netdev - Re: [Patch bpf-next] bpf: clear per_cpu pointers in bpf_prog_clone

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Date:   Wed, 17 Feb 2021 14:46:57 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Cong Wang <cong.wang@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [Patch bpf-next] bpf: clear per_cpu pointers in bpf_prog_clone_create()

On Wed, Feb 17, 2021 at 2:01 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 2/17/21 4:58 AM, Cong Wang wrote:
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > Pretty much similar to commit 1336c662474e
> > ("bpf: Clear per_cpu pointers during bpf_prog_realloc") we also need to
> > clear these two percpu pointers in bpf_prog_clone_create(), otherwise
> > would get a double free:
> >
> >   BUG: kernel NULL pointer dereference, address: 0000000000000000
> >   #PF: supervisor read access in kernel mode
> >   #PF: error_code(0x0000) - not-present page
> >   PGD 0 P4D 0
> >   Oops: 0000 [#1] SMP PTI
> >   CPU: 13 PID: 8140 Comm: kworker/13:247 Kdump: loaded Tainted: G         W   OE
> >  5.11.0-rc4.bm.1-amd64+ #1
> >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> >   test_bpf: #1 TXA
> >   Workqueue: events bpf_prog_free_deferred
> >   RIP: 0010:percpu_ref_get_many.constprop.97+0x42/0xf0
> >   Code: [...]
> >   RSP: 0018:ffffa6bce1f9bda0 EFLAGS: 00010002
> >   RAX: 0000000000000001 RBX: 0000000000000000 RCX: 00000000021dfc7b
> >   RDX: ffffffffae2eeb90 RSI: 867f92637e338da5 RDI: 0000000000000046
> >   RBP: ffffa6bce1f9bda8 R08: 0000000000000000 R09: 0000000000000001
> >   R10: 0000000000000046 R11: 0000000000000000 R12: 0000000000000280
> >   R13: 0000000000000000 R14: 0000000000000000 R15: ffff9b5f3ffdedc0
> >   FS:   0000000000000000(0000) GS:ffff9b5f2fb40000(0000) knlGS:0000000000000000
> >   CS:   0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >   CR2: 0000000000000000 CR3: 000000027c36c002 CR4: 00000000003706e0
> >   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >   Call Trace:
> >   refill_obj_stock+0x5e/0xd0
> >   free_percpu+0xee/0x550
> >   __bpf_prog_free+0x4d/0x60
> >   process_one_work+0x26a/0x590
> >   worker_thread+0x3c/0x390
> >   ? process_one_work+0x590/0x590
> >   kthread+0x130/0x150
> >   ? kthread_park+0x80/0x80
> >   ret_from_fork+0x1f/0x30
> >
> > This bug is 100% reproducible with test_kmod.sh.
> >
> > Reported-by: Jiang Wang <jiang.wang@...edance.com>
> > Fixes: 700d4796ef59 ("bpf: Optimize program stats")
> > Fixes: ca06f55b9002 ("bpf: Add per-program recursion prevention mechanism")
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > ---
> >   kernel/bpf/core.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 0ae015ad1e05..b0c11532e535 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1103,6 +1103,8 @@ static struct bpf_prog *bpf_prog_clone_create(struct bpf_prog *fp_other,
> >                * this still needs to be adapted.
> >                */
> >               memcpy(fp, fp_other, fp_other->pages * PAGE_SIZE);
> > +             fp_other->stats = NULL;
> > +             fp_other->active = NULL;
> >       }
> >
> >       return fp;
> >
>
> This is not correct. I presume if you enable blinding and stats, then this will still

Well, at least I ran all BPF selftests and found no crash. (Before my patch, the
crash happened 100%.)

> crash. The proper way to fix it is to NULL these pointers in bpf_prog_clone_free()
> since the clone can be promoted as the actual prog and the prog ptr released instead.
>

Not sure if I understand your point, but what I cleared is fp_other,
which is the original, not the clone. And of course, the original would
be overriden:

        tmp = bpf_jit_blind_constants(prog);
        if (IS_ERR(tmp))
                return orig_prog;
        if (tmp != prog) {
                tmp_blinded = true;
                prog = tmp;  // <=== HERE
        }

I think this is precisely why the crash does not happen after my patch.

However, it does seem to me patching bpf_prog_clone_free() is better,
as there would be no assumption on using the original. All I want to
say here is that both ways could fix the crash, which one is better is
arguable.

Thanks.