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

Michael Ellerman
2021-08-23T04:56:13Z

Kees Cook <keescook@chromium.org> writes:
> On Fri, Aug 20, 2021 at 05:49:35PM +1000, Michael Ellerman wrote:
>> 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));
>>
>> Not sure if you are happy doing that as part of this patch. I can always
>> do it later if not.
>
> Sounds good to me; I did that in a few other cases in the series where
> the relationships between things seemed tenuous. :) I'll add this (as
> !=) in v3.

Thanks.

cheers