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Subject[PATCH 5.0 126/137] flow_dissector: disable preemption around BPF calls
From: Eric Dumazet <>

[ Upstream commit b1c17a9a353878602fd5bfe9103e4afe5e9a3f96 ]

Various things in eBPF really require us to disable preemption
before running an eBPF program.

syzbot reported :

BUG: assuming atomic context at net/core/flow_dissector.c:737
in_atomic(): 0, irqs_disabled(): 0, pid: 24710, name: syz-executor.3
2 locks held by syz-executor.3/24710:
#0: 00000000e81a4bf1 (&tfile->napi_mutex){+.+.}, at: tun_get_user+0x168e/0x3ff0 drivers/net/tun.c:1850
#1: 00000000254afebd (rcu_read_lock){....}, at: __skb_flow_dissect+0x1e1/0x4bb0 net/core/flow_dissector.c:822
CPU: 1 PID: 24710 Comm: syz-executor.3 Not tainted 5.1.0+ #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x172/0x1f0 lib/dump_stack.c:113
__cant_sleep kernel/sched/core.c:6165 [inline]
__cant_sleep.cold+0xa3/0xbb kernel/sched/core.c:6142
bpf_flow_dissect+0xfe/0x390 net/core/flow_dissector.c:737
__skb_flow_dissect+0x362/0x4bb0 net/core/flow_dissector.c:853
skb_flow_dissect_flow_keys_basic include/linux/skbuff.h:1322 [inline]
skb_probe_transport_header include/linux/skbuff.h:2500 [inline]
skb_probe_transport_header include/linux/skbuff.h:2493 [inline]
tun_get_user+0x2cfe/0x3ff0 drivers/net/tun.c:1940
tun_chr_write_iter+0xbd/0x156 drivers/net/tun.c:2037
call_write_iter include/linux/fs.h:1872 [inline]
do_iter_readv_writev+0x5fd/0x900 fs/read_write.c:693
do_iter_write fs/read_write.c:970 [inline]
do_iter_write+0x184/0x610 fs/read_write.c:951
vfs_writev+0x1b3/0x2f0 fs/read_write.c:1015
do_writev+0x15b/0x330 fs/read_write.c:1058
__do_sys_writev fs/read_write.c:1131 [inline]
__se_sys_writev fs/read_write.c:1128 [inline]
__x64_sys_writev+0x75/0xb0 fs/read_write.c:1128
do_syscall_64+0x103/0x670 arch/x86/entry/common.c:298

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Signed-off-by: Eric Dumazet <>
Reported-by: syzbot <>
Cc: Petar Penkov <>
Cc: Stanislav Fomichev <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/core/flow_dissector.c | 3 +++
1 file changed, 3 insertions(+)

--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -786,7 +786,10 @@ bool __skb_flow_dissect(const struct sk_
flow_keys.thoff = nhoff;

bpf_compute_data_pointers((struct sk_buff *)skb);
+ preempt_disable();
result = BPF_PROG_RUN(attached, skb);
+ preempt_enable();

/* Restore state */
memcpy(cb, &cb_saved, sizeof(cb_saved));

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