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

Michael Ellerman
2021-08-20T12:14:02Z

Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> 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 ?

Gah. Yes I do :)

cheers