linux-kernel - Re: kernel BUG with 2.6.23-rc3-mm1: skb_over

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Date:	Fri, 24 Aug 2007 23:52:08 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: kernel BUG with 2.6.23-rc3-mm1: skb_over_panic

* Kay Sievers (kay.sievers@...y.org) wrote:
> On Fri, 2007-08-24 at 23:02 -0400, Mathieu Desnoyers wrote:
> > * Greg KH (greg@...ah.com) wrote:
> > > On Fri, Aug 24, 2007 at 05:44:50PM -0700, Andrew Morton wrote:
> > > > On Fri, 24 Aug 2007 20:16:38 -0400
> > > > Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > > > 
> > > > > * Andrew Morton (akpm@...ux-foundation.org) wrote:
> > > > > > On Fri, 24 Aug 2007 18:47:07 -0400
> > > > > > Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > > > > > 
> > > > > > > Hi Andrew,
> > > > > > > 
> > > > > > > I get the following BUG when booting 2.6.23-rc3-mm1 on i386. I wonder if
> > > > > > > you would have some ideas about what is causing this problem. I'll start
> > > > > > > bissecting it soon. I seems to be caused by an buggy skb_put call in
> > > > > > > kobject_uevent_env.
> 
> > > > > > hm, don't know, sorry.  Kay fixed a few things in there, but iirc pretty
> > > > > > much all of the fixes were in rc3-mm1 anyway.
> > > > > > 
> > > > > > I doubt if bisection will tell us a lot: it'll probably point at
> > > > > > gregkh-driver-driver-core-change-add_uevent_var-to-use-a-struct.patch.
> > > > > > 
> > > > > > What we _would_ like to know is which sysfs file is being written to.  We
> > > > > > used to have a debug patch to exactly address this problem but it got
> > > > > > transferred into Greg's tree from whence it mysteriously disappeared.
> > > > > > 
> > > > > 
> > > > > Ok, here it is:
> > > > > 
> > > > > filename :
> > > > > 
> > > > > /devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0/rev
> > > > 
> > > > Bah.  I've never found a sane way of going from a sysfs pathname back to the
> > > > code which implements that pathname :(
> > > > 
> > > > <greps the tree for '"rev"'>
> > > > 
> > > > <comes up with zilch>
> > > 
> > > It's a scsi file, as the above is a scsi device.  It's created in the
> > > drivers/scsi/scsi_sysfs.c file.
> 
> > I think I am slowly getting there.. it looks like an off-by-one in
> > lib/kobject_uevent.c: add_uevent_var
> > 
> > when testing the return value of vsnprintf
> > 
> > if (len + 1 >= (sizeof(env->buf) - env->buflen))
> > 
> > should be
> > 
> > if (len >= (sizeof(env->buf) - env->buflen))
> > 
> > And then the problem underneath is that the array is too short for some
> > values.
> 
> That should be changed, yes. But we should not reach the end of the
> buffer.
> 
> > Since the return value of add_uevent_var is always ignored (why?)
> 
> Because nobody added these checks, most of the callers check, some
> don't. We should fix that step by step, sure.
> 
> > from its callers, fixing the off-by-one will just fail silently, which is
> > almost worse.
> > 
> > I think we should find some better way of handling full static arrays.
> > 
> > And the bug is still there even if I fix these. So I'll continue my
> > investigation.
> 
> Yeah, before these changes it was the environment buffer which got
> corrupted, but seems nobody noticed it. Now it triggers BUG() if we run
> into problems.
> 
> This needs a fix. It may be the reason for the too small buffer, you are
> seeing.
> 
> Thanks,
> Kay
> 
> --- a/drivers/firmware/dmi-id.c
> +++ b/drivers/firmware/dmi-id.c
> @@ -155,6 +155,7 @@ static int dmi_dev_uevent(struct device *dev, struct kobj_uevent_env *env)
>  			   sizeof(env->buf) - env->buflen);
>  	if (len >= (sizeof(env->buf) - env->buflen))
>  		return -ENOMEM;
> +	env->buflen += len;
>  	return 0;
>  }
>  
> 

Yep, adding some printks, it points in that direction:

[  179.898618] DBG string ACTION=add
[  179.908565] put DBG 1 len 22 envlen 68
[  179.919807] DBG string DEVPATH=/class/dmi/id
[  179.921584] DBG req size 836
[  179.921588] DBG ali size 864
[  179.921632] DBG req size 836
[  179.921634] DBG ali size 864
[  179.966998] put DBG 2 len 14 envlen 68
[  179.978208] DBG string SUBSYSTEM=dmi
[  179.988901] put DBG 3 len 21 envlen 68
[  180.000109] DBG string MODALIAS=dSEQNUM=971

as we can see, MODALIAS=d is clearly wrong there and should have a \0.
This is why the skb_put goes over the end of the allocated skb.

[  180.012618] put DBG 4 len 11 envlen 68
[  180.023829] DBG string SEQNUM=971
[  180.033742] skb_over_panic: text:c0252ee8 len:97 put:11 head:c2afa200 data:c2afa200 tail:0xc2afa261 end:0xc2afa260 dev:<NULL>
[  180.067545] ------------[ cut here ]------------
[  180.081341] Kernel BUG at c039e1bc [verbose debug info unavailable]
[  180.100069] invalid opcode: 0000 [#1] PREEMPT SMP


-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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