Re: [PATCH] pidfd: Stop taking cred_guard_mutex

[Eric W. Biederman]

Jann Horn <jannh@xxxxxxxxxx> writes:

> On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>> During exec some file descriptors are closed and the files struct is
>> unshared.  But all of that can happen at other times and it has the
>> same protections during exec as at ordinary times.  So stop taking the
>> cred_guard_mutex as it is useless.
>>
>> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
>> prone, as it is held in serveral while waiting possibly indefinitely
>> for userspace to do something.
>
> Please don't. Just use the new exec_update_mutex like everywhere else.
>
>> Cc: Sargun Dhillon <sargun@xxxxxxxxx>
>> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
>> Cc: Arnd Bergmann <arnd@xxxxxxxx>
>> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>  kernel/pid.c | 6 ------
>>  1 file changed, 6 deletions(-)
>>
>> Christian if you don't have any objections I will take this one through
>> my tree.
>>
>> I tried to figure out why this code path takes the cred_guard_mutex and
>> the archive on lore.kernel.org was not helpful in finding that part of
>> the conversation.
>
> That was my suggestion.
>
>> diff --git a/kernel/pid.c b/kernel/pid.c
>> index 60820e72634c..53646d5616d2 100644
>> --- a/kernel/pid.c
>> +++ b/kernel/pid.c
>> @@ -577,17 +577,11 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
>>         struct file *file;
>>         int ret;
>>
>> -       ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
>> -       if (ret)
>> -               return ERR_PTR(ret);
>> -
>>         if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
>>                 file = fget_task(task, fd);
>>         else
>>                 file = ERR_PTR(-EPERM);
>>
>> -       mutex_unlock(&task->signal->cred_guard_mutex);
>> -
>>         return file ?: ERR_PTR(-EBADF);
>>  }
>
> If you make this change, then if this races with execution of a setuid
> program that afterwards e.g. opens a unix domain socket, an attacker
> will be able to steal that socket and inject messages into
> communication with things like DBus. procfs currently has the same
> race, and that still needs to be fixed, but at least procfs doesn't
> let you open things like sockets because they don't have a working
> ->open handler, and it enforces the normal permission check for
> opening files.

It isn't only exec that can change credentials.  Do we need a lock for
changing credentials?

Wouldn't it be sufficient to simply test ptrace_may_access after
we get a copy of the file?

If we need a lock around credential change let's design and build that.
Having a mismatch between what a lock is designed to do, and what
people use it for can only result in other bugs as people get confused.

Eric