ptrace_attach: fix possible deadlock schenario with irqs

Eric Biederman points out that we can't take the task_lock while holding
tasklist_lock for writing, because another CPU that holds the task lock
might take an interrupt that then tries to take tasklist_lock for writing.

Which would be a nasty deadlock, with one CPU spinning forever in an
interrupt handler (although admittedly you need to really work at
triggering it ;)

Since the ptrace_attach() code is special and very unusual, just make it
be extra careful, and use trylock+repeat to avoid the possible deadlock.

Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index b0f8da80d7d4..921c22ad16e4 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -155,8 +155,26 @@ int ptrace_attach(struct task_struct *task)
         if (task->tgid == current->tgid)
                 goto out;
 
-        write_lock_irq(&tasklist_lock);
+repeat:
+        /*
+         * Nasty, nasty.
+         *
+         * We want to hold both the task-lock and the
+         * tasklist_lock for writing at the same time.
+         * But that's against the rules (tasklist_lock
+         * is taken for reading by interrupts on other
+         * cpu's that may have task_lock).
+         */
         task_lock(task);
+        local_irq_disable();
+        if (!write_trylock(&tasklist_lock)) {
+                local_irq_enable();
+                task_unlock(task);
+                do {
+                        cpu_relax();
+                } while (!write_can_lock(&tasklist_lock));
+                goto repeat;
+        }
 
         /* the same process cannot be attached many times */
         if (task->ptrace & PT_PTRACED)