Messages in this thread | | | Subject | Re: [patch 0/8] x86/fpu: Mop up XSAVES and related damage | From | Dave Hansen <> | Date | Fri, 4 Jun 2021 10:46:24 -0700 |
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On 6/4/21 7:05 AM, Thomas Gleixner wrote: > But looking at the above, it's not clear to me why that PKRU stuff works > at all (upstream), but I'll figure it out eventually. I'm quite sure > that it does work by pure chance not by design.
The upstream flush_thread() code appears correct and even intentionally so. Just how we must eagerly load PKRU on a context switch, the fpu__clear*() code eagerly "clears" PKRU. It doesn't actually zero it, of course, but reverts the register and the fpstate back to the 'init_pkru_value':
flush_thread()->fpu__clear_all()->fpu__clear(user_only=false) copy_init_fpstate_to_fpregs() copy_kernel_to_xregs(init_fpu) // fpstate copy_init_pkru_to_fpregs() write_pkru(init_pkru_value_snapshot) // fpregs
Andy said you have a fix for this, but I think the new fpu__clear_all() is failing to do the eager write_pkru().
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