Re: [PATCH v6 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC

Mickaël Salaün
2020-07-22T19:05:07Z

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.