linux-kernel - [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu

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Date:   Tue, 16 Jun 2020 11:53:40 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     x86-ml <x86@...nel.org>, jpa@...nelbug.mail.kapsi.fi
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

Ok,

here's the fix first so that it goes in. I'll hammer on the test case later.

---
From: Petteri Aimonen <jpa@....mail.kapsi.fi>

Previously, kernel floating point code would run with the MXCSR control
register value last set by userland code by the thread that was active
on the CPU core just before kernel call. This could affect calculation
results if rounding mode was changed, or a crash if a FPU/SIMD exception
was unmasked.

Restore MXCSR to the kernel's default value.

 [ bp: Carve out from a bigger patch by Petteri, add feature check. ]

Signed-off-by: Petteri Aimonen <jpa@....mail.kapsi.fi>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=207979
---
 arch/x86/include/asm/fpu/internal.h | 5 +++++
 arch/x86/kernel/fpu/core.c          | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 42159f45bf9c..845e7481ab77 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -623,6 +623,11 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
  * MXCSR and XCR definitions:
  */
 
+static inline void ldmxcsr(u32 mxcsr)
+{
+	asm volatile("ldmxcsr %0" :: "m" (mxcsr));
+}
+
 extern unsigned int mxcsr_feature_mask;
 
 #define XCR_XFEATURE_ENABLED_MASK	0x00000000
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 06c818967bb6..f398fedc590a 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -101,6 +101,9 @@ void kernel_fpu_begin(void)
 		copy_fpregs_to_fpstate(&current->thread.fpu);
 	}
 	__cpu_invalidate_fpregs_state();
+
+	if (boot_cpu_has(X86_FEATURE_XMM))
+		ldmxcsr(MXCSR_DEFAULT);
 }
 EXPORT_SYMBOL_GPL(kernel_fpu_begin);
 
-- 
2.21.0


-- 
Regards/Gruss,
    Boris.

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