[PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

[PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

[Geyslan G. Bem]

The member 'e_ehsize' that holds the ELF header size is compared
with the elfhdr struct size. If not equal, goes out.
If 'e_phoff' holds 0 the object has no program header table, so
goes out.
Ensures the file being loaded has the correct data encoding, checking
'e_ident[EI_DATA]' against 'ELF_DATA'.

Besides the checks being in accordance with the ELF Specifications,
they increase the binary consistency reducing the use of malformed ones.

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
---
 fs/binfmt_elf.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 571a423..326ca39 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -603,14 +603,20 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	if (memcmp(loc->elf_ex.e_ident, ELFMAG, SELFMAG) != 0)
 		goto out;
 
+	if (loc->elf_ex.e_ehsize != sizeof(struct elfhdr))
+		goto out;
 	if (loc->elf_ex.e_type != ET_EXEC && loc->elf_ex.e_type != ET_DYN)
 		goto out;
 	if (!elf_check_arch(&loc->elf_ex))
 		goto out;
+	if (loc->elf_ex.e_ident[EI_DATA] != ELF_DATA)
+		goto out;
 	if (!bprm->file->f_op->mmap)
 		goto out;
 
 	/* Now read in all of the header information */
+	if (loc->elf_ex.e_phoff == 0)
+		goto out;
 	if (loc->elf_ex.e_phentsize != sizeof(struct elf_phdr))
 		goto out;
 	if (loc->elf_ex.e_phnum < 1 ||
-- 
1.8.4.2

Re: [PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

[Al Viro]

On Wed, Nov 20, 2013 at 09:34:31PM -0300, Geyslan G. Bem wrote:
> The member 'e_ehsize' that holds the ELF header size is compared
> with the elfhdr struct size. If not equal, goes out.
> If 'e_phoff' holds 0 the object has no program header table, so
> goes out.
> Ensures the file being loaded has the correct data encoding, checking
> 'e_ident[EI_DATA]' against 'ELF_DATA'.
> 
> Besides the checks being in accordance with the ELF Specifications,
> they increase the binary consistency reducing the use of malformed ones.

This is completely misguided.  We are allowed to reject such binaries,
but what's the point of doing that?

Re: [PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

[Geyslan Gregório Bem]

2013/11/20 Al Viro <viro@zeniv.linux.org.uk>:
> On Wed, Nov 20, 2013 at 09:34:31PM -0300, Geyslan G. Bem wrote:
>> The member 'e_ehsize' that holds the ELF header size is compared
>> with the elfhdr struct size. If not equal, goes out.
>> If 'e_phoff' holds 0 the object has no program header table, so
>> goes out.
>> Ensures the file being loaded has the correct data encoding, checking
>> 'e_ident[EI_DATA]' against 'ELF_DATA'.
>>
>> Besides the checks being in accordance with the ELF Specifications,
>> they increase the binary consistency reducing the use of malformed ones.
>
> This is completely misguided.  We are allowed to reject such binaries,
> but what's the point of doing that?

Viro, First of all, thanks for reply.

The security (or anti-security) guys are used to mess up with the not checked
header fields for their "benefits": anti-debugging, injection and so on.

Concerning to 'e_phoff': when it is 0 the check avoids that 'elf_phdr' been read
from a erroneous offset (ELF header). I know that without this check the binary
will goes out anyway. But it reduces wasting cpu cycles.

Regarding 'e_ident[EI_DATA]': that check also prevents a farther code reading
when the binary, although been the correct arch, is compiled with a different
data encoding (MSB vs LSB).

So checking besides increase the binary consistency, guarantee some mislead
and fewer cpu cycles.

-- 
Regards,

Geyslan G. Bem
hackingbits.com

Re: [PATCH v2] fs: binfmt_elf: Add ELF header consistency checks

[Geyslan Gregório Bem]

2013/11/21 Geyslan Gregório Bem <geyslan@gmail.com>:
> 2013/11/20 Al Viro <viro@zeniv.linux.org.uk>:
>> On Wed, Nov 20, 2013 at 09:34:31PM -0300, Geyslan G. Bem wrote:
>>> The member 'e_ehsize' that holds the ELF header size is compared
>>> with the elfhdr struct size. If not equal, goes out.
>>> If 'e_phoff' holds 0 the object has no program header table, so
>>> goes out.
>>> Ensures the file being loaded has the correct data encoding, checking
>>> 'e_ident[EI_DATA]' against 'ELF_DATA'.
>>>
>>> Besides the checks being in accordance with the ELF Specifications,
>>> they increase the binary consistency reducing the use of malformed ones.
>>
>> This is completely misguided.  We are allowed to reject such binaries,
>> but what's the point of doing that?
>
> Viro, First of all, thanks for reply.
>
> The security (or anti-security) guys are used to mess up with the not checked
> header fields for their "benefits": anti-debugging, injection and so on.
>
> Concerning to 'e_phoff': when it is 0 the check avoids that 'elf_phdr' been read
> from a erroneous offset (ELF header). I know that without this check the binary
> will goes out anyway. But it reduces wasting cpu cycles.
>
> Regarding 'e_ident[EI_DATA]': that check also prevents a farther code reading
> when the binary, although been the correct arch, is compiled with a different
> data encoding (MSB vs LSB).
>
> So checking besides increase the binary consistency, guarantee some mislead
> and fewer cpu cycles.
>
> --
> Regards,
>
> Geyslan G. Bem
> hackingbits.com

Another good reason is that ld does reject such binaries (I hex edit
one to hold MSB value in header):

uzumaki@hb ~ $ /lib/ld-linux-x86-64.so.2 ./a.out
./a.out: error while loading shared libraries: ./a.out: ELF file data
encoding not little-endian

After zeroing the phoff:

uzumaki@hb ~ $ /lib/ld-linux-x86-64.so.2 ./a.out
./a.out: error while loading shared libraries: ./a.out: object file
has no loadable segments

I really think that is a way to get a more robust binfmt consistency check.

What you think?

-- 
Regards,

Geyslan G. Bem
hackingbits.com