[tip: x86/fpu] x86/fpu: Don't store PKRU in xstate in fpu_reset_fpstate()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     0e8c54f6b2c8b1037cef9276e451522ee90ed969
Gitweb:        https://git.kernel.org/tip/0e8c54f6b2c8b1037cef9276e451522ee90ed969
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Wed, 23 Jun 2021 14:02:22 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 23 Jun 2021 19:55:16 +02:00

x86/fpu: Don't store PKRU in xstate in fpu_reset_fpstate()

PKRU for a task is stored in task->thread.pkru when the task is scheduled
out. For 'current' the authoritative source of PKRU is the hardware.

fpu_reset_fpstate() has two callers:

  1) fpu__clear_user_states() for !FPU systems. For those PKRU is irrelevant

  2) fpu_flush_thread() which is invoked from flush_thread(). flush_thread()
     resets the hardware to the kernel restrictive default value.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20210623121456.802850233@xxxxxxxxxxxxx
---
 arch/x86/kernel/fpu/core.c | 22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 470576c..5295cba 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -337,23 +337,6 @@ static inline unsigned int init_fpstate_copy_size(void)
 	return sizeof(init_fpstate.xsave);
 }
 
-/* Temporary workaround. Will be removed once PKRU and XSTATE are untangled. */
-static inline void pkru_set_default_in_xstate(struct xregs_state *xsave)
-{
-	struct pkru_state *pk;
-
-	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
-		return;
-	/*
-	 * Force XFEATURE_PKRU to be set in the header otherwise
-	 * get_xsave_addr() does not work and it also needs to be set to
-	 * make XRSTOR(S) load it.
-	 */
-	xsave->header.xfeatures |= XFEATURE_MASK_PKRU;
-	pk = get_xsave_addr(xsave, XFEATURE_PKRU);
-	pk->pkru = pkru_get_init_value();
-}
-
 /*
  * Reset current->fpu memory state to the init values.
  */
@@ -371,9 +354,12 @@ static void fpu_reset_fpstate(void)
 	 *
 	 * Do not use fpstate_init() here. Just copy init_fpstate which has
 	 * the correct content already except for PKRU.
+	 *
+	 * PKRU handling does not rely on the xstate when restoring for
+	 * user space as PKRU is eagerly written in switch_to() and
+	 * flush_thread().
 	 */
 	memcpy(&fpu->state, &init_fpstate, init_fpstate_copy_size());
-	pkru_set_default_in_xstate(&fpu->state.xsave);
 	set_thread_flag(TIF_NEED_FPU_LOAD);
 	fpregs_unlock();
 }