x86/fpu: Update XFD state where required

The IA32_XFD_MSR allows to arm #NM traps for XSTATE components which are
enabled in XCR0. The register has to be restored before the tasks XSTATE is
restored. The life time rules are the same as for FPU state.

XFD is updated on return to userspace only when the FPU state of the task
is not up to date in the registers. It's updated before the XRSTORS so
that eventually enabled dynamic features are restored as well and not
brought into init state.

Also in signal handling for restoring FPU state from user space the
correctness of the XFD state has to be ensured.

Add it to CPU initialization and resume as well.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211021225527.10184-17-chang.seok.bae@intel.com

1 files changed, 27 insertions, 1 deletions

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index b5f5b08b84d7..12ca174891dc 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -156,6 +156,23 @@ void restore_fpregs_from_fpstate(struct fpstate *fpstate, u64 mask)
 
 	if (use_xsave()) {
 		/*
+		 * Dynamically enabled features are enabled in XCR0, but
+		 * usage requires also that the corresponding bits in XFD
+		 * are cleared.  If the bits are set then using a related
+		 * instruction will raise #NM. This allows to do the
+		 * allocation of the larger FPU buffer lazy from #NM or if
+		 * the task has no permission to kill it which would happen
+		 * via #UD if the feature is disabled in XCR0.
+		 *
+		 * XFD state is following the same life time rules as
+		 * XSTATE and to restore state correctly XFD has to be
+		 * updated before XRSTORS otherwise the component would
+		 * stay in or go into init state even if the bits are set
+		 * in fpstate::regs::xsave::xfeatures.
+		 */
+		xfd_update_state(fpstate);
+
+		/*
 		 * Restoring state always needs to modify all features
 		 * which are in @mask even if the current task cannot use
 		 * extended features.
@@ -241,8 +258,17 @@ int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)
 
 	cur_fps = fpu->fpstate;
 
-	if (!cur_fps->is_confidential)
+	if (!cur_fps->is_confidential) {
+		/* Includes XFD update */
 		restore_fpregs_from_fpstate(cur_fps, XFEATURE_MASK_FPSTATE);
+	} else {
+		/*
+		 * XSTATE is restored by firmware from encrypted
+		 * memory. Make sure XFD state is correct while
+		 * running with guest fpstate
+		 */
+		xfd_update_state(cur_fps);
+	}
 
 	fpregs_mark_activate();
 	fpregs_unlock();