Re: fs: NULL deref in atime_needs_update

[Al Viro]

On Fri, Feb 26, 2016 at 10:07:59PM +0000, Al Viro wrote:
> On Fri, Feb 26, 2016 at 10:25:21PM +0100, Dmitry Vyukov wrote:
> > On Fri, Feb 26, 2016 at 10:21 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > > On Thu, Feb 25, 2016 at 04:39:27PM +0000, Al Viro wrote:
> > >> Hrm...  OK, seeing that you still seem to trigger those within an hour or
> > >> two (and *any* of remaining WARN_ON() are serious bugs - none of the
> > >> "mitigation had been triggered" remained, sorry for not making it clear),
> > >> let's try this.  Again, any WARN_ON triggered means that we'd caught something,
> > >> whether it progresses into oops or not.
> > >
> > > Any news on that one?  I'm going to carve fixes for understood bugs out of
> > > that one and put those into tonight push, but it would be nice to sort out
> > > all remaining crap lurking in that area...
> > >
> > > Another question: what about the very first trace you'd posted, with apparent
> > > GPF at 00000050?  Have you seen anything like that afterwards?
> > 
> > No, I did not have time to retest.
> > 
> > GPF at 00000050 was not mine, it was Mickaël's.
> 
> Ah, OK - his is basically a forced nd->stack[] underrun, with passing a
> never-assigned nd->link_inode to atime_needs_update(), so we are just
> passing a contents of uninitialized stack word there and while it ends
> up possible to dereference, it's not an address of struct inode and the
> first attempt to follow a pointer in what would've been a struct inode
> at that address (accessing inode->i_sb->s_flags) did blow up with GPF at
> offsetof(struct super_block, s_flags).
> 
> All right, so we basically have several understood ones with fixes plus
> something unknown that leads to lookup_fast() returning 0 with NULL in
> *inode in about an hour or two on your setup...

BTW, what kind of userland are you using?  The thing is, shared-subtree
setups differ, and if the crap is anywhere near vfsmount handling, that
could have some impact...  So far I hadn't been able to trigger any of
these WARN_ON(); setup here is debian/testing on 4-way KVM guest with 4Gb
memory given to it running on a 6-way host (Phenom II X6 1100T, 3.3GHz, 16Gb
RAM total); 4.2 with debian/stable userland on host.  What's the setup on
your reproducer?