aboutsummaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
authorDavid Howells <dhowells@redhat.com>2010-07-29 12:45:49 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2010-07-29 15:16:17 -0700
commitde09a9771a5346029f4d11e4ac886be7f9bfdd75 (patch)
treeccce9e75753c98a9f38075d54ab98aa4b89e66c7
parent540ad6b62b3a188a53b51cac81d8a60d40e29fbd (diff)
CRED: Fix get_task_cred() and task_state() to not resurrect dead credentials
It's possible for get_task_cred() as it currently stands to 'corrupt' a set of credentials by incrementing their usage count after their replacement by the task being accessed. What happens is that get_task_cred() can race with commit_creds(): TASK_1 TASK_2 RCU_CLEANER -->get_task_cred(TASK_2) rcu_read_lock() __cred = __task_cred(TASK_2) -->commit_creds() old_cred = TASK_2->real_cred TASK_2->real_cred = ... put_cred(old_cred) call_rcu(old_cred) [__cred->usage == 0] get_cred(__cred) [__cred->usage == 1] rcu_read_unlock() -->put_cred_rcu() [__cred->usage == 1] panic() However, since a tasks credentials are generally not changed very often, we can reasonably make use of a loop involving reading the creds pointer and using atomic_inc_not_zero() to attempt to increment it if it hasn't already hit zero. If successful, we can safely return the credentials in the knowledge that, even if the task we're accessing has released them, they haven't gone to the RCU cleanup code. We then change task_state() in procfs to use get_task_cred() rather than calling get_cred() on the result of __task_cred(), as that suffers from the same problem. Without this change, a BUG_ON in __put_cred() or in put_cred_rcu() can be tripped when it is noticed that the usage count is not zero as it ought to be, for example: kernel BUG at kernel/cred.c:168! invalid opcode: 0000 [#1] SMP last sysfs file: /sys/kernel/mm/ksm/run CPU 0 Pid: 2436, comm: master Not tainted 2.6.33.3-85.fc13.x86_64 #1 0HR330/OptiPlex 745 RIP: 0010:[<ffffffff81069881>] [<ffffffff81069881>] __put_cred+0xc/0x45 RSP: 0018:ffff88019e7e9eb8 EFLAGS: 00010202 RAX: 0000000000000001 RBX: ffff880161514480 RCX: 00000000ffffffff RDX: 00000000ffffffff RSI: ffff880140c690c0 RDI: ffff880140c690c0 RBP: ffff88019e7e9eb8 R08: 00000000000000d0 R09: 0000000000000000 R10: 0000000000000001 R11: 0000000000000040 R12: ffff880140c690c0 R13: ffff88019e77aea0 R14: 00007fff336b0a5c R15: 0000000000000001 FS: 00007f12f50d97c0(0000) GS:ffff880007400000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f8f461bc000 CR3: 00000001b26ce000 CR4: 00000000000006f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 Process master (pid: 2436, threadinfo ffff88019e7e8000, task ffff88019e77aea0) Stack: ffff88019e7e9ec8 ffffffff810698cd ffff88019e7e9ef8 ffffffff81069b45 <0> ffff880161514180 ffff880161514480 ffff880161514180 0000000000000000 <0> ffff88019e7e9f28 ffffffff8106aace 0000000000000001 0000000000000246 Call Trace: [<ffffffff810698cd>] put_cred+0x13/0x15 [<ffffffff81069b45>] commit_creds+0x16b/0x175 [<ffffffff8106aace>] set_current_groups+0x47/0x4e [<ffffffff8106ac89>] sys_setgroups+0xf6/0x105 [<ffffffff81009b02>] system_call_fastpath+0x16/0x1b Code: 48 8d 71 ff e8 7e 4e 15 00 85 c0 78 0b 8b 75 ec 48 89 df e8 ef 4a 15 00 48 83 c4 18 5b c9 c3 55 8b 07 8b 07 48 89 e5 85 c0 74 04 <0f> 0b eb fe 65 48 8b 04 25 00 cc 00 00 48 3b b8 58 04 00 00 75 RIP [<ffffffff81069881>] __put_cred+0xc/0x45 RSP <ffff88019e7e9eb8> ---[ end trace df391256a100ebdd ]--- Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Jiri Olsa <jolsa@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--fs/proc/array.c2
-rw-r--r--include/linux/cred.h21
-rw-r--r--kernel/cred.c25
3 files changed, 27 insertions, 21 deletions
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 9b58d38bc911..fff6572676ae 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -176,7 +176,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
if (tracer)
tpid = task_pid_nr_ns(tracer, ns);
}
- cred = get_cred((struct cred *) __task_cred(p));
+ cred = get_task_cred(p);
seq_printf(m,
"State:\t%s\n"
"Tgid:\t%d\n"
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 75c0fa881308..ce40cbc791e2 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -153,6 +153,7 @@ struct cred {
extern void __put_cred(struct cred *);
extern void exit_creds(struct task_struct *);
extern int copy_creds(struct task_struct *, unsigned long);
+extern const struct cred *get_task_cred(struct task_struct *);
extern struct cred *cred_alloc_blank(void);
extern struct cred *prepare_creds(void);
extern struct cred *prepare_exec_creds(void);
@@ -282,26 +283,6 @@ static inline void put_cred(const struct cred *_cred)
((const struct cred *)(rcu_dereference_check((task)->real_cred, rcu_read_lock_held() || lockdep_tasklist_lock_is_held())))
/**
- * get_task_cred - Get another task's objective credentials
- * @task: The task to query
- *
- * Get the objective credentials of a task, pinning them so that they can't go
- * away. Accessing a task's credentials directly is not permitted.
- *
- * The caller must make sure task doesn't go away, either by holding a ref on
- * task or by holding tasklist_lock to prevent it from being unlinked.
- */
-#define get_task_cred(task) \
-({ \
- struct cred *__cred; \
- rcu_read_lock(); \
- __cred = (struct cred *) __task_cred((task)); \
- get_cred(__cred); \
- rcu_read_unlock(); \
- __cred; \
-})
-
-/**
* get_current_cred - Get the current task's subjective credentials
*
* Get the subjective credentials of the current task, pinning them so that
diff --git a/kernel/cred.c b/kernel/cred.c
index a2d5504fbcc2..60bc8b1e32e6 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -209,6 +209,31 @@ void exit_creds(struct task_struct *tsk)
}
}
+/**
+ * get_task_cred - Get another task's objective credentials
+ * @task: The task to query
+ *
+ * Get the objective credentials of a task, pinning them so that they can't go
+ * away. Accessing a task's credentials directly is not permitted.
+ *
+ * The caller must also make sure task doesn't get deleted, either by holding a
+ * ref on task or by holding tasklist_lock to prevent it from being unlinked.
+ */
+const struct cred *get_task_cred(struct task_struct *task)
+{
+ const struct cred *cred;
+
+ rcu_read_lock();
+
+ do {
+ cred = __task_cred((task));
+ BUG_ON(!cred);
+ } while (!atomic_inc_not_zero(&((struct cred *)cred)->usage));
+
+ rcu_read_unlock();
+ return cred;
+}
+
/*
* Allocate blank credentials, such that the credentials can be filled in at a
* later date without risk of ENOMEM.

Privacy Policy