linux-hardening - Re: [PATCH] powerpc/signal32: Use struct

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Date:   Fri, 19 Nov 2021 17:35:00 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        kernel test robot <lkp@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] powerpc/signal32: Use struct_group() to zero spe regs



Le 19/11/2021 à 17:28, Kees Cook a écrit :
> On Fri, Nov 19, 2021 at 08:46:27AM +0000, LEROY Christophe wrote:
>>
>>
>> Le 18/11/2021 à 21:36, Kees Cook a écrit :
>>> 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();
>>>            |    ^~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> Reported-by: kernel test robot <lkp@...el.com>
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>
>> Reviewed-by: Christophe Leroy <christophe.leroy@...roup.eu>
>>
>> However, is it really worth adding that grouping ? Wouldn't it be
>> cleaner to handle evr[] and acc separately ? Now that we are using
>> unsafe variants of get/put user performance wouldn't be impacted.
> 
> I'm fine with whatever is desired here. I reworked an earlier version of
> this patch based on mpe's feedback, so I can certain rework it again. :)

Well, with oddities like the below, it may not be straight forward. If 
the objective is to enable FORTIFY_SOURCE, maybe that's good enough.

Let see if Michael has any opinion.


> 
>>
>> I have some doubts about things like:
>>
>> 	unsafe_copy_to_user(&frame->mc_vregs, current->thread.evr,
>> 				    ELF_NEVRREG * sizeof(u32), failed);
>>
>> Because as far as I can see, ELF_NEVRREG is 34 but mc_vregs is a table
>> of 33 u32 and is at the end of the structure:
>>
>> 	struct mcontext {
>> 		elf_gregset_t	mc_gregs;
>> 		elf_fpregset_t	mc_fregs;
>> 		unsigned long	mc_pad[2];
>> 		elf_vrregset_t	mc_vregs __attribute__((__aligned__(16)));
>> 	};
>>
>> 	typedef elf_vrreg_t elf_vrregset_t[ELF_NVRREG];
>>
>> 	# define ELF_NEVRREG	34	/* includes acc (as 2) */
>> 	# define ELF_NVRREG	33	/* includes vscr */
> 
> I don't know these internals very well -- do you want me to change this
> specifically somehow? With the BUILD_BUG_ON()s added, there's no binary
> change here -- I wanted to make sure nothing was different in the
> output.
> 

Neither do I. I was just scared by what I saw while reviewing your 
patch. A cleanup is probably required but it can be another patch.

Christophe