linux: fix grant table bug

PATCH In review Michael Abd-El-Malek 4 participants 4 messages Mar 31, 2008 – Apr 3, 2008 Xen hypervisor interface

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Michael Abd-El-Malek

author

2008-03-31 03:42 UTC #0 lore raw

PATCH 1/1 linux: fix grant table bug

A PV OS has two grant table data structures: the grant table itself and a free list. The free list is composed of an array of pages, which grow dynamically as the guest OS requires more grants. While the grant table contains 8-byte entries, the free list contains 4-byte entries. So we have half as many pages in the free list than in the grant table.

There was a bug in the free list allocation code. The free list was indexed as if it was the same size as the grant table. But it's only half as large. So memory got corrupted, and I was seeing crashes in the slab allocator later on.

Signed-off-by Michael Abd-El-Malek <[email protected]>

drivers/xen/core/gnttab.c +10 -6

diff -r 1721546b9277 drivers/xen/core/gnttab.c
@@ -379,11 +379,15 @@ static int grow_gnttab_list(unsigned int
 static int grow_gnttab_list(unsigned int more_frames)
 {
 	unsigned int new_nr_grant_frames, extra_entries, i;
+	unsigned int nr_glist_frames, new_nr_glist_frames;
 
 	new_nr_grant_frames = nr_grant_frames + more_frames;
 	extra_entries       = more_frames * GREFS_PER_GRANT_FRAME;
 
-	for (i = nr_grant_frames; i < new_nr_grant_frames; i++)
+	nr_glist_frames = (nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+	new_nr_glist_frames =
+		(new_nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+	for (i = nr_glist_frames; i < new_nr_glist_frames; i++)
 	{
 		gnttab_list[i] = (grant_ref_t *)__get_free_page(GFP_ATOMIC);
 		if (!gnttab_list[i])

@@ -406,7 +410,7 @@ static int grow_gnttab_list(unsigned int
 	return 0;
 	
 grow_nomem:
-	for ( ; i >= nr_grant_frames; i--)
+	for ( ; i >= nr_glist_frames; i--)
 		free_page((unsigned long) gnttab_list[i]);
 	return -ENOMEM;
 }

@@ -720,7 +724,7 @@ int __devinit gnttab_init(void)
 int __devinit gnttab_init(void)
 {
 	int i;
-	unsigned int max_nr_glist_frames;
+	unsigned int max_nr_glist_frames, nr_glist_frames;
 	unsigned int nr_init_grefs;
 
 	if (!is_running_on_xen())

@@ -733,15 +737,15 @@ int __devinit gnttab_init(void)
 	 * grant reference free list on the current hypervisor.
 	 */
 	max_nr_glist_frames = (boot_max_nr_grant_frames *
-			       GREFS_PER_GRANT_FRAME /
-			       (PAGE_SIZE / sizeof(grant_ref_t)));
+			       GREFS_PER_GRANT_FRAME / RPP);
 
 	gnttab_list = kmalloc(max_nr_glist_frames * sizeof(grant_ref_t *),
 			      GFP_KERNEL);
 	if (gnttab_list == NULL)
 		return -ENOMEM;
 
-	for (i = 0; i < nr_grant_frames; i++) {
+	nr_glist_frames = (nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+	for (i = 0; i < nr_glist_frames; i++) {
 		gnttab_list[i] = (grant_ref_t *)__get_free_page(GFP_KERNEL);
 		if (gnttab_list[i] == NULL)
 			goto ini_nomem;

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Keir Fraser

2008-03-31 09:53 UTC #1 lore raw

Quoted 10 lines · Michael Abd-El-Malek

A PV OS has two grant table data structures: the grant table itself and a free list. The free list is composed of an array of pages, which grow dynamically as
the guest OS requires more grants. While the grant table contains 8-byte entries, the free list contains 4-byte entries. So we have half as many pages in the free list than in the grant table.
There was a bug in the free list allocation code. The free list was indexed as if it was the same size as the grant table. But it's only half as large. So memory got corrupted, and I was seeing crashes in the slab allocator later on.

Nice catch. That code is a bit confusing!

-- Keir

Mark McLoughlin

2008-04-03 13:26 UTC #2 lore raw

PATCH 1/1 xen: Fix grant table bug

In reply to Michael Abd-El-Malek

Quoted 3 lines · Michael Abd-El-Malek

There was a bug in the free list allocation code. The free list was indexed as if it was the same size as the grant table. But it's only half as large. So memory got corrupted, and I was seeing crashes in the slab allocator later on.

Looks like an obvious candidate for upstream linux too. Rebased version below.

Cheers,
Mark.

From: Michael Abd-El-Malek <[email protected]>

Taken from:

http://xenbits.xensource.com/linux-2.6.18-xen.hg?rev/4018c0da3360

Signed-off-by Michael Abd-El-Malek <[email protected]>

Signed-off-by Mark McLoughlin <[email protected]>

---
 drivers/xen/grant-table.c |   16 ++++++++++------
 1 files changed, 10 insertions(+), 6 deletions(-)

drivers/xen/grant-table.c +10 -6

index ea94dba..d85dc6d 100644
@@ -381,11 +381,15 @@ EXPORT_SYMBOL_GPL(gnttab_cancel_free_callback);
 static int grow_gnttab_list(unsigned int more_frames)
 {
 	unsigned int new_nr_grant_frames, extra_entries, i;
+	unsigned int nr_glist_frames, new_nr_glist_frames;
 
 	new_nr_grant_frames = nr_grant_frames + more_frames;
 	extra_entries       = more_frames * GREFS_PER_GRANT_FRAME;
 
-	for (i = nr_grant_frames; i < new_nr_grant_frames; i++) {
+	nr_glist_frames = (nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+	new_nr_glist_frames =
+		(new_nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+	for (i = nr_glist_frames; i < new_nr_glist_frames; i++) {
 		gnttab_list[i] = (grant_ref_t *)__get_free_page(GFP_ATOMIC);
 		if (!gnttab_list[i])
 			goto grow_nomem;

@@ -407,7 +411,7 @@ static int grow_gnttab_list(unsigned int more_frames)
 	return 0;
 
 grow_nomem:
-	for ( ; i >= nr_grant_frames; i--)
+	for ( ; i >= nr_glist_frames; i--)
 		free_page((unsigned long) gnttab_list[i]);
 	return -ENOMEM;
 }

@@ -530,7 +534,7 @@ static int gnttab_expand(unsigned int req_entries)
 static int __devinit gnttab_init(void)
 {
 	int i;
-	unsigned int max_nr_glist_frames;
+	unsigned int max_nr_glist_frames, nr_glist_frames;
 	unsigned int nr_init_grefs;
 
 	if (!is_running_on_xen())

@@ -543,15 +547,15 @@ static int __devinit gnttab_init(void)
 	 * grant reference free list on the current hypervisor.
 	 */
 	max_nr_glist_frames = (boot_max_nr_grant_frames *
-			       GREFS_PER_GRANT_FRAME /
-			       (PAGE_SIZE / sizeof(grant_ref_t)));
+			       GREFS_PER_GRANT_FRAME / RPP);
 
 	gnttab_list = kmalloc(max_nr_glist_frames * sizeof(grant_ref_t *),
 			      GFP_KERNEL);
 	if (gnttab_list == NULL)
 		return -ENOMEM;
 
-	for (i = 0; i < nr_grant_frames; i++) {
+	nr_glist_frames = (nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+	for (i = 0; i < nr_glist_frames; i++) {
 		gnttab_list[i] = (grant_ref_t *)__get_free_page(GFP_KERNEL);
 		if (gnttab_list[i] == NULL)
 			goto ini_nomem;

Jeremy Fitzhardinge

Applied

2008-04-03 14:36 UTC #3 lore raw

Re: [Xen-devel] [PATCH] xen: Fix grant table bug

Quoted 9 lines · Mark McLoughlin

Quoted 2 lines · Michael Abd-El-Malek
There was a bug in the free list allocation code. The free list was indexed as if it was the same size as the grant table. But it's only half as large. So memory got corrupted, and I was seeing crashes in the slab allocator later on.
Looks like an obvious candidate for upstream linux too. Rebased version below.

Queued, thanks.