[Qemu-devel] [Bug 1781280] [NEW] QEMU ignores all but the first control message sent over a Unix socket

[Jonas Schievink]

Public bug reported:

I've written a test program that sends both an SCM_CREDENTIALS and an
SCM_RIGHTS cmsg in the same sendmsg call. On native x86-64, armv6 and
armv7 Linux, this works as expected (the recvmsg receives both control
messages). On QEMU (both qemu-x86_64 and qemu-arm), only the first
message is received.

I've traced the problem back to a glibc bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=13500

This means that writing control messages into an uninitialized buffer
makes CMSG_NXTHDR erroneously return NULL even though there's still
space inside the allocated buffer. QEMU's logic inside
`target_to_host_cmsg` is a bit questionable here, since it stops
encoding cmsgs as soon as *either* the host or the target buffer reaches
its end, so we lose the target's cmsgs when the host's buffer runs out.
However, the host buffer should *never* reach its end before the target
buffer does, so an assertion might be more useful there. Anyway, the
actual fix for this bug is simply zeroing out the buffer created for the
host. I've attached a patch doing that and verified that it fixes the
issue.

The test program I used can be found here: https://gist.github.com
/jonas-schievink/cb6e6584a055539d2113f22d91068e2d

** Affects: qemu
     Importance: Undecided
         Status: New

** Patch added: "0001-Zero-out-the-host-s-msg_control-buffer.patch"
   
https://bugs.launchpad.net/bugs/1781280/+attachment/5162557/+files/0001-Zero-out-the-host-s-msg_control-buffer.patch

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https://bugs.launchpad.net/bugs/1781280

Title:
  QEMU ignores all but the first control message sent over a Unix socket

Status in QEMU:
  New

Bug description:
  I've written a test program that sends both an SCM_CREDENTIALS and an
  SCM_RIGHTS cmsg in the same sendmsg call. On native x86-64, armv6 and
  armv7 Linux, this works as expected (the recvmsg receives both control
  messages). On QEMU (both qemu-x86_64 and qemu-arm), only the first
  message is received.

  I've traced the problem back to a glibc bug:
  https://sourceware.org/bugzilla/show_bug.cgi?id=13500

  This means that writing control messages into an uninitialized buffer
  makes CMSG_NXTHDR erroneously return NULL even though there's still
  space inside the allocated buffer. QEMU's logic inside
  `target_to_host_cmsg` is a bit questionable here, since it stops
  encoding cmsgs as soon as *either* the host or the target buffer
  reaches its end, so we lose the target's cmsgs when the host's buffer
  runs out. However, the host buffer should *never* reach its end before
  the target buffer does, so an assertion might be more useful there.
  Anyway, the actual fix for this bug is simply zeroing out the buffer
  created for the host. I've attached a patch doing that and verified
  that it fixes the issue.

  The test program I used can be found here: https://gist.github.com
  /jonas-schievink/cb6e6584a055539d2113f22d91068e2d

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