Re: [PATCH RFC] random: fix syzkaller fuzzer test int overflow

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Oct 24, 2017 at 03:44:17PM +0800, Chen Feng wrote:
> [pid:11940,cpu6,syz-executor][flp_ioctl]cmd[0x1]
> Restart is not permit
> =================================================================
> UBSAN: Undefined behaviour in
> kernel/linux-4.4/drivers/char/random.c:676:19
> signed integer overflow:
> 2147483645 + 268435455 cannot be represented in type 'int'
> CPU: 4 PID: 11941 Comm: syz-executor Not tainted 4.4.76+ #2

Does this also happen on 4.14-rc6?

> TGID: 11928 Comm: syz-executor
> Hardware name: hi3660 (DT)
> Call trace:
> [<ffffffa661090378>] dump_backtrace+0x0/0x314
> [<ffffffa66109077c>] show_stack+0x1c/0x24
> [<ffffffa66180cc54>] dump_stack+0xdc/0x130
> [<ffffffa6618833d0>] ubsan_epilogue+0x18/0x6c
> [<ffffffa661883c5c>] handle_overflow+0x180/0x1d4
> [<ffffffa661883cdc>] __ubsan_handle_add_overflow+0x2c/0x34
> [<ffffffa661ab75a0>] credit_entropy_bits+0x358/0x9a8
> [<ffffffa661ab85bc>] random_ioctl+0x338/0x384
> [<ffffffa661399c74>] do_vfs_ioctl+0x60c/0xa4c
> [<ffffffa66139a150>] SyS_ioctl+0x9c/0xc0
> [<ffffffa6610838b0>] el0_svc_naked+0x24/0x28
> =================================================================
> 
> Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
> Signed-off-by: Yukun Zhao <zhaoyukun@huawei.com>
> ---
>  drivers/char/random.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 1ef2640..6f2bd6a 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -699,6 +699,11 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
>  	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
>  		goto retry;
>  
> +	if (INT_MAX - nbits < r->entropy_total) {
> +		WARN_ON(1);

Why WARN_ON()?  What is that going to help with?

thanks,

greg k-h