Hunting deep-seeded kernel bugs

http://www.anchor.com.au/blog/2012/03/bugfixing-the-in-kernel-megaraid_sas-driver-from-crash-to-patch/

by kezza • 14 years ago

75 16 14 years ago

anchor.com.au

caf• 14 years ago

It's "deep-seated" (although I don't see what was wrong with just using the original article title).

rquantz• 14 years ago

Ever since I learned the name for these kinds of mistakes, I can't help but call it out every time I see them: eggcorn!

http://eggcorns.lascribe.net/english/46/seed/

topbanana• 14 years ago

This term was coined by Adam and Joe. You can listen to their their podcast here: http://www.bbc.co.uk/podcasts/series/adamandjoe (and I recommend that you do)

jwildeboer• 14 years ago

They claim to use RHEL but instead of asking for support from Red Hat they decide to try themselves? Or am I missing something? Wouldn't such a patch be beneficial to other RHEL users too? (disclaimer: I work at Red Hat)

pmjordan3• 14 years ago

Well, they have pushed the fix upstream and it's been merged into mainline. I assume you guys have people monitoring the subsystems for backportable bugfixes?

I have no doubt that you guys have great support, but since the MegaCLI utility that triggers the issue is a closed source third-party piece of software, I could imagine the process of getting Red Hat engineers to even reproduce the problem would probably take longer than just going in and fixing it themselves. I can imagine having hosts with dozens of VMs going down is a high-priority issue - OK, less so once they discovered they could work around it by disabling monitoring, and eventually by upgrading the monitoring utility, but at that point they were already close to a fix. (also: they didn't say they didn't report it to RH...)

On a more technical note, if the MegaRAID driver only does DMAs to/from 32-bit memory addresses, this could be an artificial performance limitation on systems with lots of RAM (like the 128GiB in the article) as it potentially means waiting for <4GiB physical memory buffers to become available, and copying the data. It's not clear from the article if the 32-bit-only restriction only applies to STP requests, or all I/O. As the STP stuff only seems to be used for monitoring, that's not a problem, but if all disk I/O uses bounce buffers, that's a bad thing.

I wonder if a hardware IOMMU could have caught this bug.

justincormack• 14 years ago

Lots of poor hardware has limitations like this. It does hurt performance.

kezza• 14 years ago

For what it is worth, It was reported to RedHat, but our experiences with them have proved to be fairly.. hmm.. lackluster.

We've talked about it before http://www.anchor.com.au/blog/2009/02/the-value-of-commercia...

fidddle• 14 years ago

Your comment seems strangely defensive and dismissive. That they're an RHEL customer doesn't make them incapable of being proficient systems engineers, nor of participating in the wider eco-system themselves (and being capable of doing so doesn't devalue their RHEL investment either).

munin2• 14 years ago

> MegaCli is still the ultimate cause

strongly disagree! no kernel component should introduce memory corruption, and especially not in response to an ioctl from any user mode code. they say:

> Specifically, various offsets provided from userspace are used by the driver without any checks. If these offsets are maliciously chosen, the driver can be induced to write to arbitrary kernel memory

sure hope they patched that too!

pmjordan3• 14 years ago

sure hope they patched that too!

The patch appears to discard the DMA flags coming from user space, and instead applies the ones that are correct given the kernel's knowledge of the buffer.

But yes, this is ultimately a kernel bug - specifically one of the driver's userspace API.

rwmj• 14 years ago

Bugs happen though.

And these sort are old. I wrote a super [root] exploit for OS-9 which worked by tricking the GETSTAT call [equivalent of ioctl] for some driver to write its result to a random, unchecked place in memory. That was in 1992 / OS-9 2.4.

munin2• 14 years ago

bugs DO happen.

the root cause is in the kernel component that deals with memory inappropriately though. user space should be free to blast whatever into kernel space via system calls and ioctl and rely on the operating system to not crash, and also for the kernel to continue enforcing the rules of the operating system.

this is like saying "the root cause is Dan, who typed in a `drop tables. our solution is to take away Dans keyboard"

jilebedev• 14 years ago

>MegaCli specifies the address in one format, but sets a flag indicating that it’s in another

Far as I can discern (I'm not a kernel hacker) that is the root cause. Can anyone with static code analysis experience chime in on whether it's possible to detect that kind of error by examining the source?

spydum• 14 years ago

Enjoyed the read; hats off to you guys for digging in and finding/resolving and pushing the bugfix upstream.

I do have to wonder: did this bug all of a sudden spring up on you? Seems like the potential would have always been there on this platform. Could a change management process have lead you to find that "adding megaCLI monitoring to all KVM hosts" or "upgrading to megacli 8.01.06" was the cause of the problem? Just curious if you perceive a better CM or system inventory could have allowed you to deduce the problem area with a bit more efficiency?

Hunting deep-seeded kernel bugs

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