Re: [PATCH] target/s390x: fix s390_probe_access to check PAGE_WRITE_ORG for writeability

[Thomas Huth]

On 23/04/2021 15.06, Peter Maydell wrote:
> On Fri, 23 Apr 2021 at 13:22, Cornelia Huck <cohuck@redhat.com> wrote:
> > On Thu, 22 Apr 2021 16:44:27 +0100
> > Alex Bennée <alex.bennee@linaro.org> wrote:
> >
> > > We can remove PAGE_WRITE when (internally) marking a page read-only
> > > because it contains translated code. This can get confused when we are
> > > executing signal return code on signal stacks.
> > >
> > > Fixes: e56552cf07 ("target/s390x: Implement the MVPG condition-code-option bit")
> > > Found-by: Richard Henderson <richard.henderson@linaro.org>
> > > Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> > > Cc: Cornelia Huck <cohuck@redhat.com>
> > > Cc: Thomas Huth <thuth@redhat.com>
> > > Cc: David Hildenbrand <david@redhat.com>
> > > Cc: Laurent Vivier <laurent@vivier.eu>
> > > ---
> > >   target/s390x/mem_helper.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/target/s390x/mem_helper.c b/target/s390x/mem_helper.c
> > > index 12e84a4285..f6a7d29273 100644
> > > --- a/target/s390x/mem_helper.c
> > > +++ b/target/s390x/mem_helper.c
> > > @@ -145,7 +145,7 @@ static int s390_probe_access(CPUArchState *env, 
> > > target_ulong addr, int size,
> > >
> > >   #if defined(CONFIG_USER_ONLY)
> > >       flags = page_get_flags(addr);
> > > -    if (!(flags & (access_type == MMU_DATA_LOAD ?  PAGE_READ : PAGE_WRITE))) {
> > > +    if (!(flags & (access_type == MMU_DATA_LOAD ?  PAGE_READ : 
> > > PAGE_WRITE_ORG))) {
> > >           env->__excp_addr = addr;
> > >           flags = (flags & PAGE_VALID) ? PGM_PROTECTION : PGM_ADDRESSING;
> > >           if (nonfault) {
> >
> > What's the verdict on this one? I plan to queue this to s390-next; but
> > if we end up doing an -rc5, it might qualify as a regression fix.
>
> What's your opinion? I think we do need an rc5 for the network backend
> hotplug crash. I don't want to open the doors for lots of new fixes
> just because we've got another rc, but on the other hand this one
> does look like it's a pretty small and safe fix, and letting intermittent
> crash bugs out into the wild seems like it could lead to a lot of
> annoying re-investigation of the same bug if it's reported by users
> later... So I kind of lean towards putting it in rc5.

IMHO: It's in a s390x-only file, within a #ifdef CONFIG_USER_ONLY ... so the 
damage this could do is very, very limited, indeed. Thus I'd also suggest to 
include it in a rc5.

 Thomas