Re: x86/module: Detect corrupt relocations against nonzero data

[Jessica Yu]

+++ Josh Poimboeuf [02/11/17 21:19 -0500]:
> 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@xxxxxxxxxx>
> > ---
> >  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...

Thanks for the patch Josh, btw - could you also CC the x86 folks when
you send out v2? 

Thanks!

Jessica