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References: <20181005132123.6038-1-mpe@ellerman.id.au>
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From: Jann Horn <jannh@google.com>
Date: Fri, 5 Oct 2018 16:02:42 +0200
Message-ID: <CAG48ez3OZ5iquQeeqsy_SaLQF04hf6kphTSuP=OfsWC35G+TbA@mail.gmail.com>
Subject: Re: [PATCH] powerpc: Don't print kernel instructions in
 show_user_instructions()
To: Michael Ellerman <mpe@ellerman.id.au>
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On Fri, Oct 5, 2018 at 3:21 PM Michael Ellerman <mpe@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.