Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

[zhongbaisong <zhongbaisong@huawei.com>]

On 2022/11/2 0:45, Daniel Borkmann wrote:
> [ +kfence folks ]

+ cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov

Do you have any suggestions about this problem?

Thanks,

.

> 
> On 11/1/22 5:04 AM, Baisong Zhong wrote:
>> Recently, we got a syzkaller problem because of aarch64
>> alignment fault if KFENCE enabled.
>>
>> When the size from user bpf program is an odd number, like
>> 399, 407, etc, it will cause skb shard info's alignment access,
>> as seen below:
>>
>> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 
>> net/core/skbuff.c:1032
>>
>> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
>>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
>>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
>>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
>>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
>>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
>>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
>>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
>>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
>>   bpf_prog_d3839dd9068ceb51+0x80/0x330
>>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
>>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
>>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
>>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>>
>> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, 
>> cache=kmalloc-512
>>
>> allocated by task 15074 on cpu 0 at 1342.585390s:
>>   kmalloc include/linux/slab.h:568 [inline]
>>   kzalloc include/linux/slab.h:675 [inline]
>>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
>>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
>>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
>>
>> To fix the problem, we round up allocations with kmalloc_size_roundup()
>> so that build_skb()'s use of kize() is always alignment and no special
>> handling of the memory is needed by KFENCE.
>>
>> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
>> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
>> ---
>>   net/bpf/test_run.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>> index 13d578ce2a09..058b67108873 100644
>> --- a/net/bpf/test_run.c
>> +++ b/net/bpf/test_run.c
>> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr 
>> *kattr, u32 user_size,
>>       if (user_size > size)
>>           return ERR_PTR(-EMSGSIZE);
>> +    size = kmalloc_size_roundup(size);
>>       data = kzalloc(size + headroom + tailroom, GFP_USER);
> 
> The fact that you need to do this roundup on call sites feels broken, no?
> Was there some discussion / consensus that now all k*alloc() call sites
> would need to be fixed up? Couldn't this be done transparently in k*alloc()
> when KFENCE is enabled? I presume there may be lots of other such occasions
> in the kernel where similar issue triggers, fixing up all call-sites feels
> like ton of churn compared to api-internal, generic fix.
> 
>>       if (!data)
>>           return ERR_PTR(-ENOMEM);
>>
> 
> Thanks,
> Daniel
>