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Subject: Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for
 faccessat2(2)
To:     Mimi Zohar <zohar@linux.ibm.com>, linux-kernel@vger.kernel.org
Cc:     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>,
        Kees Cook <keescook@chromium.org>,
        Lakshmi Ramasubramanian <nramas@linux.microsoft.com>,
        Matthew Garrett <mjg59@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Miklos Szeredi <mszeredi@redhat.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>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Thibaut Sautereau <thibaut.sautereau@clip-os.org>,
        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,
        Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>,
        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@linux.microsoft.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        John Johansen <john.johansen@canonical.com>
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 <75451684-58f3-b946-dca4-4760fa0d7440@digikod.net>
 <01c23b2607a7dbf734722399931473c053d9b362.camel@linux.ibm.com>
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 <fd635b544ba4f409a76047a4620656ad67738db1.camel@linux.ibm.com>
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>
Message-ID: <c2cfb75d-d166-9b97-a559-109c44363fe7@digikod.net>
Date:   Tue, 8 Sep 2020 19:21:54 +0200
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On 08/09/2020 18:44, Mimi Zohar wrote:
> On Tue, 2020-09-08 at 17:44 +0200, Micka?l Sala?n wrote:
>> On 08/09/2020 17:24, Mimi Zohar wrote:
>>> On Tue, 2020-09-08 at 14:43 +0200, Micka?l Sala?n wrote:
>>>> On 08/09/2020 14:28, Mimi Zohar wrote:
>>>>> Hi Mickael,
>>>>>
>>>>> On Tue, 2020-09-08 at 09:59 +0200, Micka?l Sala?n wrote:
>>>>>> diff --git a/fs/open.c b/fs/open.c
>>>>>> index 9af548fb841b..879bdfbdc6fa 100644
>>>>>> --- a/fs/open.c
>>>>>> +++ b/fs/open.c
>>>>>> @@ -405,9 +405,13 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
>>>>>>  	if (mode & ~S_IRWXO)	/* where's F_OK, X_OK, W_OK, R_OK? */
>>>>>>  		return -EINVAL;
>>>>>>  
>>>>>> -	if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
>>>>>> +	if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH |
>>>>>> +				AT_INTERPRETED))
>>>>>>  		return -EINVAL;
>>>>>>  
>>>>>> +	/* Only allows X_OK with AT_INTERPRETED for now. */
>>>>>> +	if ((flags & AT_INTERPRETED) && !(mode & S_IXOTH))
>>>>>> +		return -EINVAL;
>>>>>>  	if (flags & AT_SYMLINK_NOFOLLOW)
>>>>>>  		lookup_flags &= ~LOOKUP_FOLLOW;
>>>>>>  	if (flags & AT_EMPTY_PATH)
>>>>>> @@ -426,7 +430,30 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
>>>>>>  
>>>>>>  	inode = d_backing_inode(path.dentry);
>>>>>>  
>>>>>> -	if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
>>>>>> +	if ((flags & AT_INTERPRETED)) {
>>>>>> +		/*
>>>>>> +		 * For compatibility reasons, without a defined security policy
>>>>>> +		 * (via sysctl or LSM), using AT_INTERPRETED must map the
>>>>>> +		 * execute permission to the read permission.  Indeed, from
>>>>>> +		 * user space point of view, being able to execute data (e.g.
>>>>>> +		 * scripts) implies to be able to read this data.
>>>>>> +		 *
>>>>>> +		 * The MAY_INTERPRETED_EXEC bit is set to enable LSMs to add
>>>>>> +		 * custom checks, while being compatible with current policies.
>>>>>> +		 */
>>>>>> +		if ((mode & MAY_EXEC)) {
>>>>>
>>>>> Why is the ISREG() test being dropped?   Without dropping it, there
>>>>> would be no reason for making the existing test an "else" clause.
>>>>
>>>> The ISREG() is not dropped, it is just moved below with the rest of the
>>>> original code. The corresponding code (with the path_noexec call) for
>>>> AT_INTERPRETED is added with the next commit, and it relies on the
>>>> sysctl configuration for compatibility reasons.
>>>
>>> Dropping the S_ISREG() check here without an explanation is wrong and
>>> probably unsafe, as it is only re-added in the subsequent patch and
>>> only for the "sysctl_interpreted_access" case.  Adding this new test
>>> after the existing test is probably safer.  If the original test fails,
>>> it returns the same value as this test -EACCES.
>>
>> The original S_ISREG() is ANDed with a MAY_EXEC check and with
>> path_noexec(). The goal of this patch is indeed to have a different
>> behavior than the original faccessat2(2) thanks to the AT_INTERPRETED
>> flag. This can't work if we add the sysctl check after the current
>> path_noexec() check. Moreover, in this patch an exec check is translated
>> to a read check. This new behavior is harmless because using
>> AT_INTERPRETED with the current faccessat2(2) would return -EINVAL. The
>> current vanilla behavior is then unchanged.
> 
> Don't get me wrong.  I'm very interested in having this support and
> appreciate all the work you're doing on getting it upstreamed.  With
> the change in this patch, I see the MAY_EXEC being changed to MAY_READ,
> but I don't see -EINVAL being returned.  It sounds like this change is
> dependent on the faccessat2 version for -EINVAL to be returned.

No worries, unfortunately the patch format doesn't ease this review. :)
access(2) and faccessat(2) have a flag value of 0. Only faccessat2(2)
takes a flag from userspace. The -EINVAL is currently returned (by
faccessat2) if there is an unknown flag provided by userspace. With this
patch, only a mode equal to X_OK is allowed for the AT_INTERPRETED flag
(cf. second hunk in this patch). As described in the cover letter, we
could handle the other modes in the future though.

> 
>>
>> The whole point of this patch series is to have a policy which do not
>> break current systems and is easy to configure by the sysadmin through
>> sysctl. This patch series also enable LSMs to take advantage of it
>> without the current faccess* limitations. For instance, it is then
>> possible for an LSM to implement more complex policies which may allow
>> execution of data from pipes or sockets, while verifying the source of
>> this data. Enforcing S_ISREG() in this patch would forbid such policies
>> to be implemented. In the case of IMA, you may want to add the same
>> S_ISREG() check.
> 
>>>
>>>>
>>>>>
>>>>>> +			mode |= MAY_INTERPRETED_EXEC;
>>>>>> +			/*
>>>>>> +			 * For compatibility reasons, if the system-wide policy
>>>>>> +			 * doesn't enforce file permission checks, then
>>>>>> +			 * replaces the execute permission request with a read
>>>>>> +			 * permission request.
>>>>>> +			 */
>>>>>> +			mode &= ~MAY_EXEC;
>>>>>> +			/* To be executed *by* user space, files must be readable. */
>>>>>> +			mode |= MAY_READ;
>>>
>>>
> 
>