Re: linux-next: Tree for Jan 20 -- Kernel panic - Unable to mount root fs

[Guenter Roeck]

On 01/21/2015 07:54 AM, Sabrina Dubroca wrote:
> 2015-01-21, 16:39:12 +0100, Thierry Reding wrote:
> > On Wed, Jan 21, 2015 at 10:24:11AM -0500, Paul Moore wrote:
> > > On Wednesday, January 21, 2015 03:42:16 PM Thierry Reding wrote:
> > > > On Wed, Jan 21, 2015 at 12:05:39PM +0100, Sabrina Dubroca wrote:
> > > > > 2015-01-21, 04:36:38 +0000, Al Viro wrote:
> > > > > > On Tue, Jan 20, 2015 at 08:01:26PM -0800, Guenter Roeck wrote:
> > > > > > > With this patch:
> > > > > > >
> > > > > > > sys_mkdir .:40775 returned -17
> > > > > > > sys_mkdir usr:40775 returned 0
> > > > > > > sys_mkdir usr/lib:40775 returned 0
> > > > > > > sys_mkdir usr/share:40755 returned 0
> > > > > > > sys_mkdir usr/share/udhcpc:40755 returned 0
> > > > > > > sys_mkdir usr/bin:40775 returned 0
> > > > > > > sys_mkdir usr/sbin:40775 returned 0
> > > > > > > sys_mkdir mnt:40775 returned 0
> > > > > > > sys_mkdir proc:40775 returned 0
> > > > > > > sys_mkdir root:40775 returned 0
> > > > > > > sys_mkdir lib:40775 returned 0
> > > > > > > sys_mkdir lib/modules:40775 returned 0
> > > > > > > ...
> > > > > > >
> > > > > > > and the problem is fixed.
> > > > >
> > > > > This patch also works for me.
> > > > >
> > > > > > ... except that it simply confirms that something's fishy with
> > > > > > getname_kernel() of ->name of struct filename returned by getname().
> > > > > > IOW, I still do not understand the mechanism of breakage there.
> > > > >
> > > > > I'm not so sure about that.  I tried to copy name to a new string in
> > > > > do_path_lookup and that didn't help.
> > > > >
> > > > > Now, I've removed the
> > > > >
> > > > >          putname(filename);
> > > > >
> > > > > line from do_path_lookup and I don't get the panic.
> > > >
> > > > That would indicate that somehow the refcount got unbalanced. Looking
> > > > more closely it seems like the various audit_*() function do take a
> > > > reference, but maybe that's not enough.
> > >
> > > I'm thinking the same thing and I think the problem may be that
> > > __audit_reusename() is not bumping the filename->refcnt.  Can someone who is
> > > seeing this problem bump the refcnt in __audit_reusename()?
> > >
> > >    struct filename *
> > >    __audit_reusename(const __user char *uptr)
> > >    {
> > >          struct audit_context *context = current->audit_context;
> > >          struct audit_names *n;
> > >
> > >          list_for_each_entry(n, &context->names_list, list) {
> > >                  if (!n->name)
> > >                          continue;
> > >                  if (n->name->uptr == uptr) {
> > > +                       n->name->refcnt++;
> > >                          return n->name;
> > >                  }
> > >          }
> > >          return NULL;
> > >    }
> >
> > That doesn't seem to help, at least in my case.
>
> Same here.
>
> Well, it's probably not an audit issue.  I tried audit=0 on the
> commandline, and I just rebuilt a kernel with CONFIG_AUDIT=n, and it's
> still panicing.  This should have fixed any audit-related issue,
> right?
I don't have audit enabled, so I don't think that is the problem either
(the refcount increase didn't help, and a WARN(1) added to the code
at the same location did not trigger).

Wonder if we have a use-after-free case and just have been lucky all along.

Guenter

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