[PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs

[Mickaël Salaün]

On Wed, Jan 24, 2024 at 01:32:02PM -0800, Kees Cook wrote:
> On Wed, Jan 24, 2024 at 12:47:34PM -0800, Linus Torvalds wrote:
> > On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook at chromium.org> wrote:
> > >
> > > Hmpf, and frustratingly Ubuntu (and Debian) still builds with
> > > CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.
> 
> For completeness, Fedora hasn't had CONFIG_USELIB for a while now.
> 
> > Well, we could just remove the __FMODE_EXEC from uselib.
> > 
> > It's kind of wrong anyway.
> 
> Yeah.
> 
> > So I think just removing __FMODE_EXEC would just do the
> > RightThing(tm), and changes nothing for any sane situation.
> 
> Agreed about these:
> 
> - fs/fcntl.c is just doing a bitfield sanity check.
> 
> - nfs_open_permission_mask(), as you say, is only checking for
>   unreadable case.
> 
> - fsnotify would also see uselib() as a read, but afaict,
>   that's what it would see for an mmap(), so this should
>   be functionally safe.
> 
> This one, though, I need some more time to examine:
> 
> - AppArmor, TOMOYO, and LandLock will see uselib() as an
>   open-for-read, so that might still be a problem? As you
>   say, it's more of a mmap() call, but that would mean
>   adding something a call like security_mmap_file() into
>   uselib()...

If user space can emulate uselib() without opening a file with
__FMODE_EXEC, then there is no security reason to keep __FMODE_EXEC for
uselib().

Removing __FMODE_EXEC from uselib() looks OK for Landlock.  We use
__FMODE_EXEC to infer if a file is being open for execution i.e., by
execve(2).

If __FMODE_EXEC is removed from uselib(), I think it should also be
backported to all stable kernels for consistency though.


> 
> The issue isn't an insane "support uselib() under AppArmor" case, but
> rather "Can uselib() be used to bypass exec/mmap checks?"
> 
> This totally untested patch might give appropriate coverage:
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index d179abb78a1c..0c9265312c8d 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -143,6 +143,10 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>  	if (IS_ERR(file))
>  		goto out;
>  
> +	error = security_mmap_file(file, PROT_READ | PROT_EXEC, MAP_FIXED | MAP_SHARED);
> +	if (error)
> +		goto exit;
> +
>  	/*
>  	 * may_open() has already checked for this, so it should be
>  	 * impossible to trip now. But we need to be extra cautious
> 
> > Of course, as you say, not having CONFIG_USELIB enabled at all is the
> > _truly_ sane thing, but the only thing that used the FMODE_EXEC bit
> > were landlock and some special-case nfs stuff.
> 
> Do we want to attempt deprecation again? This was suggested last time:
> https://lore.kernel.org/lkml/20200518130251.zih2s32q2rxhxg6f@wittgenstein/
> 
> -Kees
> 
> -- 
> Kees Cook
>