linux-kernel - Re: 2.6.31-rc2: BUG: unable to handle kernel NULL pointer dereference

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Date:	Sun, 12 Jul 2009 17:56:10 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Parag Warudkar <parag.warudkar@...il.com>,
	linux-kernel@...r.kernel.org, thomas@...3r.de, sds@...ho.nsa.gov,
	jmorris@...ei.org, eparis@...isplace.org
Subject: Re: 2.6.31-rc2: BUG: unable to handle kernel NULL pointer
 dereference

On Sun, 2009-07-12 at 22:26 +0200, Jiri Slaby wrote:
> On 07/12/2009 07:30 PM, Parag Warudkar wrote:
> > static void selinux_write_opts(struct seq_file *m,
> > 1012                                struct security_mnt_opts *opts)
> > 1013 {
> > 1014         int i;
> > 1015         char *prefix;
> > 1016
> > 1017         for (i = 0; i < opts->num_mnt_opts; i++) {
> > 1018                 char *has_comma;
> > 1019
> > 1020                 if (opts->mnt_opts[i])
> > 1021                         has_comma = strchr(opts->mnt_opts[i], ',');
> >                                          ^^^^^^^^^^^^^^^^^^^^^^^^^
> > And that is a NULL pointer dereference - but we just checked for
> > opts->mnt_opts[i] for not NULL. 
> 
> Note, that there is not a NULL dereference. It dereferences 0x40 which
> came in as %rdi. Looks like somebody assigned garbage in there.
> 
> Or a single bit mem error. Is memtest OK with this machine?
> 
> What warning tainted the kernel before this oops is still interesting...

I just looked over the selinux code where we build the
security_mnt_opts. We can do a 0 length kmalloc, but that should hurt
aything.  I should probably not be doing any allocations and leaving the
opts->mnt_opts and opts->mnt_opts_flags == NULL, but 0x40 !=
ZERO_SIZE_PTR(0x10) nor is the security_mnt_opts structure anywhere near
large enough to hit an offset of 0x40.....

I really think I'd like to see any previous BUG/WARN messages you got
and like Jiri said, see if memtest86+ runs cleanly....

-Eric

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