Messages in this thread Patch in this message | | | From | Yu-cheng Yu <> | Subject | [PATCH 04/10] x86/xsaves: Introduce a new check that allows correct xstates copy from kernel to user directly | Date | Mon, 22 Feb 2016 10:58:48 -0800 |
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XSAVES is a kernel instruction and uses a compacted format. When working with user space, the kernel should provide standard-format, non-supervisor state data. We cannot do __copy_to_user() from a compacted- format kernel xstate area to a signal frame.
Note that the path to copy_fpstate_to_sigframe() does currently check if the thread has used FPU, but add a WARN_ONCE() there to detect any potential mis-use.
Dave Hansen proposes this method to simplify copy xstate directly to user.
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> Signed-off by: Yu-cheng Yu <yu-cheng.yu@intel.com> --- arch/x86/kernel/fpu/signal.c | 41 ++++++++++++++++++++++++++++++++++++++++- 1 file changed, 40 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index 0fbf60c..7676718 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -130,6 +130,45 @@ static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf) return err; } +static int should_save_registers_directly(void) +{ + /* + * In signal handling path, the kernel already checks if + * FPU instructions have been used before it calls + * copy_fpstate_to_sigframe(). We check this here again + * to detect any potential mis-use and saving invalid + * register values directly to a signal frame. + */ + WARN_ONCE(!current->thread.fpu.fpstate_active, + "direct FPU save with no math use\n"); + + /* + * In the case that we are using a compacted kernel + * xsave area, we can not copy the thread.fpu.state + * directly to userspace and *must* save it from the + * registers directly. + */ + if (boot_cpu_has(X86_FEATURE_XSAVES)) + return 1; + + /* + * fpregs_active() means "Can I use the FPU hardware + * without taking a device-not-available exception?" This + * means that saving the registers directly will be + * cheaper than copying their contents out of + * thread.fpu.state. + * + * Note that fpregs_active() is inherently racy and may + * become false at any time. If this race happens, we + * will take a harmless device-not-available exception + * when we attempt the FPU save instruction. + */ + if (fpregs_active()) + return 1; + + return 0; +} + /* * Save the fpu, extended register state to the user signal frame. * @@ -167,7 +206,7 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size) sizeof(struct user_i387_ia32_struct), NULL, (struct _fpstate_32 __user *) buf) ? -1 : 1; - if (fpregs_active()) { + if (should_save_registers_directly()) { /* Save the live register state to the user directly. */ if (copy_fpregs_to_sigframe(buf_fx)) return -1; -- 1.9.1
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