Re: [PATCH bpf-next] bpf: dup xlated insns with kvmalloc+memcpy

[Hao Sun <sunhao.th@gmail.com>]

> On 16 Dec 2022, at 3:03 PM, Yonghong Song <yhs@meta.com> wrote:
> 
> 
> 
> On 12/15/22 9:54 PM, Hao Sun wrote:
>> Currently, kmemdup() is used for allocating and copying xlated insns
>> in bpf_insn_prepare_dump(). The following warning can be triggered
>> when dup large amount of insns (roughly BPF_COMPLEXITY_LIMIT_INSNS/2)
>> because kmemdup() uses kmalloc() which would fail when allocing size
>> is too big, leading to failure in dump xlated insns:
>> WARNING: CPU: 2 PID: 7060 at mm/page_alloc.c:5534
>> Call Trace:
>>  <TASK>
>>  __alloc_pages_node include/linux/gfp.h:237 [inline]
>>  alloc_pages_node include/linux/gfp.h:260 [inline]
>>  __kmalloc_large_node+0x81/0x160 mm/slab_common.c:1096
>>  __do_kmalloc_node mm/slab_common.c:943 [inline]
>>  __kmalloc_node_track_caller.cold+0x5/0x5d mm/slab_common.c:975
>>  kmemdup+0x29/0x60 mm/util.c:129
>>  kmemdup include/linux/fortify-string.h:585 [inline]
>>  bpf_insn_prepare_dump kernel/bpf/syscall.c:3820 [inline]
>>  bpf_prog_get_info_by_fd+0x9a3/0x2cb0 kernel/bpf/syscall.c:3975
>>  bpf_obj_get_info_by_fd kernel/bpf/syscall.c:4297 [inline]
>>  __sys_bpf+0x3928/0x56f0 kernel/bpf/syscall.c:5004
>>  __do_sys_bpf kernel/bpf/syscall.c:5069 [inline]
>>  __se_sys_bpf kernel/bpf/syscall.c:5067 [inline]
>>  ...
>> So use kvmalloc()+memcpy() to fix this, for small size of insns,
>> this is same as kmemdup(), but this also support dup large amount
>> of xlated insns.
>> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
>> ---
>>  kernel/bpf/syscall.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 35972afb6850..06229fddac0d 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -3831,10 +3831,10 @@ static struct bpf_insn *bpf_insn_prepare_dump(const struct bpf_prog *prog,
>>   u8 code;
>>   int i;
>>  - insns = kmemdup(prog->insnsi, bpf_prog_insn_size(prog),
>> - GFP_USER);
> 
> Does kmemdup(prog->insnsi, bpf_prog_insn_size(prog), GFP_USER | __GFP_NOWARN) work?

This only suppress the warning, bpf_insn_prepare_dump() still fails because of
the failure of kmalloc() invoked by kmemdup(). 

> 
>> - if (!insns)
>> + insns = kvmalloc(bpf_prog_insn_size(prog), GFP_USER | __GFP_NOWARN);
>> + if (unlikely(!insns))
>>   return insns;
>> + memcpy(insns, prog->insnsi, bpf_prog_insn_size(prog));
>>     for (i = 0; i < prog->len; i++) {
>>   code = insns[i].code;
>> @@ -3992,7 +3992,7 @@ static int bpf_prog_get_info_by_fd(struct file *file,
>>   uinsns = u64_to_user_ptr(info.xlated_prog_insns);
>>   ulen = min_t(u32, info.xlated_prog_len, ulen);
>>   fault = copy_to_user(uinsns, insns_sanitized, ulen);
>> - kfree(insns_sanitized);
>> + kvfree(insns_sanitized);
>>   if (fault)
>>   return -EFAULT;
>>   }
>> base-commit: 0e43662e61f2569500ab83b8188c065603530785