linux-kernel - [PATCH v2] x86/mm: avoid premature success when changing page attributes

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Date:	Wed, 27 Jan 2016 04:21:30 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<mingo@...e.hu>, "Thomas Gleixner" <tglx@...utronix.de>,
	<hpa@...or.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: [PATCH v2] x86/mm: avoid premature success when changing page
 attributes

When __change_page_attr() finds it necessary to call
__cpa_process_fault(), it passes its return value directly up to its
own caller, even if this indicates success. Success to the callers,
however, means that whatever ->numpages currently holds is the count
of successfully processed pages. The cases when __change_page_attr()
calls __cpa_process_fault(), otoh, don't generally mean the entire
range got processed (as can be seen from one of the two success return
paths in __cpa_process_fault() adjusting ->numpages).

When a top level caller, like in the case of change_page_attr_set_clr()
only meaning to alter _PAGE_NX, wants to suppress alias processing, the
boolean value to indicate so results in __cpa_process_fault() taking
its other successful exit path. Since ->numpages so far didn't get
adjusted there, hitting either of the conditions that cause
__cpa_process_fault() to get called meant early termination of the
processing without having processed the entire range, yet still
reporting success.

Signed-off-by: Jan Beulich <jbeulich@...e.com>
---
v2: Completely re-written description.
---
 arch/x86/mm/pageattr.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- 4.5-rc1/arch/x86/mm/pageattr.c
+++ 4.5-rc1-x86-cpa-non-primary/arch/x86/mm/pageattr.c
@@ -1122,8 +1122,10 @@ static int __cpa_process_fault(struct cp
 	/*
 	 * Ignore all non primary paths.
 	 */
-	if (!primary)
+	if (!primary) {
+		cpa->numpages = 1;
 		return 0;
+	}

 	/*
 	 * Ignore the NULL PTE for kernel identity mapping, as it is expected