From: Kees Cook <keescook@chromium.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
Eric Biggers <ebiggers3@gmail.com>,
Dmitry Vyukov <dvyukov@google.com>,
linux-fsdevel@vger.kernel.org,
linux-security-module@vger.kernel.org, linux-api@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/4] Relocate execve() sanity checks
Date: Tue, 19 May 2020 09:26:04 -0700 [thread overview]
Message-ID: <202005190918.D2BD83F7C@keescook> (raw)
In-Reply-To: <87a724t153.fsf@x220.int.ebiederm.org>
On Tue, May 19, 2020 at 10:06:32AM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
>
> > Hi,
> >
> > While looking at the code paths for the proposed O_MAYEXEC flag, I saw
> > some things that looked like they should be fixed up.
> >
> > exec: Change uselib(2) IS_SREG() failure to EACCES
> > This just regularizes the return code on uselib(2).
> >
> > exec: Relocate S_ISREG() check
> > This moves the S_ISREG() check even earlier than it was already.
> >
> > exec: Relocate path_noexec() check
> > This adds the path_noexec() check to the same place as the
> > S_ISREG() check.
> >
> > fs: Include FMODE_EXEC when converting flags to f_mode
> > This seemed like an oversight, but I suspect there is some
> > reason I couldn't find for why FMODE_EXEC doesn't get set in
> > f_mode and just stays in f_flags.
>
> So I took a look at this series.
>
> I think the belt and suspenders approach of adding code in open and then
> keeping it in exec and uselib is probably wrong. My sense of the
> situation is a belt and suspenders approach is more likely to be
> confusing and result in people making mistakes when maintaining the code
> than to actually be helpful.
This is why I added the comments in fs/exec.c's redundant checks. When I
was originally testing this series, I had entirely removed the checks in
fs/exec.c, but then had nightmares about some kind of future VFS paths
that would somehow bypass do_open() and result in execve() working on
noexec mounts, there by allowing for the introduction of a really nasty
security bug.
The S_ISREG test is demonstrably too late (as referenced in the series),
and given the LSM hooks, I think the noexec check is too late as well.
(This is especially true for the coming O_MAYEXEC series, which will
absolutely need those tests earlier as well[1] -- the permission checking
is then in the correct place: during open, not exec.) I think the only
question is about leaving the redundant checks in fs/exec.c, which I
think are a cheap way to retain a sense of robustness.
-Kees
[1] https://lore.kernel.org/lkml/202005142343.D580850@keescook/
--
Kees Cook
next prev parent reply other threads:[~2020-05-19 16:26 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-18 5:54 [PATCH 0/4] Relocate execve() sanity checks Kees Cook
2020-05-18 5:54 ` [PATCH 1/4] exec: Change uselib(2) IS_SREG() failure to EACCES Kees Cook
2020-05-18 13:02 ` Christian Brauner
2020-05-18 14:43 ` Jann Horn
2020-05-18 14:46 ` Christian Brauner
2020-05-18 23:57 ` Eric W. Biederman
2020-05-19 8:11 ` Christian Brauner
2020-05-19 8:37 ` Andreas Schwab
2020-05-19 11:56 ` Eric W. Biederman
2020-05-19 12:12 ` Andreas Schwab
2020-05-19 12:28 ` Eric W. Biederman
2020-05-19 13:29 ` Christian Brauner
2020-05-19 14:49 ` Eric W. Biederman
2020-05-19 13:13 ` Christian Brauner
2020-05-19 14:32 ` Geert Uytterhoeven
2020-05-19 14:47 ` Christian Brauner
2020-05-18 5:54 ` [PATCH 2/4] exec: Relocate S_ISREG() check Kees Cook
[not found] ` <20200525091420.GI12456@shao2-debian>
2020-06-04 22:45 ` [exec] 166d03c9ec: ltp.execveat02.fail Kees Cook
2020-06-05 2:57 ` Kees Cook
2020-05-18 5:54 ` [PATCH 3/4] exec: Relocate path_noexec() check Kees Cook
2020-05-18 5:54 ` [PATCH 4/4] fs: Include FMODE_EXEC when converting flags to f_mode Kees Cook
2020-05-19 15:06 ` [PATCH 0/4] Relocate execve() sanity checks Eric W. Biederman
2020-05-19 16:26 ` Kees Cook [this message]
2020-05-19 17:41 ` Eric W. Biederman
2020-05-19 17:56 ` Kees Cook
2020-05-19 18:42 ` Eric W. Biederman
2020-05-19 21:17 ` Kees Cook
2020-05-19 22:58 ` John Johansen
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