[PATCH] binfmt_elf: Fix bug in loading of PIE binaries.

[PATCH] binfmt_elf: Fix bug in loading of PIE binaries.

[Michael Davidson]

With CONFIG_ARCH_BINFMT_ELF_RANDOMIZE_PIE enabled, and a normal
top-down address allocation strategy, load_elf_binary() will
attempt to map a PIE binary into an address range immediately
below mm->mmap_base.

Unfortunately, load_elf_ binary() does not take account of the
need to allocate sufficient space for the entire binary which
means that, while the first PT_LOAD segment is mapped below
mm->mmap_base, the subsequent PT_LOAD segment(s) end up being
mapped above mm->mmap_base into the are that is supposed to
be the "gap" between the stack and the binary.

Since the size of the "gap" on x86_64 is only guaranteed to be
128MB this means that binaries with large data segments > 128MB
can end up mapping part of their data segment over their stack
resulting in corruption of the stack (and the data segment once
the binary starts to run).

Any PIE binary with a data segment > 128MB is vulnerable to this
although address randomization means that the actual gap between
the stack and the end of the binary is normally greater than 128MB.
The larger the data segment of the binary the higher the probability
of failure.

Fix this by calculating the total size of the binary in the same
way as load_elf_interp().

Signed-off-by: Michael Davidson <md@google.com>
---
 fs/binfmt_elf.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 995986b..d925f55 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -862,6 +862,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	    i < loc->elf_ex.e_phnum; i++, elf_ppnt++) {
 		int elf_prot = 0, elf_flags;
 		unsigned long k, vaddr;
+		unsigned long total_size = 0;
 
 		if (elf_ppnt->p_type != PT_LOAD)
 			continue;
@@ -924,10 +925,16 @@ static int load_elf_binary(struct linux_binprm *bprm)
 #else
 			load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
 #endif
+			total_size = total_mapping_size(elf_phdata,
+							loc->elf_ex.e_phnum);
+			if (!total_size) {
+				error = -EINVAL;
+				goto out_free_dentry;
+			}
 		}
 
 		error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
-				elf_prot, elf_flags, 0);
+				elf_prot, elf_flags, total_size);
 		if (BAD_ADDR(error)) {
 			retval = IS_ERR((void *)error) ?
 				PTR_ERR((void*)error) : -EINVAL;
-- 
2.2.0.rc0.207.ga3a616c

Re: [PATCH] binfmt_elf: Fix bug in loading of PIE binaries.

[James Hogan]

Hi Michael,

On 13 April 2015 at 23:49, Michael Davidson <md@google.com> wrote:
> With CONFIG_ARCH_BINFMT_ELF_RANDOMIZE_PIE enabled, and a normal
> top-down address allocation strategy, load_elf_binary() will
> attempt to map a PIE binary into an address range immediately
> below mm->mmap_base.
>
> Unfortunately, load_elf_ binary() does not take account of the
> need to allocate sufficient space for the entire binary which
> means that, while the first PT_LOAD segment is mapped below
> mm->mmap_base, the subsequent PT_LOAD segment(s) end up being
> mapped above mm->mmap_base into the are that is supposed to
> be the "gap" between the stack and the binary.
>
> Since the size of the "gap" on x86_64 is only guaranteed to be
> 128MB this means that binaries with large data segments > 128MB
> can end up mapping part of their data segment over their stack
> resulting in corruption of the stack (and the data segment once
> the binary starts to run).
>
> Any PIE binary with a data segment > 128MB is vulnerable to this
> although address randomization means that the actual gap between
> the stack and the end of the binary is normally greater than 128MB.
> The larger the data segment of the binary the higher the probability
> of failure.
>
> Fix this by calculating the total size of the binary in the same
> way as load_elf_interp().
>
> Signed-off-by: Michael Davidson <md@google.com>
> ---
>  fs/binfmt_elf.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 995986b..d925f55 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -862,6 +862,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>             i < loc->elf_ex.e_phnum; i++, elf_ppnt++) {
>                 int elf_prot = 0, elf_flags;
>                 unsigned long k, vaddr;
> +               unsigned long total_size = 0;
>
>                 if (elf_ppnt->p_type != PT_LOAD)
>                         continue;
> @@ -924,10 +925,16 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  #else
>                         load_bias = ELF_PAGESTART(ELF_ET_DYN_BASE - vaddr);
>  #endif
> +                       total_size = total_mapping_size(elf_phdata,
> +                                                       loc->elf_ex.e_phnum);
> +                       if (!total_size) {
> +                               error = -EINVAL;

I was just printk debugging this function, and this stood out. Should
that be retval instead of error?

Cheers
James

> +                               goto out_free_dentry;
> +                       }
>                 }
>
>                 error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
> -                               elf_prot, elf_flags, 0);
> +                               elf_prot, elf_flags, total_size);
>                 if (BAD_ADDR(error)) {
>                         retval = IS_ERR((void *)error) ?
>                                 PTR_ERR((void*)error) : -EINVAL;
> --
> 2.2.0.rc0.207.ga3a616c
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/