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

[Michael Ellerman]

Christophe LEROY <christophe.leroy at c-s.fr> writes:
> 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 ?

I wanted to check against USER_DS explicitly. But maybe that was
over-paranoid of me.

cheers