[Lguest] [tip:x86/urgent] x86/cpu: Fix SMAP check in PVOPS environments

[tip-bot for Andrew Cooper]

Commit-ID:  581b7f158fe0383b492acd1ce3fb4e99d4e57808
Gitweb:     http://git.kernel.org/tip/581b7f158fe0383b492acd1ce3fb4e99d4e57808
Author:     Andrew Cooper <andrew.cooper3 at citrix.com>
AuthorDate: Wed, 3 Jun 2015 10:31:14 +0100
Committer:  Thomas Gleixner <tglx at linutronix.de>
CommitDate: Thu, 19 Nov 2015 11:07:49 +0100

x86/cpu: Fix SMAP check in PVOPS environments

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 at citrix.com>
Reviewed-by: David Vrabel <david.vrabel at citrix.com>
Tested-by: Rusty Russell <rusty at rustcorp.com.au>
Cc: Rusty Russell <rusty at rustcorp.com.au>
Cc: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky at oracle.com>
Cc: <lguest at lists.ozlabs.org>
Cc: Xen-devel <xen-devel at lists.xen.org>
CC: stable at vger.kernel.org
Link: http://lkml.kernel.org/r/1433323874-6927-1-git-send-email-andrew.cooper3@citrix.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 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 4ddd780..c2b7522 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -273,10 +273,9 @@ __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)) {