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(¤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: >
