[patch, lock validator] fix uidhash_lock <-> RCU deadlock

[Ingo Molnar <mingo@elte.hu>]

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

This bug was found by the lock validator i'm working on:

 ============================
 [ BUG: illegal lock usage! ]
 ----------------------------
 illegal {enabled-softirqs} -> {used-in-softirq} usage.
 swapper/0 [HC0[0]:SC1[2]:HE1:SE0] takes {uidhash_lock [u:25]}, at:
  [<c025e858>] _atomic_dec_and_lock+0x48/0x80
 {enabled-softirqs} state was registered at:
  [<c04d7cfd>] _write_unlock_bh+0xd/0x10
 hardirqs last enabled at: [<c015f278>] kmem_cache_free+0x78/0xb0
 softirqs last enabled at: [<c011b2da>] copy_process+0x2ca/0xe80

 other info that might help in debugging this:
 ------------------------------
 | showing all locks held by: |  (swapper/0 [c30d8790, 140]): <none>
 ------------------------------

  [<c010432d>] show_trace+0xd/0x10
  [<c0104347>] dump_stack+0x17/0x20
  [<c01371d1>] print_usage_bug+0x1e1/0x200
  [<c0137789>] mark_lock+0x259/0x290
  [<c0137c25>] debug_lock_chain_spin+0x465/0x10f0
  [<c0264abd>] _raw_spin_lock+0x2d/0x90
  [<c04d7a68>] _spin_lock+0x8/0x10
  [<c025e858>] _atomic_dec_and_lock+0x48/0x80
  [<c0127674>] free_uid+0x14/0x70
  [<c011a428>] __put_task_struct+0x38/0x130
  [<c0114afd>] __put_task_struct_cb+0xd/0x10
  [<c012f151>] __rcu_process_callbacks+0x81/0x180
  [<c012f550>] rcu_process_callbacks+0x30/0x60
  [<c0122aa4>] tasklet_action+0x54/0xd0
  [<c0122c77>] __do_softirq+0x87/0x120
  [<c0105519>] do_softirq+0x69/0xb0
  =======================
  [<c0122939>] irq_exit+0x39/0x50
  [<c010f47c>] smp_apic_timer_interrupt+0x4c/0x50
  [<c010393b>] apic_timer_interrupt+0x27/0x2c
  [<c0101c58>] cpu_idle+0x68/0x80
  [<c010e37e>] start_secondary+0x29e/0x480
  [<00000000>] 0x0
  [<c30d9fb4>] 0xc30d9fb4

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

Signed-off-by: Ingo Molnar <mingo@elte.hu>

----

Index: linux/kernel/user.c
===================================================================
--- linux.orig/kernel/user.c
+++ linux/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;
 }