[patch 1/9] sys: Fix missing rcu protection for __task_cred() access

[Thomas Gleixner]

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.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: linux-security-module@xxxxxxxxxxxxxxx
---
 kernel/sys.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6-tip/kernel/sys.c
===================================================================
--- linux-2.6-tip.orig/kernel/sys.c
+++ linux-2.6-tip/kernel/sys.c
@@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which,
 	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,
 	}
 out_unlock:
 	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 out:
 	return error;
 }


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