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

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