On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman [off-list ref] wrote:

Jann Horn [off-list ref] writes:

quoted

On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman [off-list ref] wrote:

quoted

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.

quoted

Cc: Sargun Dhillon <redacted>
Cc: Christian Brauner <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
Signed-off-by: "Eric W. Biederman" <redacted>
---
 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.

quoted

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?

Hmm, I guess so? Normally, a task that's changing credentials becomes
nondumpable at the same time (and there are explicit memory barriers
in commit_creds() and __ptrace_may_access() to enforce the ordering
for this); so you normally don't see tasks becoming ptrace-accessible
via anything other than execve(). But I guess if someone opens a
root-only file, closes it, drops privileges, and then explicitly does
prctl(PR_SET_DUMPABLE, 1), we should probably protect that, too.

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

There are also setuid helpers that can, after having done privileged
stuff, drop privileges and call execve(); after that,
ptrace_may_access() succeeds again. In particular, polkit has a helper
that does this.

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.

Hmm... what benefits do we get from making it a separate lock? I guess
it would allow us to make it a per-task lock instead of a
signal_struct-wide one? That might be helpful...