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Message-ID: <14f3bee7569f229541852f61f0a1a88fcdec7249.camel@intel.com>
Subject: Re: Random shadow stack pointer corruption
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Tony Luck <tony.luck@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>
Date:   Sat, 18 Jul 2020 16:34:05 -0700
In-Reply-To: <d4b9d6a1-d413-b044-1d4a-512bbbf06d78@intel.com>
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On Sat, 2020-07-18 at 15:41 -0700, Dave Hansen wrote:
> On 7/18/20 11:24 AM, Yu-cheng Yu wrote:
> > On Sat, 2020-07-18 at 11:00 -0700, Andy Lutomirski wrote:
> > > On Sat, Jul 18, 2020 at 10:58 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> > > > Hi,
> > > > 
> > > > My shadow stack tests start to have random shadow stack pointer corruption after
> > > > v5.7 (excluding).  The symptom looks like some locking issue or the kernel is
> > > > confused about which CPU a task is on.  In later tip/master, this can be
> > > > triggered by creating two tasks and each does continuous
> > > > pthread_create()/pthread_join().  If the kernel has max_cpus=1, the issue goes
> > > > away.  I also checked XSAVES/XRSTORS, but this does not seem to be an issue
> > > > coming from there.
> > > 
> > > What do you mean "shadow stack pointer corruption"?  Is SSP itself
> > > corrupt while running in the kernel?  Is one of the MSRs getting
> > > corrupted?  Is the memory to which the shadow stack points getting
> > > corrupted? Is the CPU rejecting an attempt to change SSP?
> > 
> > What I see is, a new thread after ret_from_fork() and iret back to ring-3, 
> > its shadow stack pointer (MSR_IA32_PL3_SSP) is corrupted.
> 
> Does corrupt mean random?  Or is it a valid stack address, just not for
> _this_ thread?  Or NULL?  Or is it a kernel address?  Have you tried
> tracing *ALL* the WRMSR's and XRSTOR's that write to the MSR?

When a shadow stack address is changed, the address appears to be other task's. 
I traced all WRMSR's and XRSTOR's.  I also verified there have not been any
XRSTORS from a wrong buffer.  When rc6 is tagged, I will re-base, test, and
share current patches.