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From: "Mickaël Salaün" <mic@digikod.net>
To: Thibaut Sautereau <thibaut.sautereau@clip-os.org>,
	Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org, "Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Eric Biggers" <ebiggers@kernel.org>,
	"Eric Chiang" <ericchiang@google.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Tetsuo Handa" <penguin-kernel@i-love.sakura.ne.jp>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v6 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC
Date: Wed, 22 Jul 2020 21:04:28 +0200	[thread overview]
Message-ID: <efb88aab-f9f9-4b66-e7ab-3aa054eec96e@digikod.net> (raw)
In-Reply-To: <20200722161639.GA24129@gandi.net>


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?
> 

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.

  reply	other threads:[~2020-07-22 19:04 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-14 18:16 [PATCH v6 0/7] Add support for O_MAYEXEC Mickaël Salaün
2020-07-14 18:16 ` [PATCH v6 1/7] exec: Change uselib(2) IS_SREG() failure to EACCES Mickaël Salaün
2020-07-14 18:16 ` [PATCH v6 2/7] exec: Move S_ISREG() check earlier Mickaël Salaün
2020-07-14 18:16 ` [PATCH v6 3/7] exec: Move path_noexec() " Mickaël Salaün
2020-07-14 18:16 ` [PATCH v6 4/7] fs: Introduce O_MAYEXEC flag for openat2(2) Mickaël Salaün
2020-07-15 20:06   ` Kees Cook
2020-07-16 14:18     ` Mickaël Salaün
2020-07-16 15:31       ` Kees Cook
2020-07-14 18:16 ` [PATCH v6 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-07-14 18:40   ` Randy Dunlap
2020-07-16 14:40     ` Mickaël Salaün
2020-07-15 20:37   ` Kees Cook
2020-07-16 14:39     ` Mickaël Salaün
2020-07-22 16:16       ` Thibaut Sautereau
2020-07-22 19:04         ` Mickaël Salaün [this message]
2020-07-22 19:40           ` Kees Cook
2020-07-14 18:16 ` [PATCH v6 6/7] selftest/openat2: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2020-07-15 20:38   ` Kees Cook
2020-07-14 18:16 ` [PATCH v6 7/7] ima: add policy support for the new file open MAY_OPENEXEC flag Mickaël Salaün
2020-07-15 20:40   ` Kees Cook
2020-07-16 14:40     ` Mickaël Salaün
2020-07-16 14:59       ` Randy Dunlap
2020-07-16 15:22         ` Mickaël Salaün
2020-07-16 19:13         ` Kees Cook
2020-07-16 19:12       ` Kees Cook

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