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Subject[tip:core/urgent] sys: Fix missing rcu protection for __task_cred() access
Commit-ID:  d4581a239a40319205762b76c01eb6363f277efa
Gitweb: http://git.kernel.org/tip/d4581a239a40319205762b76c01eb6363f277efa
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 10 Dec 2009 00:52:51 +0000
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 10 Dec 2009 23:04:11 +0100
sys: Fix missing rcu protection for __task_cred() access

commit c69e8d9 (CRED: Use RCU to access another task's creds and to
release a task's own creds) added non rcu_read_lock() protected access
to task creds of the target task in set_prio_one().

The comment above the function says:
* - the caller must hold the RCU read lock

The calling code in sys_setpriority does read_lock(&tasklist_lock) but
not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
interrupt when they see no read side critical section.

There is another instance of __task_cred() in sys_setpriority() itself
which is equally unprotected.

Wrap the whole code section into a rcu read side critical section to
fix this quick and dirty.

Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
crusade.

Oleg noted further:

This also fixes another bug here. find_task_by_vpid() is not safe
without rcu_read_lock(). I do not mean it is not safe to use the
result, just find_pid_ns() by itself is not safe.

Usually tasklist gives enough protection, but if copy_process() fails
it calls free_pid() lockless and does call_rcu(delayed_put_pid().
This means, without rcu lock find_pid_ns() can't scan the hash table
safely.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20091210004703.029784964@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

---
kernel/sys.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/kernel/sys.c b/kernel/sys.c
index 9968c5f..bc1dc61 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
if (niceval > 19)
niceval = 19;

+ rcu_read_lock();
read_lock(&tasklist_lock);
switch (which) {
case PRIO_PROCESS:
@@ -200,6 +201,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
}
out_unlock:
read_unlock(&tasklist_lock);
+ rcu_read_unlock();
out:
return error;
}

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