Re: [PATCH] proc: Update inode upon changing task security attribute

From: Munehisa Kamata
Date: Fri Dec 01 2023 - 01:11:22 EST

On Thu, 2023-11-30 16:31:11 -0800, Casey Schaufler wrote:
>
> On 11/30/2023 12:35 PM, Munehisa Kamata wrote:
> > On Thu, 2023-11-30 18:00:13 +0000, Casey Schaufler wrote:
> >> On 11/29/2023 7:07 PM, Munehisa Kamata wrote:
> >>> Hi Casey,
> >>>
> >>> On Wed, 2023-11-29 18:28:55 -0800, Casey Schaufler wrote:
> >>>> On 11/29/2023 4:37 PM, Munehisa Kamata wrote:
> >>>>> I'm not clear whether VFS is a better (or worse) place[1] to fix the
> >>>>> problem described below and would like to hear opinion.
> >>>> Please To: or at least Cc: me on all Smack related issues.
> >>> Will do that next.
> >>>
> >>>>> If the /proc/[pid] directory is bind-mounted on a system with Smack
> >>>>> enabled, and if the task updates its current security attribute, the task
> >>>>> may lose access to files in its own /proc/[pid] through the mountpoint.
> >>>>>
> >>>>> $ sudo capsh --drop=cap_mac_override --
> >>>>> # mkdir -p dir
> >>>>> # mount --bind /proc/$$ dir
> >>>>> # echo AAA > /proc/$$/task/current # assuming built-in echo
> >>>> I don't see "current" in /proc/$$/task. Did you mean /proc/$$/attr?
> >>> Ahh, yes, I meant /proc/$$/attr/current. Sorry about that...
> >>>
> >>>>> # cat /proc/$$/task/current # revalidate
> >>>>> AAA
> >>>>> # echo BBB > dir/attr/current
> >>>>> # cat dir/attr/current
> >>>>> cat: dir/attr/current: Permission denied
> >>>>> # ls dir/
> >>>>> ls: cannot access dir/: Permission denied
> >> I don't see this behavior. What kernel version are you using?
> >> I have a 6.5 kernel.
> > I verified the behavior with 6.7-rc3.
> >
> > Here is more "raw" log from my machine:
> >
> > [ec2-user@ip-10-0-32-198 ~]$ uname -r
> > 6.7.0-rc3-proc-fix+
> > [ec2-user@ip-10-0-32-198 ~]$ sudo capsh --drop=cap_mac_override --
> > [root@ip-10-0-32-198 ec2-user]# mount --bind /proc/$$ dir
> > [root@ip-10-0-32-198 ec2-user]# echo AAA > /proc/$$/attr/current
> > [root@ip-10-0-32-198 ec2-user]# cat /proc/$$/attr/current; echo
> > AAA
> > [root@ip-10-0-32-198 ec2-user]# echo BBB > dir/attr/current
> > [root@ip-10-0-32-198 ec2-user]# cat dir/attr/current
> > cat: dir/attr/current: Permission denied
> >
> > If something frequently scans /proc, such as ps, top or whatever, on your
> > machine, the inode may get updated quickly (i.e. revalidated during path
> > lookup) and then you may only have a short window to observe the behavior.
>
> I was able to reproduce the issue with a 6.5 kernel. The window seems
> to be really short.

Creating a PID namespace before the bind-mount may make the window lasts
longer (or forever).

$ sudo unshare -pf --mount-proc

> Would it be completely unreasonable for your sandboxing application to
> call syncfs(2) after writing to current?

It doesn't help. It won't revalidate dentries.

> >
> >>>>> # cat /proc/$$/attr/current # revalidate
> >>>>> BBB
> >>>>> # cat dir/attr/current
> >>>>> BBB
> >>>>> # echo CCC > /proc/$$/attr/current
> >>>>> # cat dir/attr/current
> >>>>> cat: dir/attr/current: Permission denied
> >>>>>
> >>>>> This happens because path lookup doesn't revalidate the dentry of the
> >>>>> /proc/[pid] when traversing the filesystem boundary, so the inode security
> >>>>> blob of the /proc/[pid] doesn't get updated with the new task security
> >>>>> attribute. Then, this may lead security modules to deny an access to the
> >>>>> directory. Looking at the code[2] and the /proc/pid/attr/current entry in
> >>>>> proc man page, seems like the same could happen with SELinux. Though, I
> >>>>> didn't find relevant reports.
> >>>>>
> >>>>> The steps above are quite artificial. I actually encountered such an
> >>>>> unexpected denial of access with an in-house application sandbox
> >>>>> framework; each app has its own dedicated filesystem tree where the
> >>>>> process's /proc/[pid] is bind-mounted to and the app enters into via
> >>>>> chroot.
> >>>>>
> >>>>> With this patch, writing to /proc/[pid]/attr/current (and its per-security
> >>>>> module variant) updates the inode security blob of /proc/[pid] or
> >>>>> /proc/[pid]/task/[tid] (when pid != tid) with the new attribute.
> >>>>>
> >>>>> [1] https://lkml.kernel.org/linux-fsdevel/4A2D15AF.8090000@xxxxxxx/
> >>>>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/security/selinux/hooks.c#n4220
> >>>>>
> >>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> >>>>> Signed-off-by: Munehisa Kamata <kamatam@xxxxxxxxxx>
> >>>>> ---
> >>>>> fs/proc/base.c | 23 ++++++++++++++++++++---
> >>>>> 1 file changed, 20 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
> >>>>> index dd31e3b6bf77..bdb7bea53475 100644
> >>>>> --- a/fs/proc/base.c
> >>>>> +++ b/fs/proc/base.c
> >>>>> @@ -2741,6 +2741,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
> >>>>> {
> >>>>> struct inode * inode = file_inode(file);
> >>>>> struct task_struct *task;
> >>>>> + const char *name = file->f_path.dentry->d_name.name;
> >>>>> void *page;
> >>>>> int rv;
> >>>>>
> >>>>> @@ -2784,10 +2785,26 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
> >>>>> if (rv < 0)
> >>>>> goto out_free;
> >>>>>
> >>>>> - rv = security_setprocattr(PROC_I(inode)->op.lsm,
> >>>>> - file->f_path.dentry->d_name.name, page,
> >>>>> - count);
> >>>>> + rv = security_setprocattr(PROC_I(inode)->op.lsm, name, page, count);
> >>>>> mutex_unlock(&current->signal->cred_guard_mutex);
> >>>>> +
> >>>>> + /*
> >>>>> + * Update the inode security blob in advance if the task's security
> >>>>> + * attribute was updated
> >>>>> + */
> >>>>> + if (rv > 0 && !strcmp(name, "current")) {
> >>>>> + struct pid *pid;
> >>>>> + struct proc_inode *cur, *ei;
> >>>>> +
> >>>>> + rcu_read_lock();
> >>>>> + pid = get_task_pid(current, PIDTYPE_PID);
> >>>>> + hlist_for_each_entry(cur, &pid->inodes, sibling_inodes)
> >>>>> + ei = cur;
> >>>>> + put_pid(pid);
> >>>>> + pid_update_inode(current, &ei->vfs_inode);
> >>>>> + rcu_read_unlock();
> >>>>> + }
> >>>>> +
> >>>>> out_free:
> >>>>> kfree(page);
> >>>>> out:
>