Re: /proc/<pid>/exe symlink behavior change in >=3.15.

[Miklos Szeredi]

On Fri, Sep 12, 2014 at 1:57 AM, Mateusz Guzik <mguzik@xxxxxxxxxx> wrote:
> On Thu, Sep 11, 2014 at 06:39:58PM -0500, Chuck Ebbert wrote:
>> On Sun, 7 Sep 2014 09:56:08 +0200
>> Mateusz Guzik <mguzik@xxxxxxxxxx> wrote:
>>
>> > On Sat, Sep 06, 2014 at 11:44:32PM +0200, Piotr Karbowski wrote:
>> > > Hi,
>> > >
>> > > Starting with kernel 3.15 the 'exe' symlink under /proc/<pid>/ acts diffrent
>> > > than it used to in all the pre-3.15 kernels.
>> > >
>> > > The usecase:
>> > >
>> > > run /root/testbin (app that just sleeps)
>> > > cp /root/testbin /root/testbin.new
>> > > mv /root/testbin.new /root/testbin
>> > > ls -al /proc/`pidof testbin`/exe
>> > >
>> > > <=3.14: /root/testbin (deleted)
>> > > >=3.15: /root/testbin.new (deleted)
>> > >
>> > > Was the change intentional? It does render my system unusable and I failed
>> > > to find a information about such change in the ChangeLog.

Piotr, what exactly happens?  How does this break your system?

>> > >
>> >
>> > It looks like this was already broken for "long" (> DNAME_INLINE_LEN)
>> > names.
>> >
>> > Short names share the problem since da1ce0670c14d8 "vfs: add
>> > cross-rename".
>> >
>> > The following change to switch_names is the culprit:
>> >
>> > -       memcpy(dentry->d_iname, target->d_name.name,
>> > -                   target->d_name.len + 1);
>> > -       dentry->d_name.len = target->d_name.len;
>> > -       return;
>> > +       unsigned int i;
>> > +       BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
>> > +       for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
>> > +                     swap(((long *) &dentry->d_iname)[i],
>> > +                   ((long *) &target->d_iname)[i]);
>> > +       }
>> >
>> >
>> > Dentries can have names from embedded structure or from an external buffer.
>> >
>> > If you take a look around you will see the code just swaps pointers for
>> > "both external" case. But this results in the same behavoiur you are seeing.
>> >
>>
>> Looks like the real problem here is that __d_materialise_dentry() needs the
>> old behavior of switch_names() . At least that's how it got fixed in grsecurity.
>
> No.
>
> Regression in question is an effect of swap instead of memcpy in
> switch_names, as called by d_move. Fix in grsecurity reverts to previous
> behaviour when needed and imho should be applied for the time being.

Ack for that.  Linus will happily take this on the grounds of backward
compatibility, even if the old behavior was arguably more crazy than
the new one.

>
> The real problem is that __d_move always switches parent dentry and
> calls switch_names, which actually switches names in some cases.
>
> Without the regression you get expected results only for short names
> when you move stuff around within the same directory.
>
> For instance with current code:
> mv /foo/bar/baz /1/2/3
>
> will replace the whole path.
>
> Previous behavoiur would result in /foo/bar/3 as the new path, which is
> clearly still incorrect
>
> Leaving the old dentry under the same parent would mean that the "tree"
> associated with now moved dentry will possibly need to be freed.

It's done by dput().  But callers need to hold ref to old parent
anyway (because of locking) so it's not going to go away in d_move(),
only after everything is done.

>
> In addition to that one has to deal with the need of having renamed
> dentry the new name which possibly came from an external buffer. An idea
> I came up with (atomic_t refcount; char name[0]; with ->name assigned to
> dentry) may require adding an additional field to struct dentry, which
> would be bad.

You can do that without an extra field: e.g. use container_of() to get
the refcounted struct from the name.

Consider using kref for refcounting.

Thanks,
Miklos
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