Re: [PATCH 01/13] bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Christophe Leroy wrote:
> 
> 
> Le 06/01/2022 à 12:45, Naveen N. Rao a écrit :
>> task_pt_regs() can return NULL on powerpc for kernel threads. This is
>> then used in __bpf_get_stack() to check for user mode, resulting in a
>> kernel oops. Guard against this by checking return value of
>> task_pt_regs() before trying to obtain the call chain.
> 
> I started looking at that some time ago, and I'm wondering whether it is 
> worth keeping that powerpc particularity.
> 
> We used to have a potentially different pt_regs depending on how we 
> entered kernel, especially on PPC32, but since the following commits it 
> is not the case anymore.
> 
> 06d67d54741a ("powerpc: make process.c suitable for both 32-bit and 64-bit")
> db297c3b07af ("powerpc/32: Don't save thread.regs on interrupt entry")
> b5cfc9cd7b04 ("powerpc/32: Fix critical and debug interrupts on BOOKE")
> 
> We could therefore just do like other architectures, define
> 
> #define task_pt_regs(p) ((struct pt_regs *)(THREAD_SIZE + 
> task_stack_page(p)) - 1)
> 
> And then remove the regs field we have in thread_struct.

Sure, I don't have an opinion on that, but I think this patch will still 
be needed for -stable.

> 
> 
>> 
>> Fixes: fa28dcb82a38f8 ("bpf: Introduce helper bpf_get_task_stack()")
>> Cc: stable@vger.kernel.org # v5.9+
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> ---
>>   kernel/bpf/stackmap.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 6e75bbee39f0b5..0dcaed4d3f4cec 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -525,13 +525,14 @@ BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf,
>>   	   u32, size, u64, flags)
>>   {
>>   	struct pt_regs *regs;
>> -	long res;
>> +	long res = -EINVAL;
>>   
>>   	if (!try_get_task_stack(task))
>>   		return -EFAULT;
>>   
>>   	regs = task_pt_regs(task);
>> -	res = __bpf_get_stack(regs, task, NULL, buf, size, flags);
>> +	if (regs)
>> +		res = __bpf_get_stack(regs, task, NULL, buf, size, flags);
> 
> Should there be a comment explaining that on powerpc, 'regs' can be NULL 
> for a kernel thread ?

I guess this won't be required if we end up with the change you are 
proposing above?


- Naveen