Re: [PATCH v4 1/2] fs/binfmt_elf: Fix AT_PHDR for unusual ELF files

[Kees Cook <keescook@chromium.org>]

On Mon, Dec 13, 2021 at 08:24:11AM +0900, Akira Kawata wrote:
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=197921
> 
> As pointed out in the discussion of buglink, we cannot calculate AT_PHDR
> as the sum of load_addr and exec->e_phoff.
> 
> : The AT_PHDR of ELF auxiliary vectors should point to the memory address
> : of program header. But binfmt_elf.c calculates this address as follows:
> :
> : NEW_AUX_ENT(AT_PHDR, load_addr + exec->e_phoff);
> :
> : which is wrong since e_phoff is the file offset of program header and
> : load_addr is the memory base address from PT_LOAD entry.
> :
> : The ld.so uses AT_PHDR as the memory address of program header. In normal
> : case, since the e_phoff is usually 64 and in the first PT_LOAD region, it
> : is the correct program header address.
> :
> : But if the address of program header isn't equal to the first PT_LOAD
> : address + e_phoff (e.g.  Put the program header in other non-consecutive
> : PT_LOAD region), ld.so will try to read program header from wrong address
> : then crash or use incorrect program header.
> 
> This is because exec->e_phoff
> is the offset of PHDRs in the file and the address of PHDRs in the
> memory may differ from it. This patch fixes the bug by calculating the
> address of program headers from PT_LOADs directly.
> 
> Signed-off-by: Akira Kawata <akirakawata1@gmail.com>
> Reported-by: kernel test robot <lkp@intel.com>
> ---
>  fs/binfmt_elf.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index beeb1247b5c4..828e88841cb4 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -170,8 +170,8 @@ static int padzero(unsigned long elf_bss)
>  
>  static int
>  create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
> -		unsigned long load_addr, unsigned long interp_load_addr,
> -		unsigned long e_entry)
> +		unsigned long interp_load_addr,
> +		unsigned long e_entry, unsigned long phdr_addr)
>  {
>  	struct mm_struct *mm = current->mm;
>  	unsigned long p = bprm->p;
> @@ -257,7 +257,7 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
>  	NEW_AUX_ENT(AT_HWCAP, ELF_HWCAP);
>  	NEW_AUX_ENT(AT_PAGESZ, ELF_EXEC_PAGESIZE);
>  	NEW_AUX_ENT(AT_CLKTCK, CLOCKS_PER_SEC);
> -	NEW_AUX_ENT(AT_PHDR, load_addr + exec->e_phoff);
> +	NEW_AUX_ENT(AT_PHDR, phdr_addr);
>  	NEW_AUX_ENT(AT_PHENT, sizeof(struct elf_phdr));
>  	NEW_AUX_ENT(AT_PHNUM, exec->e_phnum);
>  	NEW_AUX_ENT(AT_BASE, interp_load_addr);
> @@ -822,7 +822,7 @@ static int parse_elf_properties(struct file *f, const struct elf_phdr *phdr,
>  static int load_elf_binary(struct linux_binprm *bprm)
>  {
>  	struct file *interpreter = NULL; /* to shut gcc up */
> - 	unsigned long load_addr = 0, load_bias = 0;
> +	unsigned long load_addr, load_bias = 0, phdr_addr = 0;
>  	int load_addr_set = 0;
>  	unsigned long error;
>  	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
> @@ -1168,6 +1168,13 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  				reloc_func_desc = load_bias;
>  			}
>  		}
> +
> +		if (elf_ppnt->p_offset <= elf_ex->e_phoff &&
> +		    elf_ex->e_phoff < elf_ppnt->p_offset + elf_ppnt->p_filesz) {
> +			phdr_addr = elf_ex->e_phoff - elf_ppnt->p_offset +
> +				    elf_ppnt->p_vaddr;
> +		}

This chunk could really use a comment above it. Maybe something like:

/*
 * Figure out which segment in the file contains the Program
 * Header table, and map to the associated memory address.
 */

Some additional thoughts:

1) The ELF spec says e_phoff is 0 if there's no program header table.

The old code would just pass the load_addr as a result. This patch will
now retain the same result (phdr_addr defaults to 0). I wonder if there
is a bug in this behavior, though? (To be addressed in a different patch
if needed...)

2) This finds any matching segment, not just PT_PHDR, which is good,
since PT_PHDR isn't strictly required.

> +
>  		k = elf_ppnt->p_vaddr;
>  		if ((elf_ppnt->p_flags & PF_X) && k < start_code)
>  			start_code = k;
> @@ -1203,6 +1210,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	}
>  
>  	e_entry = elf_ex->e_entry + load_bias;
> +	phdr_addr += load_bias;
>  	elf_bss += load_bias;
>  	elf_brk += load_bias;
>  	start_code += load_bias;
> @@ -1266,8 +1274,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		goto out;
>  #endif /* ARCH_HAS_SETUP_ADDITIONAL_PAGES */
>  
> -	retval = create_elf_tables(bprm, elf_ex,
> -			  load_addr, interp_load_addr, e_entry);
> +	retval = create_elf_tables(bprm, elf_ex, interp_load_addr,
> +				   e_entry, phdr_addr);
>  	if (retval < 0)
>  		goto out;

Looks good!

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook