On Fri, Oct 5, 2018 at 3:21 PM Michael Ellerman <m...@ellerman.id.au> wrote:
> Recently we implemented show_user_instructions() which dumps the code
> around the NIP when a user space process dies with an unhandled
> signal. This was modelled on the x86 code, and we even went so far as
> to implement the exact same bug, namely that if the user process
> crashed with its NIP pointing into the kernel we will dump kernel text
> to dmesg. eg:
>
> bad-bctr[2996]: segfault (11) at c000000000010000 nip c000000000010000 lr
> 12d0b0894 code 1
> bad-bctr[2996]: code: fbe10068 7cbe2b78 7c7f1b78 fb610048 38a10028 38810020
> fb810050 7f8802a6
> bad-bctr[2996]: code: 3860001c f8010080 48242371 60000000 <7c7b1b79>
> 4082002c e8010080 eb610048
>
> This was discovered on x86 by Jann Horn and fixed in commit
> 342db04ae712 ("x86/dumpstack: Don't dump kernel memory based on usermode
> RIP").
>
> Fix it by checking the adjusted NIP value (pc) and number of
> instructions against USER_DS, and bail if we fail the check, eg:
This fix looks good to me.
In the long term, I think it is somewhat awkward to use
probe_kernel_address(), which uses set_fs(KERNEL_DS), when you
actually just want to access userspace memory. It might make sense to
provide a better helper for explicitly accessing memory with USER_DS.
