linux-kernel - [patch 40/41] x86/fpu: Dont store PKRU in xstate in fpu_reset

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20210611163115.382824362@linutronix.de>
Date:   Fri, 11 Jun 2021 18:16:03 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: [patch 40/41] x86/fpu: Dont 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@...utronix.de>
---
V4: New patch
---
 arch/x86/kernel/fpu/core.c |   22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -403,23 +403,6 @@ static inline unsigned int init_fpstate_
 	return sizeof(init_fpstate.xsave);
 }
 
-/* Temporary workaround. Will be removed once PKRU and XSTATE are distangled. */
-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.
  */
@@ -437,9 +420,12 @@ 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();
 }