Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 18 Apr 2023 18:36:43 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Waiman Long <longman@redhat.com>
cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Joe Mario <jmario@redhat.com>,
        Barry Marson <bmarson@redhat.com>,
        Rafael Aquini <aquini@redhat.com>
Subject: Re: [PATCH] mm/mmap: Map MAP_STACK to VM_STACK
In-Reply-To: <6c3c68b1-c4d4-dd82-58e8-f7013fb6c8e5@redhat.com>
Message-ID: <cffc7454-614-1939-f235-7b139dc46b41@google.com>
References: <20230418210230.3495922-1-longman@redhat.com> <20230418141852.75e551e57e97f4b522957c5c@linux-foundation.org> <6c3c68b1-c4d4-dd82-58e8-f7013fb6c8e5@redhat.com>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="-1463760895-397833526-1681868216=:9821"
Precedence: bulk

  This message is in MIME format.  The first part should be readable text,
  while the remaining parts are likely unreadable without MIME-aware tools.

---1463760895-397833526-1681868216=:9821
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: QUOTED-PRINTABLE

On Tue, 18 Apr 2023, Waiman Long wrote:
> On 4/18/23 17:18, Andrew Morton wrote:
> > On Tue, 18 Apr 2023 17:02:30 -0400 Waiman Long <longman@redhat.com> wro=
te:
> >
> >> One of the flags of mmap(2) is MAP_STACK to request a memory segment
> >> suitable for a process or thread stack. The kernel currently ignores
> >> this flags. Glibc uses MAP_STACK when mmapping a thread stack. However=
,
> >> selinux has an execstack check in selinux_file_mprotect() which disall=
ows
> >> a stack VMA to be made executable.
> >>
> >> Since MAP_STACK is a noop, it is possible for a stack VMA to be merged
> >> with an adjacent anonymous VMA. With that merging, using mprotect(2)
> >> to change a part of the merged anonymous VMA to make it executable may
> >> fail. This can lead to sporadic failure of applications that need to
> >> make those changes.
> > "Sporadic failure of applications" sounds quite serious.  Can you
> > provide more details?
>=20
> The problem boils down to the fact that it is possible for user code to m=
map a
> region of memory and then for the kernel to merge the VMA for that memory=
 with
> the VMA for one of the application's thread stacks. This is causing rando=
m
> SEGVs with one of our large customer application.
>=20
> At a high level, this is what's happening:
>=20
> =C2=A01) App runs creating lots of threads.
> =C2=A02) It mmap's 256K pages of anonymous memory.
> =C2=A03) It writes executable code to that memory.
> =C2=A04) It calls mprotect() with PROT_EXEC on that memory so
> =C2=A0=C2=A0=C2=A0 it can subsequently execute the code.
>=20
> The above mprotect() will fail if the mmap'd region's VMA gets merged wit=
h the
> VMA for one of the thread stacks.=C2=A0 That's because the default RHEL S=
ELinux
> policy is to not allow executable stacks.

Then wouldn't the bug be at the SELinux end?  VMAs may have been merged
already, but the mprotect() with PROT_EXEC of the good non-stack range
will then split that area off from the stack again - maybe the SELinux
check does not understand that must happen?

Hugh
---1463760895-397833526-1681868216=:9821--