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. 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 # cat /proc/$$/task/current # revalidate AAA # echo BBB > dir/attr/current # cat dir/attr/current cat: dir/attr/current: Permission denied # ls dir/ ls: cannot access dir/: Permission denied # cat /proc/$$/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: -- 2.40.1
