linux-kernel - Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW

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Message-Id: <20240522162324.0aeba086228eddd8aff4f628@linux-foundation.org>
Date: Wed, 22 May 2024 16:23:24 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jeff Xu <jeffxu@...gle.com>
Cc: Barnabás Pőcze <pobrn@...tonmail.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, dmitry.torokhov@...il.com,
 dverkamp@...omium.org, hughd@...gle.com, jorgelo@...omium.org,
 skhan@...uxfoundation.org, keescook@...omium.org
Subject: Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply
 `MFD_ALLOW_SEALING`

On Wed, 15 May 2024 23:11:12 -0700 Jeff Xu <jeffxu@...gle.com> wrote:

> On Mon, May 13, 2024 at 12:15 PM Barnabás Pőcze <pobrn@...tonmail.com> wrote:
> >
> > `MFD_NOEXEC_SEAL` should remove the executable bits and set
> > `F_SEAL_EXEC` to prevent further modifications to the executable
> > bits as per the comment in the uapi header file:
> >
> >   not executable and sealed to prevent changing to executable
> >
> > However, currently, it also unsets `F_SEAL_SEAL`, essentially
> > acting as a superset of `MFD_ALLOW_SEALING`. Nothing implies
> > that it should be so, and indeed up until the second version
> > of the of the patchset[0] that introduced `MFD_EXEC` and
> > `MFD_NOEXEC_SEAL`, `F_SEAL_SEAL` was not removed, however it
> > was changed in the third revision of the patchset[1] without
> > a clear explanation.
> >
> > This behaviour is suprising for application developers,
> > there is no documentation that would reveal that `MFD_NOEXEC_SEAL`
> > has the additional effect of `MFD_ALLOW_SEALING`.
> >
> Ya, I agree that there should be documentation, such as a man page. I will
> work on that.
> 
> > So do not remove `F_SEAL_SEAL` when `MFD_NOEXEC_SEAL` is requested.
> > This is technically an ABI break, but it seems very unlikely that an
> > application would depend on this behaviour (unless by accident).
> >
> > [0]: https://lore.kernel.org/lkml/20220805222126.142525-3-jeffxu@google.com/
> > [1]: https://lore.kernel.org/lkml/20221202013404.163143-3-jeffxu@google.com/
> 
> ...
>
> Reviewed-by: Jeff Xu <jeffxu@...gle.com>

It's a change to a userspace API, yes?  Please let's have a detailed
description of why this is OK.  Why it won't affect any existing users.

Also, please let's give consideration to a -stable backport so that all
kernel versions will eventually behave in the same manner.