Jann Horn [off-list ref] writes:

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?

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