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Date: Wed, 22 Jul 2020 12:40:19 -0700
From: Kees Cook <keescook@...omium.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
Alexei Starovoitov <ast@...nel.org>,
Al Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
Christian Brauner <christian.brauner@...ntu.com>,
Christian Heimes <christian@...hon.org>,
Daniel Borkmann <daniel@...earbox.net>,
Deven Bowers <deven.desai@...ux.microsoft.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Eric Biggers <ebiggers@...nel.org>,
Eric Chiang <ericchiang@...gle.com>,
Florian Weimer <fweimer@...hat.com>,
James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
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Matthew Garrett <mjg59@...gle.com>,
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Michael Kerrisk <mtk.manpages@...il.com>,
Mickaël Salaün <mickael.salaun@....gouv.fr>,
Mimi Zohar <zohar@...ux.ibm.com>,
Philippe Trébuchet <philippe.trebuchet@....gouv.fr>,
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Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>,
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linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 5/7] fs,doc: Enable to enforce noexec mounts or file
exec through O_MAYEXEC
On Wed, Jul 22, 2020 at 09:04:28PM +0200, Mickaël Salaün wrote:
>
> On 22/07/2020 18:16, Thibaut Sautereau wrote:
> > On Thu, Jul 16, 2020 at 04:39:14PM +0200, Mickaël Salaün wrote:
> >>
> >> On 15/07/2020 22:37, Kees Cook wrote:
> >>> On Tue, Jul 14, 2020 at 08:16:36PM +0200, Mickaël Salaün wrote:
> >>>> @@ -2849,7 +2855,7 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> >>>> case S_IFLNK:
> >>>> return -ELOOP;
> >>>> case S_IFDIR:
> >>>> - if (acc_mode & (MAY_WRITE | MAY_EXEC))
> >>>> + if (acc_mode & (MAY_WRITE | MAY_EXEC | MAY_OPENEXEC))
> >>>> return -EISDIR;
> >>>> break;
> >>>
> >>> (I need to figure out where "open for reading" rejects S_IFDIR, since
> >>> it's clearly not here...)
> >
> > Doesn't it come from generic_read_dir() in fs/libfs.c?
> >
> >>>
> >>>> case S_IFBLK:
> >>>> @@ -2859,13 +2865,26 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> >>>> fallthrough;
> >>>> case S_IFIFO:
> >>>> case S_IFSOCK:
> >>>> - if (acc_mode & MAY_EXEC)
> >>>> + if (acc_mode & (MAY_EXEC | MAY_OPENEXEC))
> >>>> return -EACCES;
> >>>> flag &= ~O_TRUNC;
> >>>> break;
> >>>
> >>> This will immediately break a system that runs code with MAY_OPENEXEC
> >>> set but reads from a block, char, fifo, or socket, even in the case of
> >>> a sysadmin leaving the "file" sysctl disabled.
> >>
> >> As documented, O_MAYEXEC is for regular files. The only legitimate use
> >> case seems to be with pipes, which should probably be allowed when
> >> enforcement is disabled.
> >
> > By the way Kees, while we fix that for the next series, do you think it
> > would be relevant, at least for the sake of clarity, to add a
> > WARN_ON_ONCE(acc_mode & MAY_OPENEXEC) for the S_IFSOCK case, since a
> > socket cannot be open anyway?
If it's a state that userspace should never be able to reach, then yes,
I think a WARN_ON_ONCE() would be nice.
> We just did some more tests (for the next patch series) and it turns out
> that may_open() can return EACCES before another part returns ENXIO.
>
> As a reminder, the next series will deny access to block devices,
> character devices, fifo and socket when opened with O_MAYEXEC *and* if
> any policy is enforced (via the sysctl).
>
> The question is then: do we prefer to return EACCES when a policy is
> enforced (on a socket), or do we stick to the ENXIO? The EACCES approach
> will be more consistent with devices and fifo handling, and seems safer
> (belt and suspenders) thought.
I think EACCES is correct for these cases, since it's a new flag, etc.
--
Kees Cook
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