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Date:   Sat, 4 Apr 2020 00:39:24 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        Peter Zijlstra <peterz@infradead.org>,
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        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86: signal: move save_altstack_ex out of generic
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 <20200403231611.81444-1-ndesaulniers@google.com>
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On Fri, Apr 03, 2020 at 04:16:06PM -0700, Nick Desaulniers wrote:
> In some configurations (clang+KASAN), sas_ss_reset() may emit calls to
> memset().  This is a problem for SMAP protections on x86, which should
> try to minimize calls to any function not already on short whitelist, in
> order to prevent leaking AC flags or being used as a gadget.
> 
> Linus noted that unsafe_save_altstack() only has callsites in the
> arch-specific arch/x86/kernel/signal.c, and shouldn't be defined in arch
> independent headers.
> 
> Split the logic of unsafe_save_altstack() into two, and move the definitions
> to arch/x86/include/asm/sigframe.h.  This does less work with the SMAP
> guards down.

Just move that into signal_delivered() and that's it.  SMAP or no SMAP -
doing that until the sigframe is set and we are committed to entering
the handler is wrong.