linux-kernel - Re: x86/module: Detect corrupt relocations against nonzero data

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Date:   Fri, 3 Nov 2017 11:22:13 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: x86/module: Detect corrupt relocations against nonzero data

+++ 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@...hat.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...

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

Thanks!

Jessica