From 06c38d5e36b12d040839ff224e805146c0368556 Mon Sep 17 00:00:00 2001 From: Suresh Siddha Date: Thu, 9 Apr 2009 15:24:34 -0700 Subject: [PATCH] x86-64: fix FPU corruption with signals and preemption 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 Cc: [2.6.28.x, 2.6.29.x] Signed-off-by: H. Peter Anvin --- arch/x86/kernel/xsave.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c index 0a5b04aa98f1..c5ee17e8c6d9 100644 --- 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;