Re: [RFC] kernel/panic: place an upper limit on number of oopses

[Daniel Axtens <dja@axtens.net>]

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Jann Horn <jann@thejh.net> writes:

> To prevent an attacker from turning a mostly harmless oops into an
> exploitable issue using a refcounter wraparound caused by repeated
> oopsing, limit the number of oopses.
>
> I have not experimentally verified whether the attack I describe
> in the comment works, but I don't see why it wouldn't.
> (f_count increments through fget() use atomic_long_inc_not_zero(),
> but get_file() just does a normal increment and is e.g.
> used by dup_fd().)
>
> This approach is strictly inferior to PAX_REFCOUNT, but as long
> as that's not upstreamed and turned on by default, it might make
> sense to at least use this patch.
>
> Opinions?

I'm torn between making the limit configurable and not adding to the
massive proliferation of config options.

Other comments below.
>
> Signed-off-by: Jann Horn <jann@thejh.net>
> ---
>  kernel/panic.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 4b150bc..27a480d 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -422,9 +422,37 @@ void print_oops_end_marker(void)
>   */
>  void oops_exit(void)
>  {
> +	static atomic_t oops_counter = ATOMIC_INIT(0);
> +
>  	do_oops_enter_exit();
>  	print_oops_end_marker();
>  	kmsg_dump(KMSG_DUMP_OOPS);
> +
> +	/*
> +	 * Every time the system oopses, if the oops happens while a
> +	 * reference to an object was held (e.g. in a VFS function),
> +	 * the reference leaks. If the oops doesn't also leak memory,
> +	 * repeated oopsing can cause the reference counter to wrap
> +	 * around - in particular, on 32bit systems, f_count in
> +	 * struct file is only 32 bits long and can realistically
> +	 * wrap around.
> +	 * This means that an oops, even if it's just caused by an
> +	 * unexploitable-looking NULL pointer dereference or so,
> +	 * could maybe be turned into a use-after-free through a
> +	 * counter overincrement, and a use-after-free might be
> +	 * exploitable.
> +	 * To reduce the probability that this happens, place an
> +	 * upper bound on how often the kernel may oops - after this
> +	 * limit is reached, just panic.
> +	 * The constant used as limit should be low enough to
> +	 * mitigate this kind of exploitation attempt, but high
> +	 * enough to avoid unnecessary panics.
> +	 */
> +	if (atomic_inc_return(&oops_counter) >= 0x100000 &&
> +			panic_on_oops == 0) {
Do you need to check panic_on_oops? If it was 1 you'd already have
paniced, right?
> +		pr_emerg("oopsed too often, setting panic_on_oops=1\n");
> +		panic_on_oops = 1;
Would it be easier to just panic here, rather than wait for another oops?
> +	}
>  }
>  
>  #ifdef WANT_WARN_ON_SLOWPATH

Regards,
Daniel
> -- 
> 2.1.4

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