Commit d8778e39 authored Jun 08, 2021 by Andy Lutomirski Committed by Borislav Petkov Jun 09, 2021

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



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: 1d731e73 ("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

parent 484cea4f

arch/x86/kernel/fpu/signal.c

+19 −0

Original line number	Diff line number	Diff line
		@@ -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 {
		/*