Re: [PATCH bpf] bpf: cgroup: prevent out-of-order release of cgroup bpf

On Fri, Dec 27, 2019 at 01:50:34PM -0800, Roman Gushchin wrote:

Before commit 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself") cgroup bpf structures were released with corresponding cgroup structures. It guaranteed the hierarchical order of destruction: children were always first. It preserved attached programs from being released before their propagated copies.

But with cgroup auto-detachment there are no such guarantees anymore: cgroup bpf is released as soon as the cgroup is offline and there are no live associated sockets. It means that an attached program can be detached and released, while its propagated copy is still living in the cgroup subtree. This will obviously lead to an use-after-free bug.

To reproduce the issue the following script can be used:

#!/bin/bash

CGROOT=/sys/fs/cgroup

mkdir -p ${CGROOT}/A ${CGROOT}/B ${CGROOT}/A/C sleep 1

./test_cgrp2_attach ${CGROOT}/A egress & A_PID=$! ./test_cgrp2_attach ${CGROOT}/B egress & B_PID=$!

echo $$ > ${CGROOT}/A/C/cgroup.procs iperf -s & S_PID=$! iperf -c localhost -t 100 & C_PID=$!

sleep 1

echo $$ > ${CGROOT}/B/cgroup.procs echo ${S_PID} > ${CGROOT}/B/cgroup.procs echo ${C_PID} > ${CGROOT}/B/cgroup.procs

sleep 1

rmdir ${CGROOT}/A/C rmdir ${CGROOT}/A

sleep 1

kill -9 ${S_PID} ${C_PID} ${A_PID} ${B_PID}

test_cgrp2_attach is an example from samples/bpf with the following patch applied (required to close cgroup and bpf program file descriptors after attachment):

diff --git a/samples/bpf/test_cgrp2_attach.c b/samples/bpf/test_cgrp2_attach.c index 20fbd1241db3..7c7d0e91204d 100644 --- a/samples/bpf/test_cgrp2_attach.c +++ b/samples/bpf/test_cgrp2_attach.c @@ -111,6 +111,8 @@ static int attach_filter(int cg_fd, int type, int verdict) strerror(errno)); return EXIT_FAILURE; }

•   close(cg_fd);
  

•   close(prog_fd);
    while (1) {
            key = MAP_KEY_PACKETS;
            assert(bpf_map_lookup_elem(map_fd, &key, &pkt_cnt) == 0);
  
  

On the unpatched kernel the following stacktrace can be obtained:

[ 33.619799] BUG: unable to handle page fault for address: ffffbdb4801ab002 [ 33.620677] #PF: supervisor read access in kernel mode [ 33.621293] #PF: error_code(0x0000) - not-present page [ 33.621918] PGD 236d59067 P4D 236d59067 PUD 236d5c067 PMD 236d5d067 PTE 0 [ 33.622754] Oops: 0000 [#1] SMP NOPTI [ 33.623202] CPU: 0 PID: 601 Comm: iperf Not tainted 5.5.0-rc2+ #23 [ 33.623943] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.f4 [ 33.625545] RIP: 0010:__cgroup_bpf_run_filter_skb+0x29f/0x3d0 [ 33.626231] Code: f6 0f 84 3a 01 00 00 49 8d 47 30 31 db 48 89 44 24 30 48 8b 45 08 65 48 89 05 4d 9d e0 64 48 8b d [ 33.628431] RSP: 0018:ffffbdb4802ffa90 EFLAGS: 00010246 [ 33.629051] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000034 [ 33.629906] RDX: 0000000000000000 RSI: ffff9ddf9d7a0000 RDI: ffff9ddf9b97f1c0 [ 33.630761] RBP: ffff9ddf9d4899d0 R08: ffff9ddfb67ddd80 R09: 0000000000010000 [ 33.631616] R10: 0000000000000070 R11: ffffbdb4802ffde8 R12: ffff9ddf9ba858e0 [ 33.632463] R13: 0000000000000001 R14: ffffbdb4801ab000 R15: ffff9ddf9ba858e0 [ 33.633306] FS: 00007f9d15ed9700(0000) GS:ffff9ddfb7c00000(0000) knlGS:0000000000000000 [ 33.634262] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 33.634945] CR2: ffffbdb4801ab002 CR3: 000000021b94e000 CR4: 00000000003406f0 [ 33.635809] Call Trace: [ 33.636118] ? __cgroup_bpf_run_filter_skb+0x2bf/0x3d0 [ 33.636728] ? __switch_to_asm+0x40/0x70 [ 33.637196] ip_finish_output+0x68/0xa0 [ 33.637654] ip_output+0x76/0xf0 [ 33.638046] ? __ip_finish_output+0x1c0/0x1c0 [ 33.638576] __ip_queue_xmit+0x157/0x410 [ 33.639049] __tcp_transmit_skb+0x535/0xaf0 [ 33.639557] tcp_write_xmit+0x378/0x1190 [ 33.640049] ? _copy_from_iter_full+0x8d/0x260 [ 33.640592] tcp_sendmsg_locked+0x2a2/0xdc0 [ 33.641098] ? sock_has_perm+0x10/0xa0 [ 33.641574] tcp_sendmsg+0x28/0x40 [ 33.641985] sock_sendmsg+0x57/0x60 [ 33.642411] sock_write_iter+0x97/0x100 [ 33.642876] new_sync_write+0x1b6/0x1d0 [ 33.643339] vfs_write+0xb6/0x1a0 [ 33.643752] ksys_write+0xa7/0xe0 [ 33.644156] do_syscall_64+0x5b/0x1b0 [ 33.644605] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fix this by grabbing a reference to the bpf structure of each ancestor on the initialization of the cgroup bpf structure, and dropping the reference at the end of releasing the cgroup bpf structure.

This will restore the hierarchical order of cgroup bpf releasing, without adding any operations on hot paths.

Thanks to Josef Bacik for the debugging and the initial analysis of the problem.

Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself") Reported-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Roman Gushchin guro@fb.com Cc: Alexei Starovoitov ast@kernel.org Cc: stable@vger.kernel.org

A friendly ping!

Thanks!