From f358166a9405e4f1d8e50d8f415c26d95505b6de Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Thu, 11 May 2006 11:08:49 -0700 Subject: [PATCH] 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 Cc: Eric W. Biederman Cc: Roland McGrath Signed-off-by: Linus Torvalds --- kernel/ptrace.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) 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) -- 2.20.1