[patch V2 09/52] x86/fpu: Reject invalid MXCSR values in copy_kernel_to_xstate()

[Thomas Gleixner]

Instead of masking out reserved bits, check them and reject the provided
state as invalid if not zero.

Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
V2: New patch
---
 arch/x86/kernel/fpu/xstate.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1166,6 +1166,14 @@ int copy_kernel_to_xstate(struct xregs_s
 	if (validate_user_xstate_header(&hdr))
 		return -EINVAL;
 
+	if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {
+		const u32 *mxcsr = kbuf + offsetof(struct fxregs_state, mxcsr);
+
+		/* Reserved bits in MXCSR must be zero. */
+		if (*mxcsr & ~mxcsr_feature_mask)
+			return -EINVAL;
+	}
+
 	for (i = 0; i < XFEATURE_MAX; i++) {
 		u64 mask = ((u64)1 << i);
 
@@ -1196,9 +1204,6 @@ int copy_kernel_to_xstate(struct xregs_s
 	 */
 	xsave->header.xfeatures |= hdr.xfeatures;
 
-	/* mxcsr reserved bits must be masked to zero for historical reasons. */
-	xsave->i387.mxcsr &= mxcsr_feature_mask;
-
 	return 0;
 }