[tip: x86/urgent] x86/fpu: Mask out the invalid MXCSR bits properly

[tip-bot2 for Borislav Petkov]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     b2381acd3fd9bacd2c63f53b2c610c89959b31cc
Gitweb:        https://git.kernel.org/tip/b2381acd3fd9bacd2c63f53b2c610c89959b31cc
Author:        Borislav Petkov <bp@xxxxxxx>
AuthorDate:    Fri, 15 Oct 2021 12:46:25 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Sat, 16 Oct 2021 12:37:50 +02:00

x86/fpu: Mask out the invalid MXCSR bits properly

This is a fix for the fix (yeah, /facepalm).

The correct mask to use is not the negation of the MXCSR_MASK but the
actual mask which contains the supported bits in the MXCSR register.

Reported and debugged by Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>

Fixes: d298b03506d3 ("x86/fpu: Restore the masking out of reserved MXCSR bits")
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Tested-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Tested-by: Ser Olmy <ser.olmy@xxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/YWgYIYXLriayyezv@xxxxxxxxx
---
 arch/x86/kernel/fpu/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index fa17a27..831b25c 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -385,7 +385,7 @@ static int __fpu_restore_sig(void __user *buf, void __user *buf_fx,
 				return -EINVAL;
 		} else {
 			/* Mask invalid bits out for historical reasons (broken hardware). */
-			fpu->state.fxsave.mxcsr &= ~mxcsr_feature_mask;
+			fpu->state.fxsave.mxcsr &= mxcsr_feature_mask;
 		}
 
 		/* Enforce XFEATURE_MASK_FPSSE when XSAVE is enabled */