[patch] amd-fxsave-24

amd-fxsave-24 (text/plain), 3.46 KB, created by Sune Kloppenborg Jeppesen (RETIRED) on 2006-04-15 00:55:00 UTC

i386/x86-64: Fix x87 information leak between processes

AMD K7/K8 CPUs only save/restore the FOP/FIP/FDP x87 registers in FXSAVE
when an exception is pending.  This means the value leak through
context switches and allow processes to observe some x87 instruction 
state of other processes.

This was actually documented by AMD, but nobody recognized it as
being different from Intel before.

The fix first adds an optimization: instead of unconditionally 
calling FNCLEX after each FXSAVE test if ES is pending and skip
it when not needed. Then do a dummy x87 load to clear FOP/FIP/FDP.
This means other processes always will only see a constant value
defined by the kernel.

Then it does a ffree st(7) ; fild <l1 address> 
This is executed unconditionally on FXSAVE capable systems, but has 
been benchmarked on Intel systems to be reasonably fast. 

I also had to move unlazy_fpu for 64bit to make sure the code
always executes with the data segment of the new process to prevent
leaking the old one.

Patch for both i386/x86-64.

The problem was discovered originally by Jan Beulich. Richard
Brunner provided the basic code for the workarounds with contributions
from Jan. 

This is CVE-2006-1056

Signed-off-by: Andi Kleen <[e-mail]>

Index: linux-2.4.32-work/arch/i386/kernel/i387.c
===================================================================
--- linux-2.4.32-work.orig/arch/i386/kernel/i387.c
+++ linux-2.4.32-work/arch/i386/kernel/i387.c
@@ -11,6 +11,7 @@
 #include <linux/config.h>
 #include <linux/sched.h>
 #include <linux/init.h>
+#include <linux/kernel_stat.h>
 #include <asm/processor.h>
 #include <asm/i387.h>
 #include <asm/math_emu.h>
@@ -70,8 +71,12 @@ void init_fpu(void)
 static inline void __save_init_fpu( struct task_struct *tsk )
 {
 	if ( cpu_has_fxsr ) {
-		asm volatile( "fxsave %0 ; fnclex"
+		asm volatile( "fxsave %0"
 			      : "=m" (tsk->thread.i387.fxsave) );
+		if (tsk->thread.i387.fxsave.swd & (1<<7))
+			asm volatile("fnclex");
+		/* AMD CPUs leak F?P. Clear it here */
+		asm volatile("ffree %%st(7) ; fildl %0" :: "m" (kstat.context_swtch));
 	} else {
 		asm volatile( "fnsave %0 ; fwait"
 			      : "=m" (tsk->thread.i387.fsave) );
Index: linux-2.4.32-work/include/asm-x86_64/i387.h
===================================================================
--- linux-2.4.32-work.orig/include/asm-x86_64/i387.h
+++ linux-2.4.32-work/include/asm-x86_64/i387.h
@@ -125,8 +125,12 @@ static inline void kernel_fpu_begin(void
 
 static inline void save_init_fpu( struct task_struct *tsk )
 {
-	asm volatile( "fxsave %0 ; fnclex"
+	asm volatile( "fxsave %0"
 		      : "=m" (tsk->thread.i387.fxsave));
+	if (tsk->thread.i387.fxsave.swd & (1<<7))
+		asm volatile("fnclex");
+	/* AMD CPUs leak F?P through FXSAVE. Clear it here */
+	asm volatile("ffree %st(7) ; fildl %gs:0");
 	tsk->flags &= ~PF_USEDFPU;
 	stts();
 }
Index: linux-2.4.32-work/arch/x86_64/kernel/process.c
===================================================================
--- linux-2.4.32-work.orig/arch/x86_64/kernel/process.c
+++ linux-2.4.32-work/arch/x86_64/kernel/process.c
@@ -564,8 +564,6 @@ struct task_struct *__switch_to(struct t
 				 *next = &next_p->thread;
 	struct tss_struct *tss = init_tss + smp_processor_id();
 
-	unlazy_fpu(prev_p);
-
 	/*
 	 * Reload rsp0, LDT and the page table pointer:
 	 */
@@ -583,6 +581,11 @@ struct task_struct *__switch_to(struct t
 		loadsegment(ds, next->ds);
 
 	/* 
+  	 * Must be after DS reload for AMD workaround.
+	 */
+	unlazy_fpu(prev_p);
+
+	/* 
 	 * Switch FS and GS.
 	 */
 	{