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

From: Aleksa Sarai
Date: Sat Oct 13 2018 - 05:27:37 EST

On 2018-10-13, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Oct 13, 2018 at 07:53:26PM +1100, Aleksa Sarai wrote:
> > 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

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