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

[Christophe LEROY]

Le 18/10/2018 à 13:12, Jann Horn a écrit :
> 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?
> 

But access_ok() uses current->thread.addr_limit, while USER_DS may 
provide a larger segment:

#ifdef __powerpc64__
/* We use TASK_SIZE_USER64 as TASK_SIZE is not constant */
#define USER_DS		MAKE_MM_SEG(TASK_SIZE_USER64 - 1)
#else
#define USER_DS		MAKE_MM_SEG(TASK_SIZE - 1)
#endif

Christophe