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Date:   Fri, 20 Aug 2021 09:53:41 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Sudeep Holla <sudeep.holla@....com>,
        linuxppc-dev@...ts.ozlabs.org, kernel test robot <lkp@...el.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-staging@...ts.linux.dev,
        linux-block@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 57/63] powerpc/signal32: Use struct_group() to zero spe
 regs



Le 20/08/2021 à 09:49, Michael Ellerman a écrit :
> Kees Cook <keescook@...omium.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@...erman.id.au>
>> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>> Cc: Paul Mackerras <paulus@...ba.org>
>> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
>> Cc: Sudeep Holla <sudeep.holla@....com>
>> Cc: linuxppc-dev@...ts.ozlabs.org
>> Reported-by: kernel test robot <lkp@...el.com>
>> Signed-off-by: Kees Cook <keescook@...omium.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 ?


> 
> 
> Not sure if you are happy doing that as part of this patch. I can always
> do it later if not.
> 
> cheers
> 

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