[PATCH] powerpc: Don't print kernel instructions in show_user_instructions()

[Jann Horn]

On Thu, Oct 18, 2018 at 11:28 AM Christophe LEROY
<christophe.leroy at c-s.fr> wrote:
> Le 05/10/2018 à 15:21, Michael Ellerman a écrit :
> > 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:
> >
> >    bad-bctr[2969]: segfault (11) at c000000000010000 nip c000000000010000 lr 107930894 code 1
> >    bad-bctr[2969]: Bad NIP, not dumping instructions.
> >
> > Fixes: 88b0fe175735 ("powerpc: Add show_user_instructions()")
> > Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> > ---
> >   arch/powerpc/kernel/process.c | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> >
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 913c5725cdb2..bb6ac471a784 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -1306,6 +1306,16 @@ void show_user_instructions(struct pt_regs *regs)
> >
> >       pc = regs->nip - (instructions_to_print * 3 / 4 * sizeof(int));
> >
> > +     /*
> > +      * Make sure the NIP points at userspace, not kernel text/data or
> > +      * elsewhere.
> > +      */
> > +     if (!__access_ok(pc, instructions_to_print * sizeof(int), USER_DS)) {
> > +             pr_info("%s[%d]: Bad NIP, not dumping instructions.\n",
> > +                     current->comm, current->pid);
> > +             return;
> > +     }
> > +
>
> Is there any reason for not using access_ok() here ?

It's probably more robust this way, in case someone decides to call
into this from kernel exception context at some point, or something
like that?