Messages in this thread | | | Subject | Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails | From | Yu-cheng Yu <> | Date | Thu, 19 Dec 2019 08:44:08 -0800 |
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On Thu, 2019-12-19 at 15:22 +0100, Sebastian Andrzej Siewior wrote: > On 2019-12-18 12:53:59 [-0800], Yu-cheng Yu wrote: > > I could have explained this better, sorry! I will explain the first > > case below; other cases are similar. > > > > In copy_user_to_fpregs_zeroing(), we have: > > > > if (user_xsave()) { > > ... > > if (unlikely(init_bv)) > > copy_kernel_to_xregs(&init_fpstate.xsave, init_bv); > > return copy_user_to_xregs(buf, xbv); > > ... > > } > > > > The copy_user_to_xregs() may fail, and when that happens, before going to > > the slow path, there is fpregs_unlock() and context switches may happen. > > The context switch may only happen after fpregs_unlock(). > > > However, at this point, fpu_fpregs_owner_ctx has not been changed; it could > > still be another task's FPU. > > TIF_NEED_FPU_LOAD is set for the task in __fpu__restore_sig() and its > context (__fpu_invalidate_fpregs_state()) has been invalidated. So the > FPU register may contain another task's content and > fpu_fpregs_owner_ctx points to another context. > > > For this to happen and to be detected, the user > > stack page needs to be non-present, fpu_fpregs_owner_ctx need to be another task, > > and that other task needs to be able to detect its registers are modified. > > The last factor is not easy to reproduce, and a CET control-protection fault > > helps. > > So far everything is legal. However. If there is a context switch before > fpregs_lock() then this is bad before we don't account for that. > So that: > > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c > --- a/arch/x86/kernel/fpu/signal.c > +++ b/arch/x86/kernel/fpu/signal.c > @@ -352,6 +352,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) > fpregs_unlock(); > return 0; > } > + fpregs_deactivate(fpu); > fpregs_unlock(); > } > > @@ -403,6 +404,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) > } > if (!ret) > fpregs_mark_activate(); > + else > + fpregs_deactivate(fpu); > fpregs_unlock(); > > err_out: > > > Should be enough.
Yes, this works. But then everywhere that calls copy_*_to_xregs_*() etc. needs to be checked. Are there other alternatives?
Yu-cheng
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