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

[Daniel Borkmann]

On 12/16/22 8:18 AM, Hao Sun wrote:
> > On 16 Dec 2022, at 3:03 PM, Yonghong Song <yhs@xxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >   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