[patch] x86/fpu: x86/fpu: Preserve supervisor states in sanitize_restored_user_xstate()

Thomas Gleixner
2021-06-14T20:15:15Z

sanitize_restored_user_xstate() preserves the supervisor states only
when the fx_only argument is zero, which allows unpriviledged user space
to put supervisor states back into init state.

Preserve them unconditionally.

Fixes: 5d6b6a6f9b5c ("x86/fpu/xstate: Update sanitize_restored_xstate() for supervisor xstates")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
---
arch/x86/kernel/fpu/signal.c | 26 ++++++++------------------
1 file changed, 8 insertions(+), 18 deletions(-)

--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -221,28 +221,18 @@ sanitize_restored_user_xstate(union fpre

if (use_xsave()) {
/*
- * Note: we don't need to zero the reserved bits in the
- * xstate_header here because we either didn't copy them at all,
- * or we checked earlier that they aren't set.
+ * Clear all features bit which are not set in
+ * user_xfeatures and clear all extended features
+ * for fx_only mode.
*/
+ u64 mask = fx_only ? XFEATURE_MASK_FPSSE : user_xfeatures;

/*
- * 'user_xfeatures' might have bits clear which are
- * set in header->xfeatures. This represents features that
- * were in init state prior to a signal delivery, and need
- * to be reset back to the init state. Clear any user
- * feature bits which are set in the kernel buffer to get
- * them back to the init state.
- *
- * Supervisor state is unchanged by input from userspace.
- * Ensure supervisor state bits stay set and supervisor
- * state is not modified.
+ * Supervisor state has to be preserved. The sigframe
+ * restore can only modify user features, i.e. @mask
+ * cannot contain them.
*/
- if (fx_only)
- header->xfeatures = XFEATURE_MASK_FPSSE;
- else
- header->xfeatures &= user_xfeatures |
- xfeatures_mask_supervisor();
+ header->xfeatures &= mask | xfeatures_mask_supervisor();
}

if (use_fxsr()) {