Re: [PATCH bpf] bpf: Fix kernel BUG in purge_effective_progs

[Andrii Nakryiko]

On Sat, Aug 13, 2022 at 6:11 AM Pu Lehui <pulehui@xxxxxxxxxx> wrote:
>
> Syzkaller reported kernel BUG as follows:
>
> ------------[ cut here ]------------
> kernel BUG at kernel/bpf/cgroup.c:925!
> invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> CPU: 1 PID: 194 Comm: detach Not tainted 5.19.0-14184-g69dac8e431af #8
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> RIP: 0010:__cgroup_bpf_detach+0x1f2/0x2a0
> Code: 00 e8 92 60 30 00 84 c0 75 d8 4c 89 e0 31 f6 85 f6 74 19 42 f6 84
> 28 48 05 00 00 02 75 0e 48 8b 80 c0 00 00 00 48 85 c0 75 e5 <0f> 0b 48
> 8b 0c5
> RSP: 0018:ffffc9000055bdb0 EFLAGS: 00000246
> RAX: 0000000000000000 RBX: ffff888100ec0800 RCX: ffffc900000f1000
> RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff888100ec4578
> RBP: 0000000000000000 R08: ffff888100ec0800 R09: 0000000000000040
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff888100ec4000
> R13: 000000000000000d R14: ffffc90000199000 R15: ffff888100effb00
> FS:  00007f68213d2b80(0000) GS:ffff88813bc80000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000055f74a0e5850 CR3: 0000000102836000 CR4: 00000000000006e0
> Call Trace:
>  <TASK>
>  cgroup_bpf_prog_detach+0xcc/0x100
>  __sys_bpf+0x2273/0x2a00
>  __x64_sys_bpf+0x17/0x20
>  do_syscall_64+0x3b/0x90
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f68214dbcb9
> Code: 08 44 89 e0 5b 41 5c c3 66 0f 1f 84 00 00 00 00 00 48 89 f8 48 89
> f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01
> f0 ff8
> RSP: 002b:00007ffeb487db68 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 000000000000000b RCX: 00007f68214dbcb9
> RDX: 0000000000000090 RSI: 00007ffeb487db70 RDI: 0000000000000009
> RBP: 0000000000000003 R08: 0000000000000012 R09: 0000000b00000003
> R10: 00007ffeb487db70 R11: 0000000000000246 R12: 00007ffeb487dc20
> R13: 0000000000000004 R14: 0000000000000001 R15: 000055f74a1011b0
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
>
> Repetition steps:
> For the following cgroup tree,
>
> root
>  |
> cg1
>  |
> cg2
>
> 1. attach prog2 to cg2, and then attach prog1 to cg1, both bpf progs
> attach type is NONE or OVERRIDE.
> 2. write 1 to /proc/thread-self/fail-nth for failslab.
> 3. detach prog1 for cg1, and then kernel BUG occur.
>
> Failslab injection will cause kmalloc fail and fall back to
> purge_effective_progs. The problem is that cg2 have attached another prog,
> so when go through cg2 layer, iteration will add pos to 1, and subsequent
> operations will be skipped by the following condition, and cg will meet
> NULL in the end.
>
> `if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI))`
>
> The NULL cg means no link or prog match, this is as expected, and it's not
> a bug. So here just skip the no match situation.
>
> Fixes: 4c46091ee985 ("bpf: Fix KASAN use-after-free Read in compute_effective_progs")
> Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
> ---
>  kernel/bpf/cgroup.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 59b7eb60d5b4..4a400cd63731 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -921,8 +921,10 @@ static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
>                                 pos++;
>                         }
>                 }
> +
> +               /* no link or prog match, skip the cgroup of this layer */
> +               continue;
>  found:
> -               BUG_ON(!cg);

I don't think it's necessary to remove this BUG_ON(), but it also
feels unnecessary for purge_effective_progs, so I don't mind it.

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

>                 progs = rcu_dereference_protected(
>                                 desc->bpf.effective[atype],
>                                 lockdep_is_held(&cgroup_mutex));
> --
> 2.25.1
>