[Bug 1749393] Re: sbrk() not working under qemu-user with a PIE-compiled binary?

[Christian Ehrhardt ]

FYI the release of this is slowed down by the slow verification of bug
https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1929926

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https://bugs.launchpad.net/bugs/1749393

Title:
  sbrk() not working under qemu-user with a PIE-compiled binary?

Status in QEMU:
  Fix Released
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Focal:
  Fix Committed

Bug description:
  [Impact]

   * The current space reserved can be too small and we can end up
     with no space at all for BRK. It can happen to any case, but is
     much more likely with the now common PIE binaries.

   * Backport the upstream fix which reserves a bit more space while loading
     and giving it back after interpreter and stack is loaded.

  [Test Plan]

   * On x86 run:
  sudo apt install -y qemu-user-static docker.io
  sudo docker run --rm arm64v8/debian:bullseye bash -c 'apt update && apt 
install -y wget'
  ...
  Running hooks in /etc/ca-certificates/update.d...
  done.
  Errors were encountered while processing:
   libc-bin
  E: Sub-process /usr/bin/dpkg returned an error code (1)

  
  Second test from bug 1928075

  $ sudo qemu-debootstrap --arch=arm64 bullseye bullseye-arm64
  http://ftp.debian.org/debian

  In the bad case this is failing like
  W: Failure trying to run: /sbin/ldconfig
  W: See //debootstrap/debootstrap.log for detail

  And in that log file you'll see the segfault
  $ tail -n 2 bullseye-arm64/debootstrap/debootstrap.log
  qemu: uncaught target signal 11 (Segmentation fault) - core dumped
  Segmentation fault (core dumped)

  [Where problems could occur]

   * Regressions would be around use-cases of linux-user that is
     emulation not of a system but of binaries.
     Commonly uses for cross-tests and cross-builds so that is the
     space to watch for regressions

  [Other Info]

   * n/a

  ---

  In Debian unstable, we recently switched bash to be a PIE-compiled
  binary (for hardening). Unfortunately this resulted in bash being
  broken when run under qemu-user (for all target architectures, host
  being amd64 for me).

  $ sudo chroot /srv/chroots/sid-i386/ qemu-i386-static /bin/bash
  bash: xmalloc: .././shell.c:1709: cannot allocate 10 bytes (0 bytes allocated)

  bash has its own malloc implementation based on sbrk():
  https://git.savannah.gnu.org/cgit/bash.git/tree/lib/malloc/malloc.c

  When we disable this internal implementation and rely on glibc's
  malloc, then everything is fine. But it might be that glibc has a
  fallback when sbrk() is not working properly and it might hide the
  underlying problem in qemu-user.

  This issue has also been reported to the bash upstream author and he 
suggested that the issue might be in qemu-user so I'm opening a ticket here. 
Here's the discussion with the bash upstream author:
  https://lists.gnu.org/archive/html/bug-bash/2018-02/threads.html#00080

  You can find the problematic bash binary in that .deb file:
  
http://snapshot.debian.org/archive/debian/20180206T154716Z/pool/main/b/bash/bash_4.4.18-1_i386.deb

  The version of qemu I have been using is 2.11 (Debian package qemu-
  user-static version 1:2.11+dfsg-1) but I have had reports that the
  problem is reproducible with older versions (back to 2.8 at least).

  Here are the related Debian bug reports:
  https://bugs.debian.org/889869
  https://bugs.debian.org/865599

  It's worth noting that bash used to have this problem (when compiled as a PIE 
binary) even when run directly but then something got fixed in the kernel and 
now the problem only appears when run under qemu-user:
  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1518483

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