Messages in this thread | | | Subject | Re: [patch 3/8] x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer | From | Andy Lutomirski <> | Date | Thu, 3 Jun 2021 10:30:05 -0700 |
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On 6/2/21 8:06 AM, Borislav Petkov wrote: > On Wed, Jun 02, 2021 at 11:55:46AM +0200, Thomas Gleixner wrote: >> From: Andy Lutomirski <luto@kernel.org> >> >> If XRSTOR fails due to sufficiently complicated paging errors (e.g. >> concurrent TLB invalidation), > > I can't connect "concurrent TLB invalidation" to "sufficiently > complicated paging errors". Can you elaborate pls?
Think "complex microarchitectural conditions".
How about:
As far as I can tell, both Intel and AMD consider it to be architecturally valid for XRSTOR to fail with #PF but nonetheless change user state. The actual conditions under which this might occur are unclear [1], but it seems plausible that this might be triggered if one sibling thread unmaps a page and invalidates the shared TLB while another sibling thread is executing XRSTOR on the page in question.
__fpu__restore_sig() can execute XRSTOR while the hardware registers are preserved on behalf of a different victim task (using the fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but modify the registers. If this happens, then there is a window in which __fpu__restore_sig() could schedule out and the victim task could schedule back in without reloading its own FPU registers. This would result in part of the FPU state that __fpu__restore_sig() was attempting to load leaking into the victim task's user-visible state.
Invalidate preserved FPU registers on XRSTOR failure to prevent this situation from corrupting any state.
[1] Frequent readers of the errata lists might imagine "complex microarchitectural conditions".
>> + * failed. In the event that the ucode was >> + * unfriendly and modified the registers at all, we >> + * need to make sure that we aren't corrupting an >> + * innocent non-current task's registers. >> + */ >> + __cpu_invalidate_fpregs_state(); >> + } else { >> + /* >> + * As above, we may have just clobbered current's >> + * user FPU state. We will either successfully >> + * load it or clear it below, so no action is >> + * required here. >> + */ >> + } > > I'm wondering if that comment can simply be above the TIF_NEED_FPU_LOAD > testing, standalone, instead of having it in an empty else? And then get > rid of that else.
I'm fine either way.
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