linux-kernel - Re: [PATCH mips-fixes] MIPS: fix fortify panic when copying asm exception handlers

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Message-ID: <20220301163411.GC13091@alpha.franken.de>
Date:   Tue, 1 Mar 2022 17:34:11 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Alexander Lobakin <alobakin@...me>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Mike Rapoport <rppt@...nel.org>,
        Davidlohr Bueso <dbueso@...e.de>,
        Florian Fainelli <f.fainelli@...il.com>,
        Liam Howlett <liam.howlett@...cle.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Atsushi Nemoto <anemo@....ocn.ne.jp>,
        linux-mips@...r.kernel.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH mips-fixes] MIPS: fix fortify panic when copying asm
 exception handlers

On Wed, Feb 23, 2022 at 01:30:23AM +0000, Alexander Lobakin wrote:
> With KCFLAGS="-O3", I was able to trigger a fortify-source
> memcpy() overflow panic on set_vi_srs_handler().
> Although O3 level is not supported in the mainline, under some
> conditions that may've happened with any optimization settings,
> it's just a matter of inlining luck. The panic itself is correct,
> more precisely, 50/50 false-positive and not at the same time.
> >From the one side, no real overflow happens. Exception handler
> defined in asm just gets copied to some reserved places in the
> memory.
> But the reason behind is that C code refers to that exception
> handler declares it as `char`, i.e. something of 1 byte length.
> It's obvious that the asm function itself is way more than 1 byte,
> so fortify logics thought we are going to past the symbol declared.
> The standard way to refer to asm symbols from C code which is not
> supposed to be called from C is to declare them as
> `extern const u8[]`. This is fully correct from any point of view,
> as any code itself is just a bunch of bytes (including 0 as it is
> for syms like _stext/_etext/etc.), and the exact size is not known
> at the moment of compilation.
> Adjust the type of the except_vec_vi_*() and related variables.
> Make set_handler() take `const` as a second argument to avoid
> cast-away warnings and give a little more room for optimization.
> 
> Fixes: e01402b115cc ("More AP / SP bits for the 34K, the Malta bits and things. Still wants")
> Fixes: c65a5480ff29 ("[MIPS] Fix potential latency problem due to non-atomic cpu_wait.")
> Cc: stable@...r.kernel.org # 3.10+

I like your patch, but I have a problem with these tags. If I understand
your description correctly there is no bug, but because of the way the
code is written fortify-source gets confused. So if it doesn't fix
anything, there shouldn't be Fixes tags, IMHO. If you agree, I'll
apply this patch to mips-next and remove the tags.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]