queue/x86-fpu-Reset-MXCSR-to-default-in-kernel_fpu_begin.patch

From e87892448a51b8bdbc9f96a722cd62cb675f2d31 Mon Sep 17 00:00:00 2001
From: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Date: Tue, 16 Jun 2020 11:12:57 +0200
Subject: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

commit 7ad816762f9bf89e940e618ea40c43138b479e10 upstream.

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, add
   FNINIT call too (amluto). ]

Signed-off-by: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=207979
Link: https://lkml.kernel.org/r/20200624114646.28953-2-bp@alien8.de
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 44c48e34d799..00eac7f1529b 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -619,6 +619,11 @@ 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 649fbc3fcf9f..8ba0e53dec75 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -113,6 +113,12 @@ static void __kernel_fpu_begin(void)
 		}
 	}
 	__cpu_invalidate_fpregs_state();
+
+	if (boot_cpu_has(X86_FEATURE_XMM))
+		ldmxcsr(MXCSR_DEFAULT);
+
+	if (boot_cpu_has(X86_FEATURE_FPU))
+		asm volatile ("fninit");
 }
 
 static void __kernel_fpu_end(void)
-- 
2.27.0