Re: [PATCH V2] exportfs: do not read dentry after free

[Al Viro]

On Fri, Nov 23, 2018 at 03:56:33PM +0800, Pan Bian wrote:
> The function dentry_connected calls dput(dentry) to drop the previously
> acquired reference to dentry. In this case, dentry can be released.
> After that, IS_ROOT(dentry) checks the condition
> (dentry == dentry->d_parent), which may result in a use-after-free bug.
> This patch directly compares dentry with its parent obtained before
> dropping the reference.

It's a bit more subtle than the description implies (the race has
dentry connected during dget_parent() and the child we'd reached it
through moved elsewhere during the dput()), but you are right - the
race is there and that patch fixes it.

I wonder if we could avoid those dget_parent()/dput() completely -
looks like we might be able to with rcu_read_lock() and some
care.  OTOH, that's not going to be a hot path, anyway...

Applied.