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Subject[PATCH 3/6] mempolicy: fix a race in shared_policy_replace()
Date
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

shared_policy_replace() uses sp_alloc() wrongly. 1) sp_node can't be dereferenced when
not holding sp->lock and 2) another thread can modify sp_node between spin_unlock for
restarting and next spin_lock, we shouldn't believe n->end and n->policy were not
modified.

This patch fixes them.

Note: The bug was introduced pre-git age (IOW, before 2.6.12-rc2). I believe nobody
uses this feature in production systems.

Cc: Dave Jones <davej@redhat.com>,
Cc: Mel Gorman <mgorman@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>,
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
mm/mempolicy.c | 55 ++++++++++++++++++++++++++++++++++++++-----------------
1 files changed, 38 insertions(+), 17 deletions(-)
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 9505cb9..d97d2db 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2158,6 +2158,14 @@ static void sp_delete(struct shared_policy *sp, struct sp_node *n)
kmem_cache_free(sn_cache, n);
}

+static void sp_node_init(struct sp_node *node, unsigned long start,
+ unsigned long end, struct mempolicy *pol)
+{
+ node->start = start;
+ node->end = end;
+ node->policy = pol;
+}
+
static struct sp_node *sp_alloc(unsigned long start, unsigned long end,
struct mempolicy *pol)
{
@@ -2174,10 +2182,7 @@ static struct sp_node *sp_alloc(unsigned long start, unsigned long end,
return NULL;
}
newpol->flags |= MPOL_F_SHARED;
-
- n->start = start;
- n->end = end;
- n->policy = newpol;
+ sp_node_init(n, start, end, newpol);

return n;
}
@@ -2186,7 +2191,9 @@ static struct sp_node *sp_alloc(unsigned long start, unsigned long end,
static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
unsigned long end, struct sp_node *new)
{
+ int err;
struct sp_node *n, *new2 = NULL;
+ struct mempolicy *new2_pol = NULL;

restart:
spin_lock(&sp->lock);
@@ -2202,16 +2209,16 @@ restart:
} else {
/* Old policy spanning whole new range. */
if (n->end > end) {
- if (!new2) {
- spin_unlock(&sp->lock);
- new2 = sp_alloc(end, n->end, n->policy);
- if (!new2)
- return -ENOMEM;
- goto restart;
- }
- n->end = start;
+ if (!new2)
+ goto alloc_new2;
+
+ *new2_pol = *n->policy;
+ atomic_set(&new2_pol->refcnt, 1);
+ sp_node_init(new2, n->end, end, new2_pol);
sp_insert(sp, new2);
+ n->end = start;
new2 = NULL;
+ new2_pol = NULL;
break;
} else
n->end = start;
@@ -2223,11 +2230,25 @@ restart:
if (new)
sp_insert(sp, new);
spin_unlock(&sp->lock);
- if (new2) {
- mpol_put(new2->policy);
- kmem_cache_free(sn_cache, new2);
- }
- return 0;
+ err = 0;
+
+ err_out:
+ if (new2_pol)
+ mpol_put(new2_pol);
+ if (new2)
+ kmem_cache_free(sn_cache, new2);
+ return err;
+
+ alloc_new2:
+ spin_unlock(&sp->lock);
+ err = -ENOMEM;
+ new2 = kmem_cache_alloc(sn_cache, GFP_KERNEL);
+ if (!new2)
+ goto err_out;
+ new2_pol = kmem_cache_alloc(policy_cache, GFP_KERNEL);
+ if (!new2_pol)
+ goto err_out;
+ goto restart;
}

/**
--
1.7.1


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