linux-kernel - [patch] x86/fpu: x86/fpu: Preserve supervisor states in sanitize_restored_user

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Message-ID: <877diwursj.ffs@nanos.tec.linutronix.de>
Date:   Mon, 14 Jun 2021 22:15:08 +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>
Subject: [patch] x86/fpu: x86/fpu: Preserve supervisor states in sanitize_restored_user_xstate()

sanitize_restored_user_xstate() preserves the supervisor states only
when the fx_only argument is zero, which allows unpriviledged user space
to put supervisor states back into init state.

Preserve them unconditionally.

Fixes: 5d6b6a6f9b5c ("x86/fpu/xstate: Update sanitize_restored_xstate() for supervisor xstates")
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
---
 arch/x86/kernel/fpu/signal.c |   26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -221,28 +221,18 @@ sanitize_restored_user_xstate(union fpre
 
 	if (use_xsave()) {
 		/*
-		 * Note: we don't need to zero the reserved bits in the
-		 * xstate_header here because we either didn't copy them at all,
-		 * or we checked earlier that they aren't set.
+		 * Clear all features bit which are not set in
+		 * user_xfeatures and clear all extended features
+		 * for fx_only mode.
 		 */
+		u64 mask = fx_only ? XFEATURE_MASK_FPSSE : user_xfeatures;
 
 		/*
-		 * 'user_xfeatures' might have bits clear which are
-		 * set in header->xfeatures. This represents features that
-		 * were in init state prior to a signal delivery, and need
-		 * to be reset back to the init state.  Clear any user
-		 * feature bits which are set in the kernel buffer to get
-		 * them back to the init state.
-		 *
-		 * Supervisor state is unchanged by input from userspace.
-		 * Ensure supervisor state bits stay set and supervisor
-		 * state is not modified.
+		 * Supervisor state has to be preserved. The sigframe
+		 * restore can only modify user features, i.e. @mask
+		 * cannot contain them.
 		 */
-		if (fx_only)
-			header->xfeatures = XFEATURE_MASK_FPSSE;
-		else
-			header->xfeatures &= user_xfeatures |
-					     xfeatures_mask_supervisor();
+		header->xfeatures &= mask | xfeatures_mask_supervisor();
 	}
 
 	if (use_fxsr()) {