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References: <20210316065215.23768-1-chang.seok.bae@intel.com>
 <20210316065215.23768-6-chang.seok.bae@intel.com> <CALCETrU_n+dP4GaUJRQoKcDSwaWL9Vc99Yy+N=QGVZ_tx_j3Zg@mail.gmail.com>
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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 25 Mar 2021 21:58:54 -0700
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Subject: Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal
 stack overflow
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Andrew Cooper <andrew.cooper3@citrix.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <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>,
        Jann Horn <jannh@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Carlos O'Donell" <carlos@redhat.com>,
        Tony Luck <tony.luck@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>
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I forgot to mention why I cc'd all you fine Xen folk:

On Thu, Mar 25, 2021 at 11:13 AM Andy Lutomirski <luto@kernel.org> wrote:

>
> >         } else if (IS_ENABLED(CONFIG_X86_32) &&
> >                    !onsigstack &&
> >                    regs->ss != __USER_DS &&

This bit here seems really dubious on Xen PV.  Honestly it seems
dubious everywhere, but especially on Xen PV.

--Andy