[RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)

[Mickaël Salaün]

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;
>>>
>>>
> 
>