[PATCH] 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 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: Linus Torvalds <torvalds@osdl.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index 1c44b53cb15b..fb903e65e079 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -575,8 +575,10 @@ __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. */
+           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);