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

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Date:   Fri, 16 Dec 2022 16:24:31 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Hao Sun <sunhao.th@...il.com>, Yonghong Song <yhs@...a.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, jolsa@...nel.org,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: dup xlated insns with kvmalloc+memcpy

On 12/16/22 8:18 AM, Hao Sun wrote:
> 
> 
>> On 16 Dec 2022, at 3:03 PM, Yonghong Song <yhs@...a.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@...il.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().

Ok, instead of open coding, would be nice if we add a helper to mm/util.c :

void *kvmemdup(const void *src, size_t len, gfp_t gfp)
{
         void *p;

         p = kvmalloc(len, gfp);
         if (p)
                 memcpy(p, src, len);
         return p;
}
EXPORT_SYMBOL(kvmemdup);

And then bpf and in future others could make use of it.

Thanks,
Daniel