On Wed, Feb 17, 2021 at 2:01 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 2/17/21 4:58 AM, Cong Wang wrote:
> > From: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > Fixes: 700d4796ef59 ("bpf: Optimize program stats")
> > Fixes: ca06f55b9002 ("bpf: Add per-program recursion prevention mechanism")
> > Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> > Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> > ---
> > 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.
