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Subject: Re: [PATCH v3 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec
 mount property
To:     James Morris <jmorris@namei.org>
Cc:     linux-kernel@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        Eric Chiang <ericchiang@google.com>,
        Florian Weimer <fweimer@redhat.com>, Jan Kara <jack@suse.cz>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Matthew Garrett <mjg59@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mickael.salaun@ssi.gouv.fr>,
        Mimi Zohar <zohar@linux.ibm.com>,
        =?UTF-8?Q?Philippe_Tr=c3=a9buchet?= 
        <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>,
        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-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
References: <20200428175129.634352-1-mic@digikod.net>
 <20200428175129.634352-3-mic@digikod.net>
 <alpine.LRH.2.21.2005011357290.29679@namei.org>
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>
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Date:   Fri, 1 May 2020 16:17:54 +0200
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On 01/05/2020 06:02, James Morris wrote:
> On Tue, 28 Apr 2020, Micka?l Sala?n wrote:
> 
>> An LSM doesn't get path information related to an access request to open
>> an inode.  This new (internal) MAY_EXECMOUNT flag enables an LSM to
>> check if the underlying mount point of an inode is marked as executable.
>> This is useful to implement a security policy taking advantage of the
>> noexec mount option.
>>
>> This flag is set according to path_noexec(), which checks if a mount
>> point is mounted with MNT_NOEXEC or if the underlying superblock is
>> SB_I_NOEXEC.
>>
>> Signed-off-by: Micka?l Sala?n <mic@digikod.net>
>> Reviewed-by: Philippe Tr?buchet <philippe.trebuchet@ssi.gouv.fr>
>> Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
>> Cc: Aleksa Sarai <cyphar@cyphar.com>
>> Cc: Al Viro <viro@zeniv.linux.org.uk>
>> Cc: Kees Cook <keescook@chromium.org>
> 
> Are there any existing LSMs which plan to use this aspect?

This commit message was initially for the first version of O_MAYEXEC,
which extended Yama. The current enforcement implementation is now
directly in the FS subsystem (as requested by Kees Cook). However, this
MAY_EXECMOUNT flag is still used by the current FS implementation and it
could still be useful for LSMs.