linux-kernel - [PATCH 3.12 28/91] x86/cpu: Fix SMAP check in PVOPS environments

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Message-Id: <6119d48728f749093c1b7a2ec18a125252756a5d.1452015822.git.jslaby@suse.cz>
Date:	Tue,  5 Jan 2016 18:46:12 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	lguest@...ts.ozlabs.org, Xen-devel <xen-devel@...ts.xen.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 28/91] x86/cpu: Fix SMAP check in PVOPS environments

From: Andrew Cooper <andrew.cooper3@...rix.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 581b7f158fe0383b492acd1ce3fb4e99d4e57808 upstream.

There appears to be no formal statement of what pv_irq_ops.save_fl() is
supposed to return precisely.  Native returns the full flags, while lguest and
Xen only return the Interrupt Flag, and both have comments by the
implementations stating that only the Interrupt Flag is looked at.  This may
have been true when initially implemented, but no longer is.

To make matters worse, the Xen PVOP leaves the upper bits undefined, making
the BUG_ON() undefined behaviour.  Experimentally, this now trips for 32bit PV
guests on Broadwell hardware.  The BUG_ON() is consistent for an individual
build, but not consistent for all builds.  It has also been a sitting timebomb
since SMAP support was introduced.

Use native_save_fl() instead, which will obtain an accurate view of the AC
flag.

Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
Reviewed-by: David Vrabel <david.vrabel@...rix.com>
Tested-by: Rusty Russell <rusty@...tcorp.com.au>
Cc: Rusty Russell <rusty@...tcorp.com.au>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: <lguest@...ts.ozlabs.org>
Cc: Xen-devel <xen-devel@...ts.xen.org>
Link: http://lkml.kernel.org/r/1433323874-6927-1-git-send-email-andrew.cooper3@citrix.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 arch/x86/kernel/cpu/common.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 6db4828574ef..9364936b47c2 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -280,10 +280,9 @@ __setup("nosmap", setup_disable_smap);
 
 static __always_inline void setup_smap(struct cpuinfo_x86 *c)
 {
-	unsigned long eflags;
+	unsigned long eflags = native_save_fl();
 
 	/* This should have been cleared long ago */
-	raw_local_save_flags(eflags);
 	BUG_ON(eflags & X86_EFLAGS_AC);
 
 	if (cpu_has(c, X86_FEATURE_SMAP)) {
-- 
2.6.4

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