Re: [PATCH v3 3/3] namei: aggressively check for nd->root escape on ".." resolution

On 2018-10-13, Al Viro [off-list ref] wrote:

On Sat, Oct 13, 2018 at 07:53:26PM +1100, Aleksa Sarai wrote:

quoted

I didn't know about path_is_under() -- I just checked and it appears to
not take &rename_lock? From my understanding, in order to protect
against the rename attack you need to take &rename_lock (or check
against &rename_lock at least and retry if it changed).

I could definitely use path_is_under() if you prefer, though I think
that in this case we'd need to take &rename_lock (right?). Also is there
a speed issue with taking the write-side of a seqlock when we are just
reading -- is this more efficient than doing a retry like in __d_path?

???

1) it uses is_subdir(), which does deal with rename_lock

Oh -- complete brain-fart on my part. Sorry about that.

2) what it does is taking mount_lock.lock.  I.e. the same
thing as the second retry in __d_path().  _If_ it shows
up in profiles, we can switch it to read_seqbegin_or_lock(),
but I'd like to see the profiling data first.

Sure, I'll switch it to use path_is_under().

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>