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.