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Subject[PATCH 3/5] mempolicy: fix a race in shared_policy_replace()
Date
shared_policy_replace() use of sp_alloc() is unsafe. 1) sp_node cannot
be dereferenced if sp->lock is not held and 2) another thread can modify
sp_node between spin_unlock for allocating a new sp node and next spin_lock.
The bug was introduced before 2.6.12-rc2.

Kosaki's original patch for this problem was to allocate an sp node and policy
within shared_policy_replace and initialise it when the lock is reacquired. I
was not keen on this approach because it partially duplicates sp_alloc(). As
the paths were sp->lock is taken are not that performance critical this
patch converts sp->lock to sp->mutex so it can sleep when calling sp_alloc().

[kosaki.motohiro@jp.fujitsu.com: Original patch]
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
include/linux/mempolicy.h | 2 +-
mm/mempolicy.c | 37 ++++++++++++++++---------------------
2 files changed, 17 insertions(+), 22 deletions(-)
diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index 95b738c..df08254 100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -188,7 +188,7 @@ struct sp_node {

struct shared_policy {
struct rb_root root;
- spinlock_t lock;
+ struct mutex mutex;
};

void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol);
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 8025720..d297929 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2083,7 +2083,7 @@ bool __mpol_equal(struct mempolicy *a, struct mempolicy *b)
*/

/* lookup first element intersecting start-end */
-/* Caller holds sp->lock */
+/* Caller holds sp->mutex */
static struct sp_node *
sp_lookup(struct shared_policy *sp, unsigned long start, unsigned long end)
{
@@ -2147,13 +2147,13 @@ mpol_shared_policy_lookup(struct shared_policy *sp, unsigned long idx)

if (!sp->root.rb_node)
return NULL;
- spin_lock(&sp->lock);
+ mutex_lock(&sp->mutex);
sn = sp_lookup(sp, idx, idx+1);
if (sn) {
mpol_get(sn->policy);
pol = sn->policy;
}
- spin_unlock(&sp->lock);
+ mutex_unlock(&sp->mutex);
return pol;
}

@@ -2193,10 +2193,10 @@ 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)
{
- struct sp_node *n, *new2 = NULL;
+ struct sp_node *n;
+ int ret = 0;

-restart:
- spin_lock(&sp->lock);
+ mutex_lock(&sp->mutex);
n = sp_lookup(sp, start, end);
/* Take care of old policies in the same range. */
while (n && n->start < end) {
@@ -2209,16 +2209,14 @@ restart:
} else {
/* Old policy spanning whole new range. */
if (n->end > end) {
+ struct sp_node *new2;
+ new2 = sp_alloc(end, n->end, n->policy);
if (!new2) {
- spin_unlock(&sp->lock);
- new2 = sp_alloc(end, n->end, n->policy);
- if (!new2)
- return -ENOMEM;
- goto restart;
+ ret = -ENOMEM;
+ goto out;
}
n->end = start;
sp_insert(sp, new2);
- new2 = NULL;
break;
} else
n->end = start;
@@ -2229,12 +2227,9 @@ 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;
+out:
+ mutex_unlock(&sp->mutex);
+ return ret;
}

/**
@@ -2252,7 +2247,7 @@ void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol)
int ret;

sp->root = RB_ROOT; /* empty tree == default mempolicy */
- spin_lock_init(&sp->lock);
+ mutex_init(&sp->mutex);

if (mpol) {
struct vm_area_struct pvma;
@@ -2318,7 +2313,7 @@ void mpol_free_shared_policy(struct shared_policy *p)

if (!p->root.rb_node)
return;
- spin_lock(&p->lock);
+ mutex_lock(&p->mutex);
next = rb_first(&p->root);
while (next) {
n = rb_entry(next, struct sp_node, nd);
@@ -2327,7 +2322,7 @@ void mpol_free_shared_policy(struct shared_policy *p)
mpol_put(n->policy);
kmem_cache_free(sn_cache, n);
}
- spin_unlock(&p->lock);
+ mutex_unlock(&p->mutex);
}

/* assumes fs == KERNEL_DS */
--
1.7.7


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