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

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