Re: NULL deref around blkmq in v4.0-rc1–rc7

From: Jens Axboe
Date: Thu Apr 09 2015 - 17:25:20 EST

On 04/09/2015 03:12 PM, Linus Torvalds wrote:
On Thu, Apr 9, 2015 at 11:24 AM, Jan Engelhardt <jengelh@xxxxxxx> wrote:

It's fairly consistent (reproducible?). Only 1 in 15 or so (have not kept track
really) attempts does it not die.

With frame pointers:
[<ffffffff81286d59>] scsi_queue_rq+0x2e8/0x3d2
[<ffffffff8119e64d>] __blk_mq_run_hw_queue+0x19b/0x2a2
[<ffffffff8119e901>] ? blk_mq_merge_queue_io+0x75/0x147
[<ffffffffa00fa34a>] ? __xfs_get_blocks+0x2f9/0x2f9 [xfs]
[<ffffffff8119edeb>] blk_mq_run_hw_queue+0x4f/0x99
[<ffffffff8119fab9>] blk_sq_make_request+0x163/0x170

Ok, good.

So the cmd comes from

struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);

which in turn is just

return (void *) rq + sizeof(*rq);

which in turn is written by some crazy monkey on crack. That's some
shit code. Why the hell you'd write it that way, when the natural
thing to do would be just

return rq + 1;

without the sizeof, and without the cast.

The particular crazy monkey on crack is Jens Axboe, in commit 320ae51feed5c.

Jens, really. This code is shit.

That's a bit rough on a single line of code like that, don't you think? But yes, rq + 1 is identical and cleaner...

That ->sense_buffer thing is supposed to be initialized by the
blk_mq_ops.init_request() function, which is called - if it exists =
when the array of requests ('->rqs[]') is initialized.

And that code too looks like crap. It seems to be very clever, trying
to allocaet big contiguous chunks of RAM for the requests, but then
the initialization sequence is questionable as hell. It takes that
nonzeroed allocation, and zeroes a few fields randomly. The rest will
contain whatever garbage data they used to.

Does this entirely untested patch make any difference?

And Jens, this all really looks very fishy. When I look at these kinds
of core functions, and find just *stupid* code like this, it makes me

Not sure why it isn't all zeroed, definitely the saner thing to do at init time. So patch looks fine, should be applied regardless of whether or not it fixes this issue.

But it's _always_ been like that, so it's not a change in behavior. It is fragile, so perhaps some SCSI change modified alloc behavior and it's not causing and issue.

And if this is mpt, we recently ran into some list corruption issues due to a bug in the driver. It hit on reboot, but it was scan related, so could be a boot issue as well.

Jens Axboe

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