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

Roman Gushchin
2020-01-03T15:31:18Z

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!