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

[Cornelia Huck]

On Wed, 14 Apr 2021 12:27:03 +0200
Ilya Leoshkevich <iii@linux.ibm.com> wrote:

> On Wed, 2021-04-14 at 11:19 +0200, Ilya Leoshkevich wrote:
> > 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!  
> 
> Seems like the new assertion triggers on ARM:
> 
> https://gitlab.com/cohuck/qemu/-/jobs/1178409450

Yep, I just wanted to make sure it was this patch before complaining :)

> 
> What are the rules in s390x-next-staging, can we amend the patch, or
> only commit a follow-up? 

-staging is before I merge properly, so no problem folding something in.

> In either case, I think we'll need something
> like this (untested):
> 
> --- a/target/arm/translate.c
> +++ b/target/arm/translate.c
> @@ -9060,6 +9060,7 @@ static void
> arm_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
>      unsigned int insn;
>  
>      if (arm_pre_translate_insn(dc)) {
> +        dc->base.pc_next += 4;
>          return;
>      }
> 
> 
> I'm currently trying to debug this in more detail and test the fix.
>