How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC)

[Florian Weimer]

* Kees Cook:

> On Fri, May 15, 2020 at 10:43:34AM +0200, Florian Weimer wrote:
>> * Kees Cook:
>> 
>> > Maybe I've missed some earlier discussion that ruled this out, but I
>> > couldn't find it: let's just add O_EXEC and be done with it. It actually
>> > makes the execve() path more like openat2() and is much cleaner after
>> > a little refactoring. Here are the results, though I haven't emailed it
>> > yet since I still want to do some more testing:
>> > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/o_exec/v1
>> 
>> I think POSIX specifies O_EXEC in such a way that it does not confer
>> read permissions.  This seems incompatible with what we are trying to
>> achieve here.
>
> I was trying to retain this behavior, since we already make this
> distinction between execve() and uselib() with the MAY_* flags:
>
> execve():
>         struct open_flags open_exec_flags = {
>                 .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>                 .acc_mode = MAY_EXEC,
>
> uselib():
>         static const struct open_flags uselib_flags = {
>                 .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>                 .acc_mode = MAY_READ | MAY_EXEC,
>
> I tried to retain this in my proposal, in the O_EXEC does not imply
> MAY_READ:

That doesn't quite parse for me, sorry.

The point is that the script interpreter actually needs to *read* those
files in order to execute them.

Thanks,
Florian