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

From: Thomas Huth
Subject: Re: [PATCH] target/s390x: fix s390_probe_access to check PAGE_WRITE_ORG for writeability
Date: Fri, 23 Apr 2021 15:28:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1

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 : 
          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.


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