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

[Greg KH]

On Tue, Oct 24, 2017 at 05:24:01PM +0800, Chen Feng wrote:
> 
> 
> On 2017/10/24 17:09, Greg KH wrote:
> > 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?
> 
> No, mainline also has this issue.
> > 
> >> 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@xxxxxxxxxxxxx>
> >> Signed-off-by: Yukun Zhao <zhaoyukun@xxxxxxxxxx>
> >> ---
> >>  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?
> Actually, I am not familiar with the random module....
> 
> This patch is RFC to see if some one has better idea.

Well, not spamming the kernel log for something that userspace can
trigger is a good start to modifying your patch :)

thanks,

greg k-h