x86/pkru: Write hardware init value to PKRU when xstate is init

When user space brings PKRU into init state, then the kernel handling is
broken:

  T1 user space
     xsave(state)
     state.header.xfeatures &= ~XFEATURE_MASK_PKRU;
     xrstor(state)

  T1 -> kernel
     schedule()
       XSAVE(S) -> T1->xsave.header.xfeatures[PKRU] == 0
       T1->flags |= TIF_NEED_FPU_LOAD;

       wrpkru();

     schedule()
       ...
       pk = get_xsave_addr(&T1->fpu->state.xsave, XFEATURE_PKRU);
       if (pk)
	 wrpkru(pk->pkru);
       else
	 wrpkru(DEFAULT_PKRU);

Because the xfeatures bit is 0 and therefore the value in the xsave
storage is not valid, get_xsave_addr() returns NULL and switch_to()
writes the default PKRU. -> FAIL #1!

So that wrecks any copy_to/from_user() on the way back to user space
which hits memory which is protected by the default PKRU value.

Assumed that this does not fail (pure luck) then T1 goes back to user
space and because TIF_NEED_FPU_LOAD is set it ends up in

  switch_fpu_return()
      __fpregs_load_activate()
        if (!fpregs_state_valid()) {
  	 load_XSTATE_from_task();
        }

But if nothing touched the FPU between T1 scheduling out and back in,
then the fpregs_state is still valid which means switch_fpu_return()
does nothing and just clears TIF_NEED_FPU_LOAD. Back to user space with
DEFAULT_PKRU loaded. -> FAIL #2!

The fix is simple: if get_xsave_addr() returns NULL then set the
PKRU value to 0 instead of the restrictive default PKRU value in
init_pkru_value.

 [ bp: Massage in minor nitpicks from folks. ]

Fixes: 0cecca9 ("x86/fpu: Eager switch PKRU state")
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>
Tested-by: Babu Moger <babu.moger@amd.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20210608144346.045616965@linutronix.de

arch/x86/include/asm/fpu/internal.h

@@ -579,9 +579,16 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
 	 * return to userland e.g. for a copy_to_user() operation.
 	 */
 	if (!(current->flags & PF_KTHREAD)) {
+		/*
+		 * If the PKRU bit in xsave.header.xfeatures is not set,
+		 * then the PKRU component was in init state, which means
+		 * XRSTOR will set PKRU to 0. If the bit is not set then
+		 * get_xsave_addr() will return NULL because the PKRU value
+		 * in memory is not valid. This means pkru_val has to be
+		 * set to 0 and not to init_pkru_value.
+		 */
 		pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
-		if (pk)
-			pkru_val = pk->pkru;
+		pkru_val = pk ? pk->pkru : 0;
 	}
 	__write_pkru(pkru_val);
 }