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Subject: Re: For review: seccomp_user_notif(2) manual page [v2]
To:     Jann Horn <jannh@google.com>, Kees Cook <keescook@chromium.org>
References: <63598b4f-6ce3-5a11-4552-cdfe308f68e4@gmail.com>
 <CAG48ez0fBE6AJfWh0in=WKkgt98y=KjAen=SQPyTYtvsUbF1yA@mail.gmail.com>
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <0de41eb1-e1fd-85da-61b7-fac4e3006726@gmail.com>
Date:   Thu, 29 Oct 2020 15:19:45 +0100
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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...

Thanks very much for pointing me at this!

So, I want to conform that the fix to the code is as simple as
adding a check following the pread() call, something like:

[[
     ssize_t nread = pread(procMemFd, path, len, req->data.args[argNum]);
     if (nread == -1)
        errExit("Supervisor: pread");
 
     if (nread == 0) {
        fprintf(stderr, "\tS: pread() of /proc/PID/mem "
                "returned 0 (EOF)\n");
        exit(EXIT_FAILURE);
     }
 
     if (close(procMemFd) == -1)
        errExit("Supervisor: close-/proc/PID/mem");
 
+    /* Once again check that the notification ID is still valid. The
+       case we are particularly concerned about here is that just
+       before we fetched the pathname, the target's blocked system
+       call was interrupted by a signal handler, and after the handler
+       returned, the target carried on execution (past the interrupted
+       system call). In that case, we have no guarantees about what we
+       are reading, since the target's memory may have been arbitrarily
+       changed by subsequent operations. */
+
+    if (!notificationIdIsValid(notifyFd, req->id, "post-open"))
+        return false;
+
     /* We have no guarantees about what was in the memory of the target
        process. We therefore treat the buffer returned by pread() as
        untrusted input. The buffer should be terminated by a null byte;
        if not, then we will trigger an error for the target process. */
 
     if (strnlen(path, nread) < nread)
         return true;
]]

> 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).
> 
>>            if (nread == 0) {
>>                fprintf(stderr, "\tS: pread() of /proc/PID/mem "
>>                        "returned 0 (EOF)\n");
>>                exit(EXIT_FAILURE);
>>            }
> .

I'll think over some changes to the text of the manual page.

Cheers,

Michael


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