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

[David Howells]

Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> 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: James Morris <jmorris@xxxxxxxxx>
> Cc: linux-security-module@xxxxxxxxxxxxxxx

Acked-by: David Howells <dhowells@xxxxxxxxxx>
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