4 ms·

Hunting deep-seeded kernel bugs

75 points by kezza 15y ago

caf 15y ago
It's "deep-seated" (although I don't see what was wrong with just using the original article title).
- rquantz 15y 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/ http://eggcorns.lascribe.net/english/46/seed/
  - topbanana 15y 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 http://www.bbc.co.uk/podcasts/series/adamandjoe (and I recommend that you do)
    - davidwtbuxton 15y ago
      As great as Adam and Joe are, the term pre-dates their podcasts. I always thought it was coined on the language log, http://itre.cis.upenn.edu/~myl/languagelog/archives/000018.html http://itre.cis.upenn.edu/~myl/languagelog/archives/000018.h...
jwildeboer 15y 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)
- pmjordan 15y 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 15y ago
    Lots of poor hardware has limitations like this. It does hurt performance.
    - pmjordan 15y ago
      I think it's a requirement of PCIe for devices to support DMA to memory with 64-bit physical addresses.[1] In any case, the hardware clearly supports it in the case - the bug was that a 32-bit I/O vector was being passed off to the hardware claiming to be a 64-bit one, and subsequently interpreted as such. The situation was certainly bad with classic PCI - I have an nForce5-based motherboard whose PCI bus silently truncates 64-bit DMA buffer addresses to 32 bits. The onboard ethernet chip does in principle support 64-bit DMA, so I was getting all sorts of weird behaviour after upgrading my RAM from 2 to 6 GiB. The southbridge chip in question is now blacklisted for 64-bit DMA in the Linux kernel. The 32-bit DMA cutoff isn't the only such constraint on buffer physical addresses of course; historically, ISA devices could only DMA to 20-bit addresses IIRC (or was it 24 bits?). I wrote an OS X driver for the virtio paravirtual ethernet adapter a while back[2] and its I/O buffers must be 4K page-aligned (low 12 bits 0), and its high bits are specified by a 32-bit number. This means you can only use buffers with physical addresses which fit in 44 bits. That's 16TiB, which are probably still a way off but could conceivably be reached not too far in the future. Current x86_64 CPUs certainly support 48 bit physical addresses in principle. [1] That doesn't mean hardware bugs don't exist, but at least it's part of the basic spec now, as opposed to being tagged on later as for PCI. [2] http://github.com/pmj/virtio-net-osx http://github.com/pmj/virtio-net-osx
munin 15y 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!
- pmjordan 15y 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 15y 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.
  - munin 15y 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 15y 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 15y 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?

75 points by kezza 15y ago

caf 15y ago

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

rquantz 15y 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/ http://eggcorns.lascribe.net/english/46/seed/
- topbanana 15y 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 http://www.bbc.co.uk/podcasts/series/adamandjoe (and I recommend that you do)
  - davidwtbuxton 15y ago
    As great as Adam and Joe are, the term pre-dates their podcasts. I always thought it was coined on the language log, http://itre.cis.upenn.edu/~myl/languagelog/archives/000018.html http://itre.cis.upenn.edu/~myl/languagelog/archives/000018.h...

jwildeboer 15y 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)

pmjordan 15y 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 15y ago
  Lots of poor hardware has limitations like this. It does hurt performance.
  - pmjordan 15y ago
    I think it's a requirement of PCIe for devices to support DMA to memory with 64-bit physical addresses.[1] In any case, the hardware clearly supports it in the case - the bug was that a 32-bit I/O vector was being passed off to the hardware claiming to be a 64-bit one, and subsequently interpreted as such. The situation was certainly bad with classic PCI - I have an nForce5-based motherboard whose PCI bus silently truncates 64-bit DMA buffer addresses to 32 bits. The onboard ethernet chip does in principle support 64-bit DMA, so I was getting all sorts of weird behaviour after upgrading my RAM from 2 to 6 GiB. The southbridge chip in question is now blacklisted for 64-bit DMA in the Linux kernel. The 32-bit DMA cutoff isn't the only such constraint on buffer physical addresses of course; historically, ISA devices could only DMA to 20-bit addresses IIRC (or was it 24 bits?). I wrote an OS X driver for the virtio paravirtual ethernet adapter a while back[2] and its I/O buffers must be 4K page-aligned (low 12 bits 0), and its high bits are specified by a 32-bit number. This means you can only use buffers with physical addresses which fit in 44 bits. That's 16TiB, which are probably still a way off but could conceivably be reached not too far in the future. Current x86_64 CPUs certainly support 48 bit physical addresses in principle. [1] That doesn't mean hardware bugs don't exist, but at least it's part of the basic spec now, as opposed to being tagged on later as for PCI. [2] http://github.com/pmj/virtio-net-osx http://github.com/pmj/virtio-net-osx

munin 15y 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!

pmjordan 15y 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 15y 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.
- munin 15y 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 15y 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 15y 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?