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SubjectRe: For review: seccomp_user_notif(2) manual page [v2]
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Hello Jann,

On 10/29/20 2:42 AM, Jann Horn wrote:
> On Mon, Oct 26, 2020 at 10:55 AM Michael Kerrisk (man-pages)
> <mtk.manpages@gmail.com> wrote:
>> static bool
>> getTargetPathname(struct seccomp_notif *req, int notifyFd,
>> char *path, size_t len)
>> {
>> char procMemPath[PATH_MAX];
>>
>> snprintf(procMemPath, sizeof(procMemPath), "/proc/%d/mem", req->pid);
>>
>> int procMemFd = open(procMemPath, O_RDONLY);
>> if (procMemFd == -1)
>> errExit("\tS: open");
>>
>> /* Check that the process whose info we are accessing is still alive.
>> If the SECCOMP_IOCTL_NOTIF_ID_VALID operation (performed
>> in checkNotificationIdIsValid()) succeeds, we know that the
>> /proc/PID/mem file descriptor that we opened corresponds to the
>> process for which we received a notification. If that process
>> subsequently terminates, then read() on that file descriptor
>> will return 0 (EOF). */
>>
>> checkNotificationIdIsValid(notifyFd, req->id);
>>
>> /* Read bytes at the location containing the pathname argument
>> (i.e., the first argument) of the mkdir(2) call */
>>
>> ssize_t nread = pread(procMemFd, path, len, req->data.args[0]);
>> if (nread == -1)
>> errExit("pread");
>
> 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.

Consider the following scenario, where the supervisor tries to
read the pathname argument of a target's blocked mount(2) system
call:

• From one of its functions (func()), the target calls mount(2),
which triggers a user-space notification and causes the target
to block.

• The supervisor receives the notification, opens
/proc/[tid]/mem, and (successfully) performs the
SECCOMP_IOCTL_NOTIF_ID_VALID check.

• The target receives a signal, which causes the mount(2) to
abort.

• The signal handler executes in the target, and returns.

• Upon return from the handler, the execution of func() resumes,
and it returns (and perhaps other functions are called,
overwriting the memory that had been used for the stack frame
of func()).

• Using the address provided in the notification information, the
supervisor reads from the target's memory location that used to
contain the pathname.

• The supervisor now calls mount(2) with some arbitrary bytes
obtained in the previous step.

The conclusion from the above scenario is this: since the
target's blocked system call may be interrupted by a signal
handler, the supervisor must be written to expect that the target
may abandon its system call at any time; in such an event, any
information that the supervisor obtained from the target's memory
must be considered invalid.

To prevent such scenarios, every read from the target's memory
must be separated from use of the bytes so obtained by a
SECCOMP_IOCTL_NOTIF_ID_VALID check. In the above example, the
check would be placed between the two final steps. An example of
such a check is shown in EXAMPLES.

Following on from the above, it should be clear that a write by
the supervisor into the target's memory can never be considered
safe.

Seem okay?

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.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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