[RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

[Mickaël Salaün]

On Fri, Jul 19, 2024 at 07:16:55AM -0700, Jeff Xu wrote:
> On Fri, Jul 19, 2024 at 1:45 AM Mickaël Salaün <mic at digikod.net> wrote:
> >
> > On Thu, Jul 18, 2024 at 06:29:54PM -0700, Jeff Xu wrote:
> > > On Thu, Jul 18, 2024 at 5:24 AM Mickaël Salaün <mic at digikod.net> wrote:
> > > >
> > > > On Wed, Jul 17, 2024 at 07:08:17PM -0700, Jeff Xu wrote:
> > > > > On Wed, Jul 17, 2024 at 3:01 AM Mickaël Salaün <mic at digikod.net> wrote:
> > > > > >
> > > > > > On Tue, Jul 16, 2024 at 11:33:55PM -0700, Jeff Xu wrote:
> > > > > > > On Thu, Jul 4, 2024 at 12:02 PM Mickaël Salaün <mic at digikod.net> wrote:
> > > > > > > >
> > > > > > > > Add a new AT_CHECK flag to execveat(2) to check if a file would be
> > > > > > > > allowed for execution.  The main use case is for script interpreters and
> > > > > > > > dynamic linkers to check execution permission according to the kernel's
> > > > > > > > security policy. Another use case is to add context to access logs e.g.,
> > > > > > > > which script (instead of interpreter) accessed a file.  As any
> > > > > > > > executable code, scripts could also use this check [1].
> > > > > > > >
> > > > > > > > This is different than faccessat(2) which only checks file access
> > > > > > > > rights, but not the full context e.g. mount point's noexec, stack limit,
> > > > > > > > and all potential LSM extra checks (e.g. argv, envp, credentials).
> > > > > > > > Since the use of AT_CHECK follows the exact kernel semantic as for a
> > > > > > > > real execution, user space gets the same error codes.
> > > > > > > >
> > > > > > > So we concluded that execveat(AT_CHECK) will be used to check the
> > > > > > > exec, shared object, script and config file (such as seccomp config),
> >
> > > > > > > I think binfmt_elf.c in the kernel needs to check the ld.so to make
> > > > > > > sure it passes AT_CHECK, before loading it into memory.
> > > > > >
> > > > > > All ELF dependencies are opened and checked with open_exec(), which
> > > > > > perform the main executability checks (with the __FMODE_EXEC flag).
> > > > > > Did I miss something?
> > > > > >
> > > > > I mean the ld-linux-x86-64.so.2 which is loaded by binfmt in the kernel.
> > > > > The app can choose its own dynamic linker path during build, (maybe
> > > > > even statically link one ?)  This is another reason that relying on a
> > > > > userspace only is not enough.
> > > >
> > > > The kernel calls open_exec() on all dependencies, including
> > > > ld-linux-x86-64.so.2, so these files are checked for executability too.
> > > >
> > > This might not be entirely true. iiuc, kernel  calls open_exec for
> > > open_exec for interpreter, but not all its dependency (e.g. libc.so.6)
> >
> > Correct, the dynamic linker is in charge of that, which is why it must
> > be enlighten with execveat+AT_CHECK and securebits checks.
> >
> > > load_elf_binary() {
> > >    interpreter = open_exec(elf_interpreter);
> > > }
> > >
> > > libc.so.6 is opened and mapped by dynamic linker.
> > > so the call sequence is:
> > >  execve(a.out)
> > >   - open exec(a.out)
> > >   - security_bprm_creds(a.out)
> > >   - open the exec(ld.so)
> > >   - call open_exec() for interruptor (ld.so)
> > >   - call execveat(AT_CHECK, ld.so) <-- do we want ld.so going through
> > > the same check and code path as libc.so below ?
> >
> > open_exec() checks are enough.  LSMs can use this information (open +
> > __FMODE_EXEC) if needed.  execveat+AT_CHECK is only a user space
> > request.
> >
> Then the ld.so doesn't go through the same security_bprm_creds() check
> as other .so.

Indeed, but...

> 
> As my previous email, the ChromeOS LSM restricts executable mfd
> through security_bprm_creds(), the end result is that ld.so can still
> be executable memfd, but not other .so.

The chromeOS LSM can check that with the security_file_open() hook and
the __FMODE_EXEC flag, see Landlock's implementation.  I think this
should be the only hook implementation that chromeOS LSM needs to add.

> 
> One way to address this is to refactor the necessary code from
> execveat() code patch, and make it available to call from both kernel
> and execveat() code paths., but if we do that, we might as well use
> faccessat2(AT_CHECK)

That's why I think it makes sense to rely on the existing __FMODE_EXEC
information.

> 
> 
> > >   - transfer the control to ld.so)
> > >   - ld.so open (libc.so)
> > >   - ld.so call execveat(AT_CHECK,libc.so) <-- proposed by this patch,
> > > require dynamic linker change.
> > >   - ld.so mmap(libc.so,rx)
> >
> > Explaining these steps is useful. I'll include that in the next patch
> > series.