Re: [PATCH] x86/module: Detect corrupt relocations against nonzero data

[Josh Poimboeuf <jpoimboe@redhat.com>]

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