Re: For review: seccomp_user_notif(2) manual page [v2]

Jann Horn
2020-10-30T19:24:35Z

On Thu, Oct 29, 2020 at 8:14 PM Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> On 10/29/20 2:42 AM, Jann Horn wrote:
> > As discussed at
> > <https://lore.kernel.org/r/CAG48ez0m4Y24ZBZCh+Tf4ORMm9_q4n7VOzpGjwGF7_Fe8EQH=Q@mail.gmail.com>,
> > we need to re-check checkNotificationIdIsValid() after reading remote
> > memory but before using the read value in any way. Otherwise, the
> > syscall could in the meantime get interrupted by a signal handler, the
> > signal handler could return, and then the function that performed the
> > syscall could free() allocations or return (thereby freeing buffers on
> > the stack).
> >
> > In essence, this pread() is (unavoidably) a potential use-after-free
> > read; and to make that not have any security impact, we need to check
> > whether UAF read occurred before using the read value. This should
> > probably be called out elsewhere in the manpage, too...
> >
> > Now, of course, **reading** is the easy case. The difficult case is if
> > we have to **write** to the remote process... because then we can't
> > play games like that. If we write data to a freed pointer, we're
> > screwed, that's it. (And for somewhat unrelated bonus fun, consider
> > that /proc/$pid/mem is originally intended for process debugging,
> > including installing breakpoints, and will therefore happily write
> > over "readonly" private mappings, such as typical mappings of
> > executable code.)
> >
> > So, uuuuh... I guess if anyone wants to actually write memory back to
> > the target process, we'd better come up with some dedicated API for
> > that, using an ioctl on the seccomp fd that magically freezes the
> > target process inside the syscall while writing to its memory, or
> > something like that? And until then, the manpage should have a big fat
> > warning that writing to the target's memory is simply not possible
> > (safely).
>
> Thank you for your very clear explanation! It turned out to be
> trivially easy to demonstrate this issue with a slightly modified
> version of my program.
>
> As well as the change to the code example that I already mentioned
> my reply of a few hours ago, I've added the following text to the
> page:
>
> Caveats regarding the use of /proc/[tid]/mem
> The discussion above noted the need to use the
> SECCOMP_IOCTL_NOTIF_ID_VALID ioctl(2) when opening the
> /proc/[tid]/mem file of the target to avoid the possibility of
> accessing the memory of the wrong process in the event that the
> target terminates and its ID is recycled by another (unrelated)
> thread. However, the use of this ioctl(2) operation is also
> necessary in other situations, as explained in the following
> pargraphs.

(nit: paragraphs)

> Consider the following scenario, where the supervisor tries to
> read the pathname argument of a target's blocked mount(2) system
> call:
[...]
> Seem okay?

Yeah, sounds good.

> By the way, is there any analogous kind of issue concerning
> pidfd_getfd()? I'm thinking not, but I wonder if I've missed
> something.

When it is used by a seccomp supervisor, you mean? I think basically
the same thing applies - when resource identifiers (such as memory
addresses or file descriptors) are passed to a syscall, it generally
has to be assumed that those identifiers may become invalid and be
reused as soon as the syscall has returned.