Messages in this thread Patch in this message | | | Subject | [patch] x64: fix FPU corruption with signals and preemption | From | Suresh Siddha <> | Date | Thu, 09 Apr 2009 15:24:34 -0700 |
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From: Suresh Siddha <suresh.b.siddha@intel.com> Subject: x64: fix FPU corruption with signals and preemption
Impact: fix FPU state corruption
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.
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com> Cc: stable@kernel.org [2.6.28.x, 2.6.29.x] ---
Index: tip/arch/x86/kernel/xsave.c =================================================================== --- tip.orig/arch/x86/kernel/xsave.c +++ tip/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;
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