[PATCH bpf] bpf: Fix kernel BUG in purge_effective_progs

[Pu Lehui]

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