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

Rik van Riel
2017-09-21T20:41:36Z

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On Thu, 2017-09-21 at 11:52 -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.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@redhat.com>

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