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 > > # 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: >
