On Wed, Jan 25, 2006 at 03:23:07PM +0100, Ingo Molnar wrote:
> RCU task-struct freeing can call free_uid(), which is taking
> uidhash_lock - while other users of uidhash_lock are softirq-unsafe.
I guess I get to feel doubly stupid today. Good catch, great tool!!!
Thanx, Paul
Acked-by <[email protected]>
> 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 <[email protected]>
>
> ----
>
> 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;
> }
>
-
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