Re: [PATCH 1/2] memfd: Fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

[Andrew Morton]

On Thu, 7 Nov 2019 21:06:14 -0500 Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:

> On Thu, Nov 07, 2019 at 05:00:23PM -0800, Andrew Morton wrote:
> > On Thu,  7 Nov 2019 14:53:54 -0500 "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> wrote:
> > 
> > > F_SEAL_FUTURE_WRITE has unexpected behavior when used with MAP_PRIVATE:
> > > A private mapping created after the memfd file that gets sealed with
> > > F_SEAL_FUTURE_WRITE loses the copy-on-write at fork behavior, meaning
> > > children and parent share the same memory, even though the mapping is
> > > private.
> > 
> > That sounds fairly serious.  Should this be backported into -stable kernels?
> 
> Yes, it should be.

I added

Fixes: ab3948f58ff84 ("mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd")
Cc: <stable@xxxxxxxxxxxxxxx>

> The F_SEAL_FUTURE_WRITE feature was introduced in v5.1 so
> v5.3.x stable kernels would need a backport. I can submit a backport tomorrow
> unless we are Ok with stable automatically picking it up (I believe the
> stable folks "auto select" fixes which should detect this is a fix since I
> have said it is a fix in the subject line).

The Cc:stable tag should trigger the appropriate actions, assisted by
the Fixes:.  I doubt if "fix" in the Subject has much effect.