From: Kees Cook <keescook@chromium.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Ariadne Conill <ariadne@dereferenced.org>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Eric Biederman <ebiederm@xmission.com>,
Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()
Date: Wed, 26 Jan 2022 08:40:49 -0800 [thread overview]
Message-ID: <202201260832.CCC8BB9@keescook> (raw)
In-Reply-To: <YfFh6O2JS6MybamT@casper.infradead.org>
On Wed, Jan 26, 2022 at 02:59:52PM +0000, Matthew Wilcox wrote:
> On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:
> > Interestingly, Michael Kerrisk opened an issue about this in 2008[1],
> > but there was no consensus to support fixing this issue then.
> > Hopefully now that CVE-2021-4034 shows practical exploitative use
> > of this bug in a shellcode, we can reconsider.
> >
> > [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> > [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
>
> Having now read 8408 ... if ABI change is a concern (and I really doubt
> it is), we could treat calling execve() with a NULL argv as if the
> caller had passed an array of length 1 with the first element set to
> NULL. Just like we reopen fds 0,1,2 for suid execs if they were closed.
I was having similar thoughts this morning. We can't actually change the
argc, though, because of the various tests (see the debian code search
links) that explicitly tests for argc == 0 in the child. But, the flaw
is not the count, but rather that argv == argp in the argc == 0 case.
(Or that argv NULL-checking iteration begins at argv[1].)
But that would could fix easily by just adding an extra NULL. e.g.:
Currently:
argc = 1
argv = "foo", NULL
envp = "bar=baz", ..., NULL
argc = 0
argv = NULL
envp = "bar=baz", ..., NULL
We could just make the argc = 0 case be:
argc = 0
argv = NULL, NULL
envp = "bar=baz", ..., NULL
We need to be careful with the stack utilization counts, though, so I'm
thinking we could actually make this completely unconditional and just
pad envp by 1 NULL on the user stack:
argv = "what", "ever", NULL
NULL
envp = "bar=baz", ..., NULL
My only concern there is that there may be some code out there that
depends on envp immediately following the trailing argv NULL, so I think
my preference would be to pad only in the argc == 0 case and correctly
manage the stack utilization.
--
Kees Cook
next prev parent reply other threads:[~2022-01-26 16:40 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-26 11:44 [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common() Ariadne Conill
2022-01-26 14:40 ` Matthew Wilcox
2022-01-26 17:41 ` Ariadne Conill
2022-01-26 14:59 ` Matthew Wilcox
2022-01-26 16:40 ` Kees Cook [this message]
2022-01-26 16:57 ` Eric W. Biederman
2022-01-26 17:32 ` Ariadne Conill
2022-01-26 18:03 ` Matthew Wilcox
2022-01-26 18:38 ` Ariadne Conill
2022-01-26 20:09 ` Kees Cook
2022-01-26 20:23 ` Ariadne Conill
2022-01-26 20:56 ` Kees Cook
2022-01-26 21:13 ` Ariadne Conill
2022-01-26 21:25 ` Kees Cook
2022-01-26 21:30 ` Ariadne Conill
2022-01-26 22:49 ` Kees Cook
2022-01-26 23:07 ` Ariadne Conill
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=202201260832.CCC8BB9@keescook \
--to=keescook@chromium.org \
--cc=ariadne@dereferenced.org \
--cc=ebiederm@xmission.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=viro@zeniv.linux.org.uk \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).