This is a note to let you know that I've just added the patch titled x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer to the 5.12-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: x86-fpu-invalidate-fpu-state-after-a-failed-xrstor-from-a-user-buffer.patch and it can be found in the queue-5.12 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From d8778e393afa421f1f117471144f8ce6deb6953a Mon Sep 17 00:00:00 2001 From: Andy Lutomirski <luto@xxxxxxxxxx> Date: Tue, 8 Jun 2021 16:36:19 +0200 Subject: x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer From: Andy Lutomirski <luto@xxxxxxxxxx> commit d8778e393afa421f1f117471144f8ce6deb6953a upstream. Both Intel and AMD consider it to be architecturally valid for XRSTOR to fail with #PF but nonetheless change the register 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". Fixes: 1d731e731c4c ("x86/fpu: Add a fastpath to __fpu__restore_sig()") Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Acked-by: Rik van Riel <riel@xxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Link: https://lkml.kernel.org/r/20210608144345.758116583@xxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kernel/fpu/signal.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -369,6 +369,25 @@ static int __fpu__restore_sig(void __use fpregs_unlock(); return 0; } + + /* + * The above did an FPU restore operation, restricted to + * the user portion of the registers, and failed, but the + * microcode might have modified the FPU registers + * nevertheless. + * + * If the FPU registers do not belong to current, then + * invalidate the FPU register state otherwise the task might + * preempt current and return to user space with corrupted + * FPU registers. + * + * In case current owns the FPU registers then no further + * action is required. The fixup below will handle it + * correctly. + */ + if (test_thread_flag(TIF_NEED_FPU_LOAD)) + __cpu_invalidate_fpregs_state(); + fpregs_unlock(); } else { /* Patches currently in stable-queue which might be from luto@xxxxxxxxxx are queue-5.12/x86-fpu-invalidate-fpu-state-after-a-failed-xrstor-from-a-user-buffer.patch