linux-kernel - [PATCH v3] x86/mm: avoid premature success when changing page attributes

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Message-Id: <56BB0AD402000078000D05BF@prv-mh.provo.novell.com>
Date:	Wed, 10 Feb 2016 02:03:00 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<mingo@...e.hu>, "Thomas Gleixner" <tglx@...utronix.de>,
	<hpa@...or.com>
Cc:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	<linux-kernel@...r.kernel.org>
Subject: [PATCH v3] x86/mm: avoid premature success when changing page
 attributes

set_memory_nx() (and set_memory_x()) currently differ in behavior from
all other set_memory_*() functions when encountering a virtual address
space hole within the kernel address range: They stop processing at the
hole, but nevertheless report success (making the caller believe the
operation was carried out on the entire range). While observed to be a
problem - triggering the CONFIG_DEBUG_WX warning - only with out of
tree code, I suspect (but didn't check) that on x86-64 the
CONFIG_DEBUG_PAGEALLOC logic in free_init_pages() would, when called
from free_initmem(), have the same effect on the set_memory_nx() called
from mark_rodata_ro().

This unexpected behavior is a result of change_page_attr_set_clr()
special casing changes to only the NX bit, in that it passes "false" as
the "checkalias" argument to __change_page_attr_set_clr(). Since this
flag becomes the "primary" argument of both __change_page_attr() and
__cpa_process_fault(), the latter would so far return success without
adjusting cpa->numpages. Success to the higher level callers, however,
means that whatever cpa->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() already adjusting ->numpages).

Signed-off-by: Jan Beulich <jbeulich@...e.com>
---
v3: Mostly re-written description.
v2: Completely re-written description.
---
 arch/x86/mm/pageattr.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- 4.5-rc3/arch/x86/mm/pageattr.c
+++ 4.5-rc3-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