[PATCH] x86: Reset FPU on exec

[Andi Kleen <andi@firstfloor.org>]

From: Andi Kleen <ak@linux.intel.com>

Currently we don't reset FPU state on exec. This can be seen as a
(minor) security issue. The bigger issue however is that the
AVX state also does not get reset. So a program that uses SSE
without VZEROUPPER may get a large penalty.

Always set the FPU to the init state at exec time.

For the eager FPU case this restores the init state,
for non eager it forces an init on the next FPU use.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 arch/x86/include/asm/elf.h | 4 ++++
 arch/x86/kernel/xsave.c    | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index ca3347a..56ab629 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -90,6 +90,8 @@ extern unsigned int vdso32_enabled;
 
 #include <asm/processor.h>
 
+extern void reset_fpu(void);
+
 #ifdef CONFIG_X86_32
 #include <asm/desc.h>
 
@@ -110,6 +112,7 @@ extern unsigned int vdso32_enabled;
 	_r->bx = 0; _r->cx = 0; _r->dx = 0;	\
 	_r->si = 0; _r->di = 0; _r->bp = 0;	\
 	_r->ax = 0;				\
+	reset_fpu();				\
 } while (0)
 
 /*
@@ -178,6 +181,7 @@ static inline void elf_common_init(struct thread_struct *t,
 	t->fs = t->gs = 0;
 	t->fsindex = t->gsindex = 0;
 	t->ds = t->es = ds;
+	reset_fpu();
 }
 
 #define ELF_PLAT_INIT(_r, load_addr)			\
diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index cdc6cf9..520e505 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -741,3 +741,8 @@ void *get_xsave_addr(struct xsave_struct *xsave, int xstate)
 	return (void *)xsave + xstate_comp_offsets[feature];
 }
 EXPORT_SYMBOL_GPL(get_xsave_addr);
+
+void reset_fpu(void)
+{
+	drop_init_fpu(current);
+}
-- 
1.9.3