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x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

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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
This commit is contained in:
Petteri Aimonen 2020-06-16 11:12:57 +02:00 committed by Borislav Petkov
parent 48778464bb
commit 7ad816762f
2 changed files with 11 additions and 0 deletions
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5
arch/x86/include/asm/fpu/internal.h
View File

@ -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

6
arch/x86/kernel/fpu/core.c
View File

@ -101,6 +101,12 @@ 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);
if (boot_cpu_has(X86_FEATURE_FPU))
asm volatile ("fninit");
}
EXPORT_SYMBOL_GPL(kernel_fpu_begin);