[PATCH 5.12 145/178] x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

Greg Kroah-Hartman
2021-06-21T16:49:17Z

From: Andy Lutomirski <luto@kernel.org>

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>
---
arch/x86/kernel/fpu/signal.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -369,6 +369,25 @@ static int __fpu__restore_sig(void __use
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 {
/*