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SubjectRe: usercopy whitelist woe in scsi_sense_cache
Hi.

09.04.2018 22:30, Kees Cook wrote:
>> echo 1 | tee /sys/block/sd*/queue/nr_requests
>
> I can't get this below "4".

Oops, yeah. It cannot be less than BLKDEV_MIN_RQ (which is 4), so it is
enforced explicitly in queue_requests_store(). It is the same for me.

>> echo 1 | tee /sys/block/sd*/device/queue_depth
>
> I've got this now too.
> Ah! dm-crypt too. I'll see if I can get that added easily to my tests.
> And XFS! You love your corner cases. ;)

Yeah, so far this wonderful configuration has allowed me to uncover a
bunch of bugs, and see, we are not done yet ;).

> Two other questions, since you can reproduce this easily:
> - does it reproduce _without_ hardened usercopy? (I would assume yes,
> but you'd just not get any warning until the hangs started.) If it
> does reproduce without hardened usercopy, then a new bisect run could
> narrow the search even more.

Looks like it cannot be disabled via kernel cmdline, so I have to
re-compile the kernel, right? I can certainly do that anyway.

> - does it reproduce with Linus's current tree?

Will try this too.

> What would imply missing locking, yes? Yikes. But I'd expect
> use-after-free or something, or bad data, not having the pointer slip
> forward?

I still think this has something to do with blk-mq re-queuing capability
and how BFQ implements it, because there are no sings of issue popping
up with Kyber so far.

> Quick update: I added dm-crypt (with XFS on top) and it hung my system
> almost immediately. I got no warnings at all, though.

Did your system hang on smartctl hammering too? Have you got some stack
traces to compare with mine ones?

Regards,
Oleksandr

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