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

[Kees Cook]

On Wed, Jul 22, 2020 at 09:04:28PM +0200, Mickaël Salaün wrote:
> 
> 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?

If it's a state that userspace should never be able to reach, then yes,
I think a WARN_ON_ONCE() would be nice.

> 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.

I think EACCES is correct for these cases, since it's a new flag, etc.

-- 
Kees Cook