From 1447399b3e34af016c368b4178db7ef0e04e15b0 Mon Sep 17 00:00:00 2001 From: Daniel J Blueman Date: Tue, 9 Nov 2010 21:33:02 +0100 Subject: [PATCH] ioprio: fix RCU locking around task dereference With 2.6.37-rc1, I observe sys_ioprio_set not taking the RCU lock [1] across access to the task credentials. Inspecting the code in fs/ioprio.c, the tasklist_lock is held for read across the __task_cred call, which is presumably sufficient to prevent the task credentials becoming stale. =================================================== [ INFO: suspicious rcu_dereference_check() usage. ] --------------------------------------------------- kernel/pid.c:419 invoked rcu_dereference_check() without protection! other info that might help us debug this: rcu_scheduler_active = 1, debug_locks = 1 1 lock held by start-stop-daem/2246: #0: (tasklist_lock){.?.?..}, at: [] sys_ioprio_set+0x8a/0x400 stack backtrace: Pid: 2246, comm: start-stop-daem Not tainted 2.6.37-rc1-330cd+ #2 Call Trace: [] lockdep_rcu_dereference+0xa4/0xc0 [] find_task_by_pid_ns+0x81/0x90 [] find_task_by_vpid+0x1d/0x20 [] sys_ioprio_set+0x3f0/0x400 [] ? trace_hardirqs_on_thunk+0x3a/0x3f [] system_call_fastpath+0x16/0x1b Take the RCU lock for read across acquiring the pointer to the task credentials and dereferencing it. Signed-off-by: Daniel J Blueman Fixed up by Jens to fix missing rcu_read_unlock() on mismatches. Signed-off-by: Jens Axboe --- fs/ioprio.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/fs/ioprio.c b/fs/ioprio.c index 748cfb9..8def14e 100644 --- a/fs/ioprio.c +++ b/fs/ioprio.c @@ -139,7 +139,12 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio) break; do_each_thread(g, p) { - if (__task_cred(p)->uid != who) + int match; + + rcu_read_lock(); + match = __task_cred(p)->uid == who; + rcu_read_unlock(); + if (!match) continue; ret = set_task_ioprio(p, ioprio); if (ret) @@ -232,7 +237,12 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who) break; do_each_thread(g, p) { - if (__task_cred(p)->uid != user->uid) + int match; + + rcu_read_lock(); + match = __task_cred(p)->uid == user->uid; + rcu_read_unlock(); + if (!match) continue; tmpio = get_task_ioprio(p); if (tmpio < 0) -- 2.7.4