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

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 23 Apr 2021 15:28:19 +0200
Thomas Huth <thuth@redhat.com> wrote:

> 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.

Exactly, the benefits outweigh the risk IMHO.

Peter, do you want to pick this one directly, or should I send you a pull req?