linux-kernel - Re: [regression] commit d298b03506d3 ("x86/fpu: Restore the masking out of reserved MXCSR bits")

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Message-ID: <YWlgPJwxmFL5nX4c@zn.tnic>
Date:   Fri, 15 Oct 2021 13:04:28 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Ser Olmy <ser.olmy@...tonmail.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [regression] commit d298b03506d3 ("x86/fpu: Restore the masking
 out of reserved MXCSR bits")

Ok, here it is.

Ser, I'd appreciate you running it too, to make sure your box is still
ok.

Thx.

---
From: Borislav Petkov <bp@...e.de>
Date: Fri, 15 Oct 2021 12:46:25 +0200
Subject: [PATCH] x86/fpu: Mask out the invalid MXCSR bits properly
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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@...ux.intel.com>

Fixes: d298b03506d3 ("x86/fpu: Restore the masking out of reserved MXCSR bits")
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: <stable@...r.kernel.org>
Link: https://lore.kernel.org/r/YWgYIYXLriayyezv@intel.com
---
 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 fa17a27390ab..831b25c5e705 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 */
-- 
2.29.2

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette