Re: [bug report] out-of-bound array access in drivers/rtc/lib.c rtc_month_days

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

Hello,

On 19/02/2021 13:51:12-0500, charley.ashbringer@gmail.com wrote:
> Hi Alessandro and Alexandre,
> Greetings, I'm a 2nd year PhD student who is interested in using UBSan to
> the kernel.
> Through some experiment, I found a out-of-bound array access in function
> rtc_month_days.
> More specifically, the through the call chain of 
> davinci_rtc_set_time/davinci_rtc_set_alarm -> convert2days ->
> rtc_month_days,
> since davinci_rtc_set_time/davinci_rtc_set_alarm are ioctl functions,
> thus the 2nd parameter, struct rtc_time *tm, is passed in purely from
> user-space which can be any value.

This part is not true and is probably what you are missing, the
userspace input is sanitized by the core, see the rtc_valid_tm calls
here:
https://elixir.bootlin.com/linux/v5.11/source/drivers/rtc/interface.c#L130
and here:
https://elixir.bootlin.com/linux/v5.11/source/drivers/rtc/interface.c#L457

> And such a value, tm->tm_mon is used directly as an index to a fixed length
> array, rtc_ydays.
> This looks very fishy to me.
> 
> Although I know that, syzkaller has applied UBSan to this driver before, and
> such a simple error cannot evade its detection, I'm still wondering if this
> is a true error,
> and more importantly, if it's not, then why, this will help me understand
> linux a lot.
> 
> Looking forward to your valued response!
> 

Regards,

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com