[RFC] vfs: security: Parse dev_name before calling security_sb_mount

[Song Liu]

> On Jul 16, 2025, at 1:31 AM, Christian Brauner <brauner at kernel.org> wrote:
> 
> On Tue, Jul 15, 2025 at 10:31:39PM +0000, Song Liu wrote:
>> 
>>> On Jul 15, 2025, at 3:18 AM, Christian Brauner <brauner at kernel.org> wrote:
>>> On Mon, Jul 14, 2025 at 03:10:57PM +0000, Song Liu wrote:
>> 
>> 
>> [...]
>> 
>>>>> If you place a new security hook into __do_loopback() the only thing
>>>>> that I'm not excited about is that we're holding the global namespace
>>>>> semaphore at that point. And I want to have as little LSM hook calls
>>>>> under the namespace semaphore as possible.
>>>> 
>>>> do_loopback() changed a bit since [1]. But if we put the new hook 
>>>> in do_loopback() before lock_mount(), we don’t have the problem with
>>>> the namespace semaphore, right? Also, this RFC doesn’t seem to have 
>>>> this issue either.
>>> 
>>> While the mount isn't locked another mount can still be mounted on top
>>> of it. lock_mount() will detect this and lookup the topmost mount and
>>> use that. IOW, the value of old_path->mnt may have changed after
>>> lock_mount().
>> 
>> I am probably confused. Do you mean path->mnt (instead of old_path->mnt) 
>> may have changed after lock_mount()?
> 
> I mean the target path. I forgot that the code uses @old_path to mean
> the source path not the target path. And you're interested in the source
> path, not the target path.

Both security_sb_mount and security_move_mount has the overmount issue 
for target path. 

[...]

>> 
>> It appears to me that do_loopback() has the tricky issue:
>> 
>> static int do_loopback(struct path *path, ...)
>> {
>> ...
>> /* 
>> * path may still change, so not a good point to add
>> * security hook 
>> */
>> mp = lock_mount(path);
>> if (IS_ERR(mp)) {
>> /* ... */
>> }
>> /* 
>> * namespace_sem is locked, so not a good point to add
>> * security hook
>> */
>> ...
>> }
>> 
>> Basically, without major work with locking, there is no good 
>> spot to insert a security hook into do_loopback(). Or, maybe 
>> we can add a hook somewhere in lock_mount()?
> 
> You can't really because the lookup_mnt() call in lock_mount() happens
> under the namespace semaphore already and if it's the topmost mount it
> won't be dropped again and you can't drop it again without risking
> overmounts again.

We probably have to accept the overmount issue for security_ hooks 
that covers the new mount APIs. 

> But again, as long as you are interested in the source mount you should
> be fine.

For the source path, we are back to the issue we want to address 
in this RFC: to get struct path of dev_name (source path) for bind 
mount. Among these proposals:

1. Introduce bpf_kern_path kfunc.
We will probably limit this kfunc to only work on security_sb_mount.

2. Add new hook(s), such as [1].
[1] https://lore.kernel.org/linux-security-module/20250110021008.2704246-1-enlightened@chromium.org/
This is not a complete solution. However, given security_sb_mount 
as-is handles so many different cases, we will likely split it in 
the future. Therefore, this new hook can be a reasonable incremental 
step. 

3. Something like this patch.

Does any proposal look acceptable?

Thanks,
Song