Re: [PATCH v2 57/63] powerpc/signal32: Use struct_group() to zero spe regs

Christophe Leroy
2021-08-20T07:53:50Z

Le 20/08/2021 à 09:49, Michael Ellerman a écrit :
> Kees Cook <keescook@chromium.org> writes:
>> In preparation for FORTIFY_SOURCE performing compile-time and run-time
>> field bounds checking for memset(), avoid intentionally writing across
>> neighboring fields.
>>
>> Add a struct_group() for the spe registers so that memset() can correctly reason
>> about the size:
>>
>> In function 'fortify_memset_chk',
>> inlined from 'restore_user_regs.part.0' at arch/powerpc/kernel/signal_32.c:539:3:
>>>> include/linux/fortify-string.h:195:4: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
>> 195 | __write_overflow_field();
>> | ^~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
>> Cc: Sudeep Holla <sudeep.holla@arm.com>
>> Cc: linuxppc-dev@lists.ozlabs.org
>> Reported-by: kernel test robot <lkp@intel.com>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> arch/powerpc/include/asm/processor.h | 6 ++++--
>> arch/powerpc/kernel/signal_32.c | 6 +++---
>> 2 files changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
>> index f348e564f7dd..05dc567cb9a8 100644
>> --- a/arch/powerpc/include/asm/processor.h
>> +++ b/arch/powerpc/include/asm/processor.h
>> @@ -191,8 +191,10 @@ struct thread_struct {
>> int used_vsr; /* set if process has used VSX */
>> #endif /* CONFIG_VSX */
>> #ifdef CONFIG_SPE
>> - unsigned long evr[32]; /* upper 32-bits of SPE regs */
>> - u64 acc; /* Accumulator */
>> + struct_group(spe,
>> + unsigned long evr[32]; /* upper 32-bits of SPE regs */
>> + u64 acc; /* Accumulator */
>> + );
>> unsigned long spefscr; /* SPE & eFP status */
>> unsigned long spefscr_last; /* SPEFSCR value on last prctl
>> call or trap return */
>> diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
>> index 0608581967f0..77b86caf5c51 100644
>> --- a/arch/powerpc/kernel/signal_32.c
>> +++ b/arch/powerpc/kernel/signal_32.c
>> @@ -532,11 +532,11 @@ static long restore_user_regs(struct pt_regs *regs,
>> regs_set_return_msr(regs, regs->msr & ~MSR_SPE);
>> if (msr & MSR_SPE) {
>> /* restore spe registers from the stack */
>> - unsafe_copy_from_user(current->thread.evr, &sr->mc_vregs,
>> - ELF_NEVRREG * sizeof(u32), failed);
>> + unsafe_copy_from_user(&current->thread.spe, &sr->mc_vregs,
>> + sizeof(current->thread.spe), failed);
>
> This makes me nervous, because the ABI is that we copy ELF_NEVRREG *
> sizeof(u32) bytes, not whatever sizeof(current->thread.spe) happens to
> be.
>
> ie. if we use sizeof an inadvertent change to the fields in
> thread_struct could change how many bytes we copy out to userspace,
> which would be an ABI break.
>
> And that's not that hard to do, because it's not at all obvious that the
> size and layout of fields in thread_struct affects the user ABI.
>
> At the same time we don't want to copy the right number of bytes but
> the wrong content, so from that point of view using sizeof is good :)
>
> The way we handle it in ptrace is to have BUILD_BUG_ON()s to verify that
> things match up, so maybe we should do that here too.
>
> ie. add:
>
> BUILD_BUG_ON(sizeof(current->thread.spe) == ELF_NEVRREG * sizeof(u32));

You mean != I guess ?

>
>
> Not sure if you are happy doing that as part of this patch. I can always
> do it later if not.
>
> cheers
>