linux-kernel - Re: [kernel-hardening] [PATCH v3 3/3] x86/fpu: reinitialize FPU registers if restoring FPU state fails

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Message-ID: <1506026489.5486.25.camel@redhat.com>
Date:   Thu, 21 Sep 2017 16:41:29 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Eric Biggers <ebiggers3@...il.com>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...nel.org>, Kevin Hao <haokexin@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [kernel-hardening] [PATCH v3 3/3] x86/fpu: reinitialize FPU
 registers if restoring FPU state fails

On Thu, 2017-09-21 at 11:52 -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Userspace can change the FPU state of a task using the ptrace() or
> rt_sigreturn() system calls.  Because reserved bits in the FPU state
> can
> cause the XRSTOR instruction to fail, the kernel has to carefully
> validate that no reserved bits or other invalid values are being set.
> 
> Unfortunately, there have been bugs in this validation code.  For
> example, we were not checking that the 'xcomp_bv' field in the
> xstate_header was 0.  As-is, such bugs are exploitable to read the
> FPU
> registers of other processes on the system.  To do so, an attacker
> can
> create a task, assign to it an invalid FPU state, then spin in a loop
> and monitor the values of the FPU registers.  Because the task's FPU
> registers are not being restored, sometimes the FPU registers will
> have
> the values from another process.
> 

Reviewed-by: Rik van Riel <riel@...hat.com>

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