Re: lib/test_overflow.c causes WARNING and tainted kernel

[Randy Dunlap]

On 5/28/19 3:47 PM, Kees Cook wrote:
> On Mon, May 27, 2019 at 09:53:33AM +0200, Rasmus Villemoes wrote:
>> On 25/05/2019 17.33, Randy Dunlap wrote:
>>> On 3/13/19 7:53 PM, Kees Cook wrote:
>>>> Hi!
>>>>
>>>> On Wed, Mar 13, 2019 at 2:29 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>> This is v5.0-11053-gebc551f2b8f9, MAR-12 around 4:00pm PT.
>>>>>
>>>>> In the first test_kmalloc() in test_overflow_allocation():
>>>>>
>>>>> [54375.073895] test_overflow: ok: (s64)(0 << 63) == 0
>>>>> [54375.074228] WARNING: CPU: 2 PID: 5462 at ../mm/page_alloc.c:4584 __alloc_pages_nodemask+0x33f/0x540
>>>>> [...]
>>>>> [54375.079236] ---[ end trace 754acb68d8d1a1cb ]---
>>>>> [54375.079313] test_overflow: kmalloc detected saturation
>>>>
>>>> Yup! This is expected and operating as intended: it is exercising the
>>>> allocator's detection of insane allocation sizes. :)
>>>>
>>>> If we want to make it less noisy, perhaps we could add a global flag
>>>> the allocators could check before doing their WARNs?
>>>>
>>>> -Kees
>>>
>>> I didn't like that global flag idea.  I also don't like the kernel becoming
>>> tainted by this test.
>>
>> Me neither. Can't we pass __GFP_NOWARN from the testcases, perhaps with
>> a module parameter to opt-in to not pass that flag? That way one can
>> make the overflow module built-in (and thus run at boot) without
>> automatically tainting the kernel.
>>
>> The vmalloc cases do not take gfp_t, would they still cause a warning?
> 
> They still warn, but they don't seem to taint. I.e. this patch:
> 
> diff --git a/lib/test_overflow.c b/lib/test_overflow.c
> index fc680562d8b6..c922f0d86181 100644
> --- a/lib/test_overflow.c
> +++ b/lib/test_overflow.c
> @@ -486,11 +486,12 @@ static int __init test_overflow_shift(void)
>   * Deal with the various forms of allocator arguments. See comments above
>   * the DEFINE_TEST_ALLOC() instances for mapping of the "bits".
>   */
> -#define alloc010(alloc, arg, sz) alloc(sz, GFP_KERNEL)
> -#define alloc011(alloc, arg, sz) alloc(sz, GFP_KERNEL, NUMA_NO_NODE)
> +#define alloc_GFP	(GFP_KERNEL | __GFP_NOWARN)
> +#define alloc010(alloc, arg, sz) alloc(sz, alloc_GFP)
> +#define alloc011(alloc, arg, sz) alloc(sz, alloc_GFP, NUMA_NO_NODE)
>  #define alloc000(alloc, arg, sz) alloc(sz)
>  #define alloc001(alloc, arg, sz) alloc(sz, NUMA_NO_NODE)
> -#define alloc110(alloc, arg, sz) alloc(arg, sz, GFP_KERNEL)
> +#define alloc110(alloc, arg, sz) alloc(arg, sz, alloc_GFP | __GFP_NOWARN)
>  #define free0(free, arg, ptr)	 free(ptr)
>  #define free1(free, arg, ptr)	 free(arg, ptr)
>  
> will remove the tainting behavior but is still a bit "noisy". I can't
> find a way to pass __GFP_NOWARN to a vmalloc-based allocation, though.
> 
> Randy, is removing taint sufficient for you?

Yes it is.  Thanks.

>> BTW, I noticed that the 'wrap to 8K' depends on 64 bit and
>> pagesize==4096; for 32 bit the result is 20K, while if the pagesize is
>> 64K one gets 128K and 512K for 32/64 bit size_t, respectively. Don't
>> know if that's a problem, but it's easy enough to make it independent of
>> pagesize (just make it 9*4096 explicitly), and if we use 5 instead of 9
>> it also becomes independent of sizeof(size_t) (wrapping to 16K).
> 
> Ah! Yes, all excellent points. I've adjusted that too now. I'll send
> the result to Andrew.
> 
> Thanks!
> 


-- 
~Randy