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. > > 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? > # 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:
