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/$$/attr/current; echo AAA [root@ip-10-0-32-198 ec2-user]# echo BBB > dir/attr/current [root@ip-10-0-32-198 ec2-user]# cat dir/attr/current cat: dir/attr/current: Permission denied If something frequently scans /proc, such as ps, top or whatever, on your machine, the inode may get updated quickly (i.e. revalidated during path lookup) and then you may only have a short window to observe the behavior. > >>> # cat /proc/$$/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(¤t->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: >
