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    SubjectRe: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state
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    On 6/16/21 9:27 AM, Dave Hansen wrote:
    > On 5/23/21 12:32 PM, Chang S. Bae wrote:
    >> @@ -571,6 +612,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
    >>
    >> set_thread_flag(TIF_NEED_FPU_LOAD);
    >>
    >> + xfd_switch(old_fpu, new_fpu);
    >
    > This seems fragile.
    >
    > XSAVE* will decline to write out state for feature i when XFD[i]=1 and
    > will instead write out the init state. That means that, at this point,
    > we switch XFD and yet leave state for feature i in place.
    >
    > That means this *MUST* come before any copy_fpregs_to_fpstate() occurs.
    >
    > Could we please add some FPU_WARN_ON() checks to ensure that no XSAVE*
    > is ever performed with XINUSE=1 *and* XFD armed?
    >

    Wait, really? I somehow thought that XSAVE* would write out the actual
    state even if XFD=1.

    This seems like it's asking for some kind of bug.

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