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x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

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commit d8778e393afa421f1f117471144f8ce6deb6953a upstream.

Both Intel and AMD consider it to be architecturally valid for XRSTOR to
fail with #PF but nonetheless change the register state.  The actual
conditions under which this might occur are unclear [1], but it seems
plausible that this might be triggered if one sibling thread unmaps a page
and invalidates the shared TLB while another sibling thread is executing
XRSTOR on the page in question.

__fpu__restore_sig() can execute XRSTOR while the hardware registers
are preserved on behalf of a different victim task (using the
fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but
modify the registers.

If this happens, then there is a window in which __fpu__restore_sig()
could schedule out and the victim task could schedule back in without
reloading its own FPU registers. This would result in part of the FPU
state that __fpu__restore_sig() was attempting to load leaking into the
victim task's user-visible state.

Invalidate preserved FPU registers on XRSTOR failure to prevent this
situation from corrupting any state.

[1] Frequent readers of the errata lists might imagine "complex
    microarchitectural conditions".

Fixes: 1d731e731c4c ("x86/fpu: Add a fastpath to __fpu__restore_sig()")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Rik van Riel <riel@surriel.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20210608144345.758116583@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Andy Lutomirski 2021-06-08 16:36:19 +02:00 committed by Greg Kroah-Hartman
parent 076f732b16
commit a7748e021b
1 changed files with 19 additions and 0 deletions
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19
arch/x86/kernel/fpu/signal.c
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@ -369,6 +369,25 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
fpregs_unlock();
return 0;
}
/*
* The above did an FPU restore operation, restricted to
* the user portion of the registers, and failed, but the
* microcode might have modified the FPU registers
* nevertheless.
*
* If the FPU registers do not belong to current, then
* invalidate the FPU register state otherwise the task might
* preempt current and return to user space with corrupted
* FPU registers.
*
* In case current owns the FPU registers then no further
* action is required. The fixup below will handle it
* correctly.
*/
if (test_thread_flag(TIF_NEED_FPU_LOAD))
__cpu_invalidate_fpregs_state();
fpregs_unlock();
} else {
/*