Hi Tetsuo,
On Thu, May 28, 2020 at 8:57 AM Tetsuo Handa
<[email protected]> wrote:
> syzbot found a NULL pointer dereference bug inside mptcp_recvmsg() due to
> ssock == NULL, but this bug manifested inside selinux_socket_recvmsg()
> because pr_debug() was no-op [1].
>
> pr_debug("fallback-read subflow=%p",
> mptcp_subflow_ctx(ssock->sk));
> copied = sock_recvmsg(ssock, msg, flags);
>
> Thus, let's allow fuzzers to always evaluate pr_devel()/pr_debug()
> messages, by redirecting no-op pr_devel()/pr_debug() calls to snprintf().
>
> [1]
> https://syzkaller.appspot.com/bug?id=12be9aa373be9d8727cdd172f190de39528a413a
>
> Signed-off-by: Tetsuo Handa <[email protected]>
Thanks for your patch!
> --- a/lib/Kconfig.twist
> +++ b/lib/Kconfig.twist
> @@ -12,10 +12,22 @@ if TWIST_KERNEL_BEHAVIOR
>
> config TWIST_FOR_SYZKALLER_TESTING
> bool "Select all twist options suitable for syzkaller testing"
> + select TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS
> select TWIST_DISABLE_KBD_K_SPEC_HANDLER
> help
> Say N unless you are building kernels for syzkaller's testing.
>
> +config TWIST_ALWAYS_EVALUATE_PRINTK_ARGUMENTS
> + bool "Always evaluate printk() arguments"
> + help
> + Currently, only format string of printk() arguments is checked
> + by compiler if pr_devel()/pr_debug() are disabled. Therefore,
> + fuzz testing cannot catch runtime bugs (e.g. NULL pointer
> + dereference, use-after-free/out-of-bounds/uninitialized read)
> + in disabled printk() calls. This option redirects disabled
> + printk(...) to snprintf(NULL, 0, ...) in order to evaluate
> + arguments without printing.
> +
> config TWIST_DISABLE_KBD_K_SPEC_HANDLER
> bool "Disable k_spec() function in drivers/tty/vt/keyboard.c"
> help
Can't you just enable CONFIG_DYNAMIC_DEBUG in your fuzzer
config instead?
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
