Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex

Bernd Edlinger
2020-03-06T11:46:07Z

Am 06.03.20 um 06:17 schrieb Eric W. Biederman:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>
>> On 3/5/20 10:16 PM, Eric W. Biederman wrote:
>>>
>>> The cred_guard_mutex is problematic. The cred_guard_mutex is held
>>> over the userspace accesses as the arguments from userspace are read.
>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>> threads are killed. The cred_guard_mutex is held over
>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>
>>> Any of those can result in deadlock, as the cred_guard_mutex is held
>>> over a possible indefinite userspace waits for userspace.
>>>
>>> Add exec_update_mutex that is only held over exec updating process
>>> with the new contents of exec, so that code that needs not to be
>>> confused by exec changing the mm and the cred in ways that can not
>>> happen during ordinary execution of a process can take.
>>>
>>> The plan is to switch the users of cred_guard_mutex to
>>> exed_udpate_mutex one by one. This lets us move forward while still
>>> being careful and not introducing any regressions.
>>>
>>> Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/
>>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
>>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
>>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
>>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
>>> Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>>> Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>>> ---
>>> fs/exec.c | 4 ++++
>>> include/linux/sched/signal.h | 9 ++++++++-
>>> kernel/fork.c | 1 +
>>> 3 files changed, 13 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/exec.c b/fs/exec.c
>>> index c243f9660d46..ad7b518f906d 100644
>>> --- a/fs/exec.c
>>> +++ b/fs/exec.c
>>> @@ -1182,6 +1182,7 @@ static int de_thread(struct linux_binprm *bprm, struct task_struct *tsk)
>>> release_task(leader);
>>> }
>>>
>>> + mutex_lock(&current->signal->exec_update_mutex);

And by the way, could you make this mutex_lock_killable?

Bernd.