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

From: Kees Cook <[email-masked]>
Date: Fri, 20 Aug 2021 08:55:56 -0700

On Fri, Aug 20, 2021 at 05:49:35PM +1000, Michael Ellerman wrote:
> Kees Cook <[email-masked]> 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 <[email-masked]>
> > Cc: Benjamin Herrenschmidt <[email-masked]>
> > Cc: Paul Mackerras <[email-masked]>
> > Cc: Christophe Leroy <[email-masked]>
> > Cc: Sudeep Holla <[email-masked]>
> > Cc: [email-masked]
> > Reported-by: kernel test robot <[email-masked]>
> > Signed-off-by: Kees Cook <[email-masked]>
> > ---
> >  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!

-- 
Kees Cook