From f362b73244fb16ea4ae127ced1467dd8adaa7733 Mon Sep 17 00:00:00 2001 From: Daniel J Blueman Date: Tue, 17 Aug 2010 23:56:55 +0100 Subject: [PATCH] Fix unprotected access to task credentials in waitid() Using a program like the following: #include #include #include #include int main() { id_t id; siginfo_t infop; pid_t res; id = fork(); if (id == 0) { sleep(1); exit(0); } kill(id, SIGSTOP); alarm(1); waitid(P_PID, id, &infop, WCONTINUED); return 0; } to call waitid() on a stopped process results in access to the child task's credentials without the RCU read lock being held - which may be replaced in the meantime - eliciting the following warning: =================================================== [ INFO: suspicious rcu_dereference_check() usage. ] --------------------------------------------------- kernel/exit.c:1460 invoked rcu_dereference_check() without protection! other info that might help us debug this: rcu_scheduler_active = 1, debug_locks = 1 2 locks held by waitid02/22252: #0: (tasklist_lock){.?.?..}, at: [] do_wait+0xc5/0x310 #1: (&(&sighand->siglock)->rlock){-.-...}, at: [] wait_consider_task+0x19a/0xbe0 stack backtrace: Pid: 22252, comm: waitid02 Not tainted 2.6.35-323cd+ #3 Call Trace: [] lockdep_rcu_dereference+0xa4/0xc0 [] wait_consider_task+0xaf1/0xbe0 [] do_wait+0xf5/0x310 [] sys_waitid+0x86/0x1f0 [] ? child_wait_callback+0x0/0x70 [] system_call_fastpath+0x16/0x1b This is fixed by holding the RCU read lock in wait_task_continued() to ensure that the task's current credentials aren't destroyed between us reading the cred pointer and us reading the UID from those credentials. Furthermore, protect wait_task_stopped() in the same way. We don't need to keep holding the RCU read lock once we've read the UID from the credentials as holding the RCU read lock doesn't stop the target task from changing its creds under us - so the credentials may be outdated immediately after we've read the pointer, lock or no lock. Signed-off-by: Daniel J Blueman Signed-off-by: David Howells Acked-by: Paul E. McKenney Acked-by: Oleg Nesterov Signed-off-by: Linus Torvalds --- kernel/exit.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 671ed56e0a4..03120229db2 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1386,8 +1386,7 @@ static int wait_task_stopped(struct wait_opts *wo, if (!unlikely(wo->wo_flags & WNOWAIT)) *p_code = 0; - /* don't need the RCU readlock here as we're holding a spinlock */ - uid = __task_cred(p)->uid; + uid = task_uid(p); unlock_sig: spin_unlock_irq(&p->sighand->siglock); if (!exit_code) @@ -1460,7 +1459,7 @@ static int wait_task_continued(struct wait_opts *wo, struct task_struct *p) } if (!unlikely(wo->wo_flags & WNOWAIT)) p->signal->flags &= ~SIGNAL_STOP_CONTINUED; - uid = __task_cred(p)->uid; + uid = task_uid(p); spin_unlock_irq(&p->sighand->siglock); pid = task_pid_vnr(p); -- 2.11.4.GIT