Re: [PATCH v2] target/s390x: Fix translation exception on illegal instruction

[Ilya Leoshkevich]

On Wed, 2021-04-14 at 10:38 +0200, Cornelia Huck wrote:
> On Tue, 13 Apr 2021 18:52:57 +0200
> Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> 
> > Hitting an uretprobe in a s390x TCG guest causes a SIGSEGV. What
> > happens is:
> > 
> > * uretprobe maps a userspace page containing an invalid
> > instruction.
> > * uretprobe replaces the target function's return address with the
> >   address of that page.
> > * When tb_gen_code() is called on that page, tb->size ends up being
> > 0
> >   (because the page starts with the invalid instruction), which
> > causes
> >   virt_page2 to point to the previous page.
> > * The previous page is not mapped, so this causes a spurious
> >   translation exception.
> > 
> > The bug is that tb->size must never be 0: even if there is an
> > illegal
> > instruction, the instruction bytes that have been looked at must
> > count
> > towards tb->size. So adjust s390x's translate_one() to act this way
> > for both illegal instructions and instructions that are known to
> > generate exceptions.
> > 
> > Also add an assertion to tb_gen_code() in order to detect such
> > situations in future.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> > 
> > v1: 
> > https://lists.nongnu.org/archive/html/qemu-devel/2021-04/msg02037.html
> > v1 -> v2: Fix target/s390x instead of trying to tolerate tb->size
> > == 0
> >           in tb_gen_code().
> > 
> >  accel/tcg/translate-all.c |  1 +
> >  target/s390x/translate.c  | 16 +++++++++++-----
> >  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> I assume this bug is not usually hit during normal usage, right? It's
> probably not release critical, so I'll line it up for 6.1 instead.

Yes, I saw it only with uprobes, and then it leads only to a process
crash, not to a kernel crash. Thanks!