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References: <20201223015312.4882-1-chang.seok.bae@intel.com> <20201223015312.4882-4-chang.seok.bae@intel.com>
In-Reply-To: <20201223015312.4882-4-chang.seok.bae@intel.com>
From:   Jann Horn <jannh@google.com>
Date:   Wed, 23 Dec 2020 07:55:20 +0100
Message-ID: <CAG48ez1yZBYPXvqxW8k7uH+jAFkkLK1KNijgPPw+Kp-8gnp-sQ@mail.gmail.com>
Subject: Re: [PATCH v3 3/4] x86/signal: Prevent an alternate stack overflow
 before a signal delivery
To:     "Chang S. Bae" <chang.seok.bae@intel.com>
Cc:     Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Len Brown <len.brown@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Dave Martin <Dave.Martin@arm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Tony Luck <tony.luck@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        libc-alpha@sourceware.org, linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>
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On Wed, Dec 23, 2020 at 2:57 AM Chang S. Bae <chang.seok.bae@intel.com> wrote:
> The kernel pushes data on the userspace stack when entering a signal. If
> using a sigaltstack(), the kernel precisely knows the user stack size.
>
> When the kernel knows that the user stack is too small, avoid the overflow
> and do an immediate SIGSEGV instead.
>
> This overflow is known to occur on systems with large XSAVE state. The
> effort to increase the size typically used for altstacks reduces the
> frequency of these overflows, but this approach is still useful for legacy
> binaries.
>
> Suggested-by: Jann Horn <jannh@google.com>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Reviewed-by: Len Brown <len.brown@intel.com>
> Cc: Jann Horn <jannh@google.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org

Reviewed-by: Jann Horn <jannh@google.com>