[PATCH] fix uidhash_lock <-> RCU deadlock

RCU task-struct freeing can call free_uid(), which is taking
uidhash_lock - while other users of uidhash_lock are softirq-unsafe.

The fix is to always take the uidhash_spinlock in a softirq-safe manner.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 8 deletions

diff --git a/kernel/user.c b/kernel/user.c
index 89e562feb1b1..d1ae2349347e 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -13,6 +13,7 @@
 #include <linux/slab.h>
 #include <linux/bitops.h>
 #include <linux/key.h>
+#include <linux/interrupt.h>
 
 /*
  * UID task count cache, to get fast user lookup in "alloc_uid"
@@ -27,6 +28,12 @@
 
 static kmem_cache_t *uid_cachep;
 static struct list_head uidhash_table[UIDHASH_SZ];
+
+/*
+ * The uidhash_lock is mostly taken from process context, but it is
+ * occasionally also taken from softirq/tasklet context, when
+ * task-structs get RCU-freed. Hence all locking must be softirq-safe.
+ */
 static DEFINE_SPINLOCK(uidhash_lock);
 
 struct user_struct root_user = {
@@ -83,14 +90,15 @@ struct user_struct *find_user(uid_t uid)
 {
         struct user_struct *ret;
 
-        spin_lock(&uidhash_lock);
+        spin_lock_bh(&uidhash_lock);
         ret = uid_hash_find(uid, uidhashentry(uid));
-        spin_unlock(&uidhash_lock);
+        spin_unlock_bh(&uidhash_lock);
         return ret;
 }
 
 void free_uid(struct user_struct *up)
 {
+        local_bh_disable();
         if (up && atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
                 uid_hash_remove(up);
                 key_put(up->uid_keyring);
@@ -98,6 +106,7 @@ void free_uid(struct user_struct *up)
                 kmem_cache_free(uid_cachep, up);
                 spin_unlock(&uidhash_lock);
         }
+        local_bh_enable();
 }
 
 struct user_struct * alloc_uid(uid_t uid)
@@ -105,9 +114,9 @@ struct user_struct * alloc_uid(uid_t uid)
         struct list_head *hashent = uidhashentry(uid);
         struct user_struct *up;
 
-        spin_lock(&uidhash_lock);
+        spin_lock_bh(&uidhash_lock);
         up = uid_hash_find(uid, hashent);
-        spin_unlock(&uidhash_lock);
+        spin_unlock_bh(&uidhash_lock);
 
         if (!up) {
                 struct user_struct *new;
@@ -137,7 +146,7 @@ struct user_struct * alloc_uid(uid_t uid)
                  * Before adding this, check whether we raced
                  * on adding the same user already..
                  */
-                spin_lock(&uidhash_lock);
+                spin_lock_bh(&uidhash_lock);
                 up = uid_hash_find(uid, hashent);
                 if (up) {
                         key_put(new->uid_keyring);
@@ -147,7 +156,7 @@ struct user_struct * alloc_uid(uid_t uid)
                         uid_hash_insert(new, hashent);
                         up = new;
                 }
-                spin_unlock(&uidhash_lock);
+                spin_unlock_bh(&uidhash_lock);
 
         }
         return up;
@@ -183,9 +192,9 @@ static int __init uid_cache_init(void)
                 INIT_LIST_HEAD(uidhash_table + n);
 
         /* Insert the root user immediately (init already runs as root) */
-        spin_lock(&uidhash_lock);
+        spin_lock_bh(&uidhash_lock);
         uid_hash_insert(&root_user, uidhashentry(0));
-        spin_unlock(&uidhash_lock);
+        spin_unlock_bh(&uidhash_lock);
 
         return 0;
 }