[PATCH] i386/x86-64: Fix x87 information leak between processes (CVE-2006-1056)

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 x87 load from a kernel variable to clear FOP/FIP/FDP.

This means other processes always will only see a constant value defined
by the kernel in their FP state.

I took some pain to make sure to chose a variable that's already in L1
during context switch to make the overhead of this low.

Also alternative() is used to patch away the new code on CPUs who don't
need it.

Patch for both i386/x86-64.

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

This is CVE-2006-1056

Cc: richard.brunner@amd.com
Cc: jbeulich@novell.com
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 10 insertions, 2 deletions

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index 22a05dec81a..818ab9e66ae 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -527,8 +527,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	int cpu = smp_processor_id();  
 	struct tss_struct *tss = &per_cpu(init_tss, cpu);
 
-	unlazy_fpu(prev_p);
-
 	/*
 	 * Reload esp0, LDT and the page table pointer:
 	 */
@@ -591,6 +589,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	prev->userrsp = read_pda(oldrsp); 
 	write_pda(oldrsp, next->userrsp); 
 	write_pda(pcurrent, next_p); 
+
+ 	/* This must be here to ensure both math_state_restore() and
+	   kernel_fpu_begin() work consistently.
+	   And the AMD workaround requires it to be after DS reload. */
+	unlazy_fpu(prev_p);
+
 	write_pda(kernelstack,
 		  task_stack_page(next_p) + THREAD_SIZE - PDA_STACKOFFSET);
 
diff --git a/arch/x86_64/kernel/setup.c b/arch/x86_64/kernel/setup.c
index aa55e3cec66..a4a0bb5fb48 100644
--- a/arch/x86_64/kernel/setup.c
+++ b/arch/x86_64/kernel/setup.c
@@ -909,6 +909,10 @@ static int __init init_amd(struct cpuinfo_x86 *c)
 	if (c->x86 == 15 && ((level >= 0x0f48 && level < 0x0f50) || level >= 0x0f58))
 		set_bit(X86_FEATURE_REP_GOOD, &c->x86_capability);
 
+	/* Enable workaround for FXSAVE leak */
+	if (c->x86 >= 6)
+		set_bit(X86_FEATURE_FXSAVE_LEAK, &c->x86_capability);
+
 	r = get_model_name(c);
 	if (!r) { 
 		switch (c->x86) {