The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: d8778e393afa421f1f117471144f8ce6deb6953a Gitweb: https://git.kernel.org/tip/d8778e393afa421f1f117471144f8ce6deb6953a Author: Andy Lutomirski luto@kernel.org AuthorDate: Tue, 08 Jun 2021 16:36:19 +02:00 Committer: Borislav Petkov bp@suse.de CommitterDate: Wed, 09 Jun 2021 09:49:38 +02:00
x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer
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@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Borislav Petkov bp@suse.de Acked-by: Dave Hansen dave.hansen@linux.intel.com Acked-by: Rik van Riel riel@surriel.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20210608144345.758116583@linutronix.de --- arch/x86/kernel/fpu/signal.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index d5bc96a..4ab9aeb 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -369,6 +369,25 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) 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 { /*