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Date: Tue, 8 Sep 2020 19:21:54 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Mimi Zohar <zohar@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc: 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>,
Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Miklos Szeredi <mszeredi@...hat.com>,
Philippe Trébuchet <philippe.trebuchet@....gouv.fr>,
Scott Shell <scottsh@...rosoft.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Shuah Khan <shuah@...nel.org>, Steve Dower <steve.dower@...hon.org>,
Steve Grubb <sgrubb@...hat.com>,
Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
Vincent Strubel <vincent.strubel@....gouv.fr>,
kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Mickaël Salaün <mic@...ux.microsoft.com>,
Stephen Smalley <stephen.smalley.work@...il.com>,
John Johansen <john.johansen@...onical.com>
Subject: Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for
faccessat2(2)
On 08/09/2020 18:44, Mimi Zohar wrote:
> On Tue, 2020-09-08 at 17:44 +0200, Mickaël Salaün wrote:
>> On 08/09/2020 17:24, Mimi Zohar wrote:
>>> On Tue, 2020-09-08 at 14:43 +0200, Mickaël Salaün wrote:
>>>> On 08/09/2020 14:28, Mimi Zohar wrote:
>>>>> Hi Mickael,
>>>>>
>>>>> On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün 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;
>>>
>>>
>
>
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