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ptrace: do not use task_lock() for attach 

Remove the "Nasty, nasty" lock dance in ptrace_attach()/ptrace_traceme() -
from now task_lock() has nothing to do with ptrace at all.

With the recent changes nobody uses task_lock() to serialize with ptrace,
but in fact it was never needed and it was never used consistently.

However ptrace_attach() calls __ptrace_may_access() and needs task_lock()
to pin task->mm for get_dumpable().  But we can call __ptrace_may_access()
before we take tasklist_lock, ->cred_exec_mutex protects us against
do_execve() path which can change creds and MMF_DUMP* flags.

(ugly, but we can't use ptrace_may_access() because it hides the error
code, so we have to take task_lock() and use __ptrace_may_access()).

NOTE: this change assumes that LSM hooks, security_ptrace_may_access() and
security_ptrace_traceme(), can be called without task_lock() held.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2009-06-17 16:27:33 -07:00 committed by Linus Torvalds
parent f2f0b00ad6
commit 4b105cbbaf
1 changed files with 13 additions and 46 deletions
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59
kernel/ptrace.c
View File

@ -167,7 +167,6 @@ bool ptrace_may_access(struct task_struct *task, unsigned int mode)
int ptrace_attach(struct task_struct *task)
{
int retval;
unsigned long flags;
audit_ptrace(task);
@ -185,34 +184,19 @@ int ptrace_attach(struct task_struct *task)
retval = mutex_lock_interruptible(&task->cred_guard_mutex);
if (retval < 0)
goto out;
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);
if (!write_trylock_irqsave(&tasklist_lock, flags)) {
task_unlock(task);
do {
cpu_relax();
} while (!write_can_lock(&tasklist_lock));
goto repeat;
}
retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH);
task_unlock(task);
if (retval)
goto bad;
goto unlock_creds;
write_lock_irq(&tasklist_lock);
retval = -EPERM;
if (unlikely(task->exit_state))
goto bad;
goto unlock_tasklist;
if (task->ptrace)
goto bad;
goto unlock_tasklist;
task->ptrace = PT_PTRACED;
if (capable(CAP_SYS_PTRACE))
@ -222,9 +206,9 @@ repeat:
send_sig_info(SIGSTOP, SEND_SIG_FORCED, task);
retval = 0;
bad:
write_unlock_irqrestore(&tasklist_lock, flags);
task_unlock(task);
unlock_tasklist:
write_unlock_irq(&tasklist_lock);
unlock_creds:
mutex_unlock(&task->cred_guard_mutex);
out:
return retval;
@ -240,26 +224,10 @@ int ptrace_traceme(void)
{
int ret = -EPERM;
/*
* Are we already being traced?
*/
repeat:
task_lock(current);
write_lock_irq(&tasklist_lock);
/* Are we already being traced? */
if (!current->ptrace) {
/*
* See ptrace_attach() comments about the locking here.
*/
unsigned long flags;
if (!write_trylock_irqsave(&tasklist_lock, flags)) {
task_unlock(current);
do {
cpu_relax();
} while (!write_can_lock(&tasklist_lock));
goto repeat;
}
ret = security_ptrace_traceme(current->parent);
/*
* Check PF_EXITING to ensure ->real_parent has not passed
* exit_ptrace(). Otherwise we don't report the error but
@ -269,10 +237,9 @@ repeat:
current->ptrace = PT_PTRACED;
__ptrace_link(current, current->real_parent);
}
write_unlock_irqrestore(&tasklist_lock, flags);
}
task_unlock(current);
write_unlock_irq(&tasklist_lock);
return ret;
}