[PATCH v3 1/4] binfmt_elf: Support segments with 0 filesz and misaligned starts

Kees Cook
2023-09-27T04:35:17Z

From: "Eric W. Biederman" <ebiederm@xmission.com>

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.

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

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.

And notably, 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.

Reported-by: Sebastian Ott <sebott@redhat.com>
Reported-by: Thomas Weißschuh <linux@weissschuh.net>
Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/87sf71f123.fsf@email.froward.int.ebiederm.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
1 file changed, 48 insertions(+), 63 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7b3d2d491407..2a615f476e44 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 {
+ map_addr = 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,
--
2.34.1