Re: fs: NULL deref in atime_needs_update

From: Al Viro
Date: Fri Feb 26 2016 - 17:08:10 EST

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...