk = k;
-		}
 	}
 
 	e_entry = elf_ex->e_entry + load_bias;
@@ -1232,18 +1228,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	start_data += load_bias;
 	end_data += load_bias;
 
-	/* Calling set_brk effectively mmaps the pages that we need
-	 * for the bss and break sections.  We must do this before
-	 * mapping in the interpreter, to make sure it doesn't wind
-	 * up getting placed where the bss needs to go.
-	 */
-	retval = set_brk(elf_bss, elf_brk, bss_prot);
-	if (retval)
-		goto out_free_dentry;
-	if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
-		retval = -EFAULT; /* Nobody gets to see this, but.. */
-		goto out_free_dentry;
-	}
+	current->mm->start_brk = current->mm->brk = ELF_PAGEALIGN(elf_brk);
 
 	if (interpreter) {
 		elf_entry = load_elf_interp(interp_elf_ex,
-- 
2.41.0

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[kernel test robot]

Hi,

Thanks for your patch.

FYI: kernel test robot notices the stable kernel rule is not satisfied.

The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html/#option-1

Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree.
Subject: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts
Link: https://lore.kernel.org/stable/87jzsemmsd.fsf_-_%40email.froward.int.ebiederm.org

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Eric W. Biederman]

kernel test robot <lkp@intel.com> writes:

> Hi,
>
> Thanks for your patch.
>
> FYI: kernel test robot notices the stable kernel rule is not satisfied.
>
> The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html/#option-1
>
> Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree.
> Subject: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts
> Link: https://lore.kernel.org/stable/87jzsemmsd.fsf_-_%40email.froward.int.ebiederm.org

My apologies kernel test robot I had realized stable was cc'd on this
conversation.

This patch as is, is most definitely not stable fodder.  Maybe after
being tested.  It is definitely not a regression fix, and I am not
certain even after being tested it could be considered a fix rather than
just a new feature. AKA "Support very weird compiler generated
exectuables".

Eric

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Sebastian Ott]

[-- Attachment #1: Type: text/plain, Size: 881 bytes --]

On Mon, 25 Sep 2023, Eric W. Biederman wrote:
>
> Implement a helper elf_load that wraps elf_map and performs all
> of the necessary work to ensure that when "memsz > filesz"
> the bytes described by "memsz > filesz" are zeroed.
>
> Link: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
> Reported-by: Sebastian Ott <sebott@redhat.com>
> Reported-by: Thomas Weißschuh <linux@weissschuh.net>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
> fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
> 1 file changed, 48 insertions(+), 63 deletions(-)
>
> Can you please test this one?
>

That one did the trick! The arm box booted successful, ran the binaries
that were used for the repo of this issue, and ran the nolibc compiled
binaries from kselftests that initially triggered the loader issues.

Thanks,
Sebastian

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Kees Cook]

On Mon, Sep 25, 2023 at 05:27:12PM +0200, Sebastian Ott wrote:
> On Mon, 25 Sep 2023, Eric W. Biederman wrote:
> > 
> > Implement a helper elf_load that wraps elf_map and performs all
> > of the necessary work to ensure that when "memsz > filesz"
> > the bytes described by "memsz > filesz" are zeroed.
> > 
> > Link: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
> > Reported-by: Sebastian Ott <sebott@redhat.com>
> > Reported-by: Thomas Weißschuh <linux@weissschuh.net>
> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> > ---
> > fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
> > 1 file changed, 48 insertions(+), 63 deletions(-)
> > 
> > Can you please test this one?

Eric thanks for doing this refactoring! This does look similar to the
earlier attempt:
https://lore.kernel.org/lkml/20221106021657.1145519-1-pedro.falcato@gmail.com/
and it's a bit easier to review.

> That one did the trick! The arm box booted successful, ran the binaries
> that were used for the repo of this issue, and ran the nolibc compiled
> binaries from kselftests that initially triggered the loader issues.

Thanks for testing! I need to dig out the other "weird" binaries (like
the mentioned ppc32 case) and see if I can get those tested too.

Pedro, are you able to test ppc64le musl libc with this patch too?

-Kees

-- 
Kees Cook

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Eric W. Biederman]

Kees Cook <keescook@chromium.org> writes:

> On Mon, Sep 25, 2023 at 05:27:12PM +0200, Sebastian Ott wrote:
>> On Mon, 25 Sep 2023, Eric W. Biederman wrote:
>> > 
>> > Implement a helper elf_load that wraps elf_map and performs all
>> > of the necessary work to ensure that when "memsz > filesz"
>> > the bytes described by "memsz > filesz" are zeroed.
>> > 
>> > Link: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
>> > Reported-by: Sebastian Ott <sebott@redhat.com>
>> > Reported-by: Thomas Weißschuh <linux@weissschuh.net>
>> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> > ---
>> > fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
>> > 1 file changed, 48 insertions(+), 63 deletions(-)
>> > 
>> > Can you please test this one?
>
> Eric thanks for doing this refactoring! This does look similar to the
> earlier attempt:
> https://lore.kernel.org/lkml/20221106021657.1145519-1-pedro.falcato@gmail.com/
> and it's a bit easier to review.

I need to context switch away for a while so Kees if you will
I will let you handle the rest of this one.


A couple of thoughts running through my head for anyone whose ambitious
might include cleaning up binfmt_elf.c

The elf_bss variable in load_elf_binary can be removed.

Work for a follow on patch is using my new elf_load in load_elf_interp
and possibly in load_elf_library.  (More code size reduction).

An outstanding issue is if the first segment has filesz 0, and has a
randomized locations.  But that is the same as today.

There is a whole question does it make sense for the elf loader
to have it's own helper vm_brk_flags in mm/mmap.c or would it
make more sense for binfmt_elf to do what binfmt_elf_fdpic does and
have everything to go through vm_mmap.

I think replacing vm_brk_flags with vm_mmap would allow fixing the
theoretical issue of filesz 0 and randomizing locations.



In this change I replaced an open coded padzero that did not clear
all of the way to the end of the page, with padzero that does.

I also stopped checking the return of padzero as there is at least
one known case where testing for failure is the wrong thing to do.
It looks like binfmt_elf_fdpic may have the proper set of tests
for when error handling can be safely completed.

I found a couple of commits in the old history
https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git,
that look very interesting in understanding this code.

commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail")
commit c6e2227e4a3e ("[SPARC64]: Missing user access return value checks in fs/binfmt_elf.c and fs/compat.c")
commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2")

Looking at commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail"):
>  commit 39b56d902bf35241e7cba6cc30b828ed937175ad
>  Author: Pavel Machek <pavel@ucw.cz>
>  Date:   Wed Feb 9 22:40:30 2005 -0800
> 
>     [PATCH] binfmt_elf: clearing bss may fail
>     
>     So we discover that Borland's Kylix application builder emits weird elf
>     files which describe a non-writeable bss segment.
>     
>     So remove the clear_user() check at the place where we zero out the bss.  I
>     don't _think_ there are any security implications here (plus we've never
>     checked that clear_user() return value, so whoops if it is a problem).
>     
>     Signed-off-by: Pavel Machek <pavel@suse.cz>
>     Signed-off-by: Andrew Morton <akpm@osdl.org>
>     Signed-off-by: Linus Torvalds <torvalds@osdl.org>

It seems pretty clear that binfmt_elf_fdpic with skipping clear_user
for non-writable segments and otherwise calling clear_user (aka padzero)
and checking it's return code is the right thing to do.

I just skipped the error checking as that avoids breaking things.

It looks like Borland's Kylix died in 2005 so it might be safe to
just consider read-only segments with memsz > filesz an error.


Looking at commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2") the
binfmt_elf.c bits confirm my guess that the weird structure is because
before that point binfmt_elf.c assumed there would be only a single
segment with memsz > filesz.  Which is why the code was structured so
weirdly.

Looking a little farther it looks like the binfmt_elf.c was introduced
in Linux v1.0, with essentially the same structure in load_elf_binary as
it has now.  Prior to that Linux hard coded support for a.out binaries
in execve.  So if someone wants to add a Fixes tag it should be
"Fixes: v1.0"

Which finally explains to me why the code is so odd.  For the most part
the code has only received maintenance for the last 30 years or so.
Strictly 29 years, but 30 has a better ring to it.

Anyway those are my rambling thoughts that might help someone.
For now I will be happy if we can get my elf_load helper tested
to everyone's satisfaction and merged.

Eric

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Kees Cook]

On Mon, Sep 25, 2023 at 10:27:02PM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > On Mon, Sep 25, 2023 at 05:27:12PM +0200, Sebastian Ott wrote:
> >> On Mon, 25 Sep 2023, Eric W. Biederman wrote:
> >> > 
> >> > Implement a helper elf_load that wraps elf_map and performs all
> >> > of the necessary work to ensure that when "memsz > filesz"
> >> > the bytes described by "memsz > filesz" are zeroed.
> >> > 
> >> > Link: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
> >> > Reported-by: Sebastian Ott <sebott@redhat.com>
> >> > Reported-by: Thomas Weißschuh <linux@weissschuh.net>
> >> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> > ---
> >> > fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
> >> > 1 file changed, 48 insertions(+), 63 deletions(-)
> >> > 
> >> > Can you please test this one?
> >
> > Eric thanks for doing this refactoring! This does look similar to the
> > earlier attempt:
> > https://lore.kernel.org/lkml/20221106021657.1145519-1-pedro.falcato@gmail.com/
> > and it's a bit easier to review.
> 
> I need to context switch away for a while so Kees if you will
> I will let you handle the rest of this one.
> 
> 
> A couple of thoughts running through my head for anyone whose ambitious
> might include cleaning up binfmt_elf.c
> 
> The elf_bss variable in load_elf_binary can be removed.
> 
> Work for a follow on patch is using my new elf_load in load_elf_interp
> and possibly in load_elf_library.  (More code size reduction).
> 
> An outstanding issue is if the first segment has filesz 0, and has a
> randomized locations.  But that is the same as today.
> 
> There is a whole question does it make sense for the elf loader
> to have it's own helper vm_brk_flags in mm/mmap.c or would it
> make more sense for binfmt_elf to do what binfmt_elf_fdpic does and
> have everything to go through vm_mmap.
> 
> I think replacing vm_brk_flags with vm_mmap would allow fixing the
> theoretical issue of filesz 0 and randomizing locations.
> 
> 
> 
> In this change I replaced an open coded padzero that did not clear
> all of the way to the end of the page, with padzero that does.

Yeah, the resulting code is way more readable now.

> I also stopped checking the return of padzero as there is at least
> one known case where testing for failure is the wrong thing to do.
> It looks like binfmt_elf_fdpic may have the proper set of tests
> for when error handling can be safely completed.
> 
> I found a couple of commits in the old history
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git,
> that look very interesting in understanding this code.
> 
> commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail")
> commit c6e2227e4a3e ("[SPARC64]: Missing user access return value checks in fs/binfmt_elf.c and fs/compat.c")
> commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2")
> 
> Looking at commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail"):
> >  commit 39b56d902bf35241e7cba6cc30b828ed937175ad
> >  Author: Pavel Machek <pavel@ucw.cz>
> >  Date:   Wed Feb 9 22:40:30 2005 -0800
> > 
> >     [PATCH] binfmt_elf: clearing bss may fail
> >     
> >     So we discover that Borland's Kylix application builder emits weird elf
> >     files which describe a non-writeable bss segment.
> >     
> >     So remove the clear_user() check at the place where we zero out the bss.  I
> >     don't _think_ there are any security implications here (plus we've never
> >     checked that clear_user() return value, so whoops if it is a problem).
> >     
> >     Signed-off-by: Pavel Machek <pavel@suse.cz>
> >     Signed-off-by: Andrew Morton <akpm@osdl.org>
> >     Signed-off-by: Linus Torvalds <torvalds@osdl.org>
> 
> It seems pretty clear that binfmt_elf_fdpic with skipping clear_user
> for non-writable segments and otherwise calling clear_user (aka padzero)
> and checking it's return code is the right thing to do.
> 
> I just skipped the error checking as that avoids breaking things.
> 
> It looks like Borland's Kylix died in 2005 so it might be safe to
> just consider read-only segments with memsz > filesz an error.

I really feel like having a read-only BSS is a pathological state that
should be detected early?

> Looking at commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2") the
> binfmt_elf.c bits confirm my guess that the weird structure is because
> before that point binfmt_elf.c assumed there would be only a single
> segment with memsz > filesz.  Which is why the code was structured so
> weirdly.

Agreed.

> Looking a little farther it looks like the binfmt_elf.c was introduced
> in Linux v1.0, with essentially the same structure in load_elf_binary as
> it has now.  Prior to that Linux hard coded support for a.out binaries
> in execve.  So if someone wants to add a Fixes tag it should be
> "Fixes: v1.0"
> 
> Which finally explains to me why the code is so odd.  For the most part
> the code has only received maintenance for the last 30 years or so.
> Strictly 29 years, but 30 has a better ring to it.
> 
> Anyway those are my rambling thoughts that might help someone.
> For now I will be happy if we can get my elf_load helper tested
> to everyone's satisfaction and merged.

I'm probably going to pull most of this email into the commit log for
the v2 patch -- there's good history here worth capturing.

-- 
Kees Cook

Re: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Dan Carpenter]

Hi Eric,

kernel test robot noticed the following build warnings:

https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Eric-W-Biederman/binfmt_elf-Support-segments-with-0-filesz-and-misaligned-starts/20230925-210022
base:   https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/execve
patch link:    https://lore.kernel.org/r/87jzsemmsd.fsf_-_%40email.froward.int.ebiederm.org
patch subject: [PATCH] binfmt_elf: Support segments with 0 filesz and misaligned starts
config: i386-randconfig-141-20230926 (https://download.01.org/0day-ci/archive/20230926/202309261925.QvgPAYL7-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce: (https://download.01.org/0day-ci/archive/20230926/202309261925.QvgPAYL7-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
| Closes: https://lore.kernel.org/r/202309261925.QvgPAYL7-lkp@intel.com/

smatch warnings:
fs/binfmt_elf.c:431 elf_load() error: uninitialized symbol 'map_addr'.

vim +/map_addr +431 fs/binfmt_elf.c

a6409120b31666 Eric W. Biederman 2023-09-25  390  static unsigned long elf_load(struct file *filep, unsigned long addr,
a6409120b31666 Eric W. Biederman 2023-09-25  391  		const struct elf_phdr *eppnt, int prot, int type,
a6409120b31666 Eric W. Biederman 2023-09-25  392  		unsigned long total_size)
a6409120b31666 Eric W. Biederman 2023-09-25  393  {
a6409120b31666 Eric W. Biederman 2023-09-25  394  	unsigned long zero_start, zero_end;
a6409120b31666 Eric W. Biederman 2023-09-25  395  	unsigned long map_addr;
a6409120b31666 Eric W. Biederman 2023-09-25  396  
a6409120b31666 Eric W. Biederman 2023-09-25  397  	if (eppnt->p_filesz) {
a6409120b31666 Eric W. Biederman 2023-09-25  398  		map_addr = elf_map(filep, addr, eppnt, prot, type, total_size);
a6409120b31666 Eric W. Biederman 2023-09-25  399  		if (BAD_ADDR(map_addr))
a6409120b31666 Eric W. Biederman 2023-09-25  400  			return map_addr;
a6409120b31666 Eric W. Biederman 2023-09-25  401  		if (eppnt->p_memsz > eppnt->p_filesz) {
a6409120b31666 Eric W. Biederman 2023-09-25  402  			zero_start = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
a6409120b31666 Eric W. Biederman 2023-09-25  403  				eppnt->p_filesz;
a6409120b31666 Eric W. Biederman 2023-09-25  404  			zero_end = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
a6409120b31666 Eric W. Biederman 2023-09-25  405  				eppnt->p_memsz;
a6409120b31666 Eric W. Biederman 2023-09-25  406  
a6409120b31666 Eric W. Biederman 2023-09-25  407  			/* Zero the end of the last mapped page */
a6409120b31666 Eric W. Biederman 2023-09-25  408  			padzero(zero_start);
a6409120b31666 Eric W. Biederman 2023-09-25  409  		}
a6409120b31666 Eric W. Biederman 2023-09-25  410  	} else {
a6409120b31666 Eric W. Biederman 2023-09-25  411  		zero_start = ELF_PAGESTART(addr);
a6409120b31666 Eric W. Biederman 2023-09-25  412  		zero_end = zero_start + ELF_PAGEOFFSET(eppnt->p_vaddr) +
a6409120b31666 Eric W. Biederman 2023-09-25  413  			eppnt->p_memsz;

For this else path, map_addr is only set if there is an error.

a6409120b31666 Eric W. Biederman 2023-09-25  414  	}
a6409120b31666 Eric W. Biederman 2023-09-25  415  	if (eppnt->p_memsz > eppnt->p_filesz) {
a6409120b31666 Eric W. Biederman 2023-09-25  416  		/*
a6409120b31666 Eric W. Biederman 2023-09-25  417  		 * Map the last of the segment.
a6409120b31666 Eric W. Biederman 2023-09-25  418  		 * If the header is requesting these pages to be
a6409120b31666 Eric W. Biederman 2023-09-25  419  		 * executable, honour that (ppc32 needs this).
a6409120b31666 Eric W. Biederman 2023-09-25  420  		 */
a6409120b31666 Eric W. Biederman 2023-09-25  421  		int error;
a6409120b31666 Eric W. Biederman 2023-09-25  422  
a6409120b31666 Eric W. Biederman 2023-09-25  423  		zero_start = ELF_PAGEALIGN(zero_start);
a6409120b31666 Eric W. Biederman 2023-09-25  424  		zero_end = ELF_PAGEALIGN(zero_end);
a6409120b31666 Eric W. Biederman 2023-09-25  425  
a6409120b31666 Eric W. Biederman 2023-09-25  426  		error = vm_brk_flags(zero_start, zero_end - zero_start,
a6409120b31666 Eric W. Biederman 2023-09-25  427  				     prot & PROT_EXEC ? VM_EXEC : 0);
a6409120b31666 Eric W. Biederman 2023-09-25  428  		if (error)
a6409120b31666 Eric W. Biederman 2023-09-25  429  			map_addr = error;
a6409120b31666 Eric W. Biederman 2023-09-25  430  	}
a6409120b31666 Eric W. Biederman 2023-09-25 @431  	return map_addr;
a6409120b31666 Eric W. Biederman 2023-09-25  432  }

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki