Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Andy Lutomirski]

> On Jan 1, 2020, at 11:44 PM, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> 
> ïOn 2020-01-01, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>>> On Wed, Jan 01, 2020 at 12:54:46AM +0000, Al Viro wrote:
>>> Note, BTW, that lookup_last() (aka walk_component()) does just
>>> that - we only hit step_into() on LAST_NORM.  The same goes
>>> for do_last().  mountpoint_last() not doing the same is _not_
>>> intentional - it's definitely a bug.
>>> 
>>> Consider your testcase; link points to . here.  So the only
>>> thing you could expect from trying to follow it would be
>>> the directory 'link' lives in.  And you don't have it
>>> when you reach the fscker via /proc/self/fd/3; what happens
>>> instead is nd->path set to ./link (by nd_jump_link()) *AND*
>>> step_into() called, pushing the same ./link onto stack.
>>> It violates all kinds of assumptions made by fs/namei.c -
>>> when pushing a symlink onto stack nd->path is expected to
>>> contain the base directory for resolving it.
>>> 
>>> I'm fairly sure that this is the cause of at least some
>>> of the insanity you've caught; there always could be
>>> something else, of course, but this hole needs to be
>>> closed in any case.
>> 
>> ... and with removal of now unused local variable, that's
>> 
>> mountpoint_last(): fix the treatment of LAST_BIND
>> 
>> step_into() should be attempted only in LAST_NORM
>> case, when we have the parent directory (in nd->path).
>> We get away with that for LAST_DOT and LOST_DOTDOT,
>> since those can't be symlinks, making step_init() and
>> equivalent of path_to_nameidata() - we do a bit of
>> useless work, but that's it.  For LAST_BIND (i.e.
>> the case when we'd just followed a procfs-style
>> symlink) we really can't go there - result might
>> be a symlink and we really can't attempt following
>> it.
>> 
>> lookup_last() and do_last() do handle that properly;
>> mountpoint_last() should do the same.
>> 
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> 
> Thanks, this fixes the issue for me (and also fixes another reproducer I
> found -- mounting a symlink on top of itself then trying to umount it).
> 
> Reported-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> Tested-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> 
> As for the original topic of bind-mounting symlinks -- given this is a
> supported feature, would you be okay with me sending an updated
> O_EMPTYPATH series?

FWIW, I have an actual use case for mounting over a symlink: replacing /etc/resolv.conf.  My virtme tool is presented with somewhat arbitrary crud in /etc, where /etc/resolv.conf might be a plain file or a symlink, but, regardless, has inappropriate contents. If itâs a file, I can mount a new file over it. If itâs a symlink and the kernel properly supported it, I could also mount over it.

Yes, I could also use overlayfs.  Maybe I should regardless.