x86/fpu: Add a fastpath to __fpu__restore_sig()

The previous commits refactor the restoration of the FPU registers so
that they can be loaded from in-kernel memory. This overhead can be
avoided if the load can be performed without a pagefault.

Attempt to restore FPU registers by invoking
copy_user_to_fpregs_zeroing(). If it fails try the slowpath which can
handle pagefaults.

 [ bp: Add a comment over the fastpath to be able to find one's way
   around the function. ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: kvm ML <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-25-bigeasy@linutronix.de

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 6df1f15e0cd5cac51c9df181082a1bd2d8e6844d..a1bd7be70206135d67e455b10ecc027683fc05dd 100644 (file)
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -242,10 +242,10 @@ sanitize_restored_xstate(union fpregs_state *state,
 /*
  * Restore the extended state if present. Otherwise, restore the FP/SSE state.
  */
-static inline int copy_user_to_fpregs_zeroing(void __user *buf, u64 xbv, int fx_only)
+static int copy_user_to_fpregs_zeroing(void __user *buf, u64 xbv, int fx_only)
 {
        if (use_xsave()) {
-               if ((unsigned long)buf % 64 || fx_only) {
+               if (fx_only) {
                        u64 init_bv = xfeatures_mask & ~XFEATURE_MASK_FPSSE;
                        copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
                        return copy_user_to_fxregs(buf);
@@ -327,8 +327,27 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
                if (ret)
                        goto err_out;
                envp = &env;
+       } else {
+               /*
+                * Attempt to restore the FPU registers directly from user
+                * memory. For that to succeed, the user access cannot cause
+                * page faults. If it does, fall back to the slow path below,
+                * going through the kernel buffer with the enabled pagefault
+                * handler.
+                */
+               fpregs_lock();
+               pagefault_disable();
+               ret = copy_user_to_fpregs_zeroing(buf_fx, xfeatures, fx_only);
+               pagefault_enable();
+               if (!ret) {
+                       fpregs_mark_activate();
+                       fpregs_unlock();
+                       return 0;
+               }
+               fpregs_unlock();
        }
 
+
        if (use_xsave() && !fx_only) {
                u64 init_bv = xfeatures_mask & ~xfeatures;