[Lguest] [PATCH 3.10 17/31] x86/cpu: Fix SMAP check in PVOPS environments

Greg Kroah-Hartman gregkh at linuxfoundation.org
Tue Dec 8 01:05:11 AEDT 2015


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

------------------

From: Andrew Cooper <andrew.cooper3 at citrix.com>

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 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>
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>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>

---
 arch/x86/kernel/cpu/common.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- 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)) {




More information about the Lguest mailing list