x86-64: fix FPU corruption with signals and preemption

commit 06c38d5e36b12d040839ff224e805146c0368556 upstream.

In 64bit signal delivery path, clear_used_math() was happening before saving
the current active FPU state on to the user stack for signal handling. Between
clear_used_math() and the state store on to the user stack, potentially we
can get a page fault for the user address and can block. Infact, while testing
we were hitting the might_fault() in __clear_user() which can do a schedule().

At a later point in time, we will schedule back into this process and
resume the save state (using "xsave/fxsave" instruction) which can lead
to DNA fault. And as used_math was cleared before, we will reinit the FP state
in the DNA fault and continue. This reinit will result in loosing the
FPU state of the process.

Move clear_used_math() to a point after the FPU state has been stored
onto the user stack.

This issue is present from a long time (even before the xsave changes
and the x86 merge). But it can easily be exposed in 2.6.28.x and 2.6.29.x
series because of the __clear_user() in this path, which has an explicit
__cond_resched() leading to a context switch with CONFIG_PREEMPT_VOLUNTARY.

[ Impact: fix FPU state corruption ]

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 2b54fe002e94e490db9a0e8ec3d4b89ec76524ab..aa8bc45fe505113a26b1f737c9ba17856157c7bd 100644 (file)
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -89,7 +89,7 @@ int save_i387_xstate(void __user *buf)
 
        if (!used_math())
                return 0;
-       clear_used_math(); /* trigger finit */
+
        if (task_thread_info(tsk)->status & TS_USEDFPU) {
                /*
                 * Start with clearing the user buffer. This will present a
@@ -114,6 +114,8 @@ int save_i387_xstate(void __user *buf)
                        return -1;
        }
 
+       clear_used_math(); /* trigger finit */
+
        if (task_thread_info(tsk)->status & TS_XSAVE) {
                struct _fpstate __user *fx = buf;
                struct _xstate __user *x = buf;