Re: [Qemu-devel] [PULL 12/13] page_unprotect(): handle calls to pages that are PAGE_WRITE

[Laurent Vivier]

Le 23/01/2018 à 15:48, Laurent Vivier a écrit :
> From: Peter Maydell <address@hidden>
> 
> If multiple guest threads in user-mode emulation write to a
> page which QEMU has marked read-only because of cached TCG
> translations, the threads can race in page_unprotect:
> 
>  * threads A & B both try to do a write to a page with code in it at
>    the same time (ie which we've made non-writeable, so SEGV)
>  * they race into the signal handler with this faulting address
>  * thread A happens to get to page_unprotect() first and takes the
>    mmap lock, so thread B sits waiting for it to be done
>  * A then finds the page, marks it PAGE_WRITE and mprotect()s it writable
>  * A can then continue OK (returns from signal handler to retry the
>    memory access)
>  * ...but when B gets the mmap lock it finds that the page is already
>    PAGE_WRITE, and so it exits page_unprotect() via the "not due to
>    protected translation" code path, and wrongly delivers the signal
>    to the guest rather than just retrying the access
> 
> In particular, this meant that trying to run 'javac' in user-mode
> emulation would fail with a spurious guest SIGSEGV.
> 
> Handle this by making page_unprotect() assume that a call for a page
> which is already PAGE_WRITE is due to a race of this sort and return
> a "fault handled" indication.
> 
> Since this would cause an infinite loop if we ever called
> page_unprotect() for some other kind of fault than "write failed due
> to bad access permissions", tighten the condition in
> handle_cpu_signal() to check the signal number and si_code, and add a
> comment so that if somebody does ever find themselves debugging an
> infinite loop of faults they have some clue about why.
> 
> (The trick for identifying the correct setting for
> current_tb_invalidated for thread B (needed to handle the precise-SMC
> case) is due to Richard Henderson.  Paolo Bonzini suggested just
> relying on si_code rather than trying anything more complicated.)
> 
> Signed-off-by: Peter Maydell <address@hidden>
> Message-Id: <address@hidden>
> Signed-off-by: Laurent Vivier <address@hidden>
> ---
>  accel/tcg/translate-all.c | 50 
> +++++++++++++++++++++++++++++------------------
>  accel/tcg/user-exec.c     | 13 +++++++++++-
>  2 files changed, 43 insertions(+), 20 deletions(-)
> 

It seems this patch breaks something in linux-user mode emulation for
m68k (32bit BE) on ppc (32bit BE).

What I have:

  ~/chroot$ sudo QEMU_CPU=m68040 chroot m68k/sid/
  I have no address@hidden:/# ls
  bin   debootstrap  etc         lib   qemu-m68k  run   sys  usr
  boot  dev        home  proc  root       sbin  tmp  var
  qemu: uncaught target signal 11 (Segmentation fault) - core dumped
  ~/chroot$

It seems "bash" crashes on "ls" exit.

My chroot has been installed with:

  ARCH=m68k
  TARGET=sid
  CHROOT=$HOME/chroot/m68k/sid/
  REPOT=http://cdn-fastly.deb.debian.org/debian-ports/
  debootstrap --arch=$ARCH --foreign --variant=minbase \
              --no-check-gpg $TARGET $CHROOT $REPO

I didn't investigate more.

Thanks,
Laurent