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SubjectRe: [PATCH] x86/module: Detect corrupt relocations against nonzero data
On Thu, Nov 02, 2017 at 04:57:11PM -0500, Josh Poimboeuf wrote:
> There have been some cases where external tooling (e.g., kpatch-build)
> creates a corrupt relocation which targets the wrong address. This is a
> silent failure which can corrupt memory in unexpected places.
>
> On x86, the bytes of data being overwritten by relocations are always
> initialized to zero beforehand. Use that knowledge to add sanity checks
> to detect such cases before they corrupt memory.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
> arch/x86/kernel/module.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
> index 62e7d70aadd5..a69b12617820 100644
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -172,19 +172,27 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
> case R_X86_64_NONE:
> break;
> case R_X86_64_64:
> + if (*(u64 *)loc != 0)
> + goto nonzero;
> *(u64 *)loc = val;
> break;
> case R_X86_64_32:
> + if (*(u32 *)loc != 0)
> + goto nonzero;
> *(u32 *)loc = val;
> if (val != *(u32 *)loc)
> goto overflow;
> break;
> case R_X86_64_32S:
> + if (*(s32 *)loc != 0)
> + goto nonzero;
> *(s32 *)loc = val;
> if ((s64)val != *(s32 *)loc)
> goto overflow;
> break;
> case R_X86_64_PC32:
> + if (*(u64 *)loc != 0)
> + goto nonzero;
> val -= (u64)loc;
> *(u32 *)loc = val;

NACK - this last bit is obviously a bug, not sure how it passed my
testing without module load failures...

--
Josh

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