[Bug 1883739] Re: ide_dma_cb: Assertion `prep_size >= 0 && prep_size <= n * 512' failed.

[John Snow]

ACK. I do not have time to fix this bug at the moment under the belief
that it's likely low-priority and only "misbehaving guests" can trigger
it. Some advice:

1. Do not use IDE in production deployments after initial installation,
if you can help it. Use a performant virtio solution.

2. If anyone would like to fix this problem, I will be more than happy
to point you to the exact lines of code that cause the problem. I think
the fix will be easy, but testing will be time-consuming as it involves
understanding error behavior of real hardware that I don't personally
have the setup to quickly test or verify.

>From memory: the problem is that ide_dma_cb expects that it was able to
prepare at least one sector's worth of scatter-gather list to begin DMA,
but it's possible to give malformed SG lists where IDE is unable to
process the remainder of a sector in a list.

It was not clear to me at the time when I first investigated this what a
real controller would do if it got an incomplete sector and how it
should signal that.

It was also not clear to me if the sg_prepare function for the pci bmdma
controller would ever encounter a situation where further entries in the
list might be received "later" and we should "wait" for them.

If this bug is more dangerous than a self-inflicted DOS, please let me
know and I'll re-prioritize. Patches, email and IRC chats welcome.

--js

** Changed in: qemu
       Status: New => Confirmed

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1883739

Title:
  ide_dma_cb: Assertion `prep_size >= 0 && prep_size <= n * 512' failed.

Status in QEMU:
  Confirmed

Bug description:
  To reproduce run the QEMU with the following command line:
  ```
  qemu-system-x86_64 -cdrom hypertrash.iso -nographic -m 100 -enable-kvm -net 
none -drive id=disk,file=hda.img,if=none -device ahci,id=ahci -device 
ide-hd,drive=disk,bus=ahci.0
  ```

  QEMU Version:
  ```
  # qemu-5.0.0
  $ ./configure --target-list=x86_64-softmmu --enable-sanitizers; make
  $ x86_64-softmmu/qemu-system-x86_64 --version
  QEMU emulator version 5.0.0
  Copyright (c) 2003-2020 Fabrice Bellard and the QEMU Project developers
  ```

  To create disk image run:
  ```
  dd if=/dev/zero of=hda.img bs=1024 count=1024
  ```

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1883739/+subscriptions