f7c83bcbfaf5 ("net: xfrm: use __this_cpu_read per-cpu helper") added a __this_cpu_read() call inside ipcomp_alloc_tfms(). Since this call was introduced, the rules around per-cpu accessors have been tightened and __this_cpu_read() cannot be used in a preemptible context.

syzkaller reported this leading to the following kernel BUG while fuzzing sendmsg:

BUG: using __this_cpu_read() in preemptible [00000000] code: repro/3101 caller is ipcomp_init_state+0x185/0x990 CPU: 3 PID: 3101 Comm: repro Not tainted 4.16.0-rc4-00123-g86f84779d8e9 #154 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014 Call Trace: dump_stack+0xb9/0x115 check_preemption_disabled+0x1cb/0x1f0 ipcomp_init_state+0x185/0x990 ? __xfrm_init_state+0x876/0xc20 ? lock_downgrade+0x5e0/0x5e0 ipcomp4_init_state+0xaa/0x7c0 __xfrm_init_state+0x3eb/0xc20 xfrm_init_state+0x19/0x60 pfkey_add+0x20df/0x36f0 ? pfkey_broadcast+0x3dd/0x600 ? pfkey_sock_destruct+0x340/0x340 ? pfkey_seq_stop+0x80/0x80 ? __skb_clone+0x236/0x750 ? kmem_cache_alloc+0x1f6/0x260 ? pfkey_sock_destruct+0x340/0x340 ? pfkey_process+0x62a/0x6f0 pfkey_process+0x62a/0x6f0 ? pfkey_send_new_mapping+0x11c0/0x11c0 ? mutex_lock_io_nested+0x1390/0x1390 pfkey_sendmsg+0x383/0x750 ? dump_sp+0x430/0x430 sock_sendmsg+0xc0/0x100 ___sys_sendmsg+0x6c8/0x8b0 ? copy_msghdr_from_user+0x3b0/0x3b0 ? pagevec_lru_move_fn+0x144/0x1f0 ? find_held_lock+0x32/0x1c0 ? do_huge_pmd_anonymous_page+0xc43/0x11e0 ? lock_downgrade+0x5e0/0x5e0 ? get_kernel_page+0xb0/0xb0 ? _raw_spin_unlock+0x29/0x40 ? do_huge_pmd_anonymous_page+0x400/0x11e0 ? __handle_mm_fault+0x553/0x2460 ? __fget_light+0x163/0x1f0 ? __sys_sendmsg+0xc7/0x170 __sys_sendmsg+0xc7/0x170 ? SyS_shutdown+0x1a0/0x1a0 ? __do_page_fault+0x5a0/0xca0 ? lock_downgrade+0x5e0/0x5e0 SyS_sendmsg+0x27/0x40 ? __sys_sendmsg+0x170/0x170 do_syscall_64+0x19f/0x640 entry_SYSCALL_64_after_hwframe+0x42/0xb7 RIP: 0033:0x7f0ee73dfb79 RSP: 002b:00007ffe14fc15a8 EFLAGS: 00000207 ORIG_RAX: 000000000000002e RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f0ee73dfb79 RDX: 0000000000000000 RSI: 00000000208befc8 RDI: 0000000000000004 RBP: 00007ffe14fc15b0 R08: 00007ffe14fc15c0 R09: 00007ffe14fc15c0 R10: 0000000000000000 R11: 0000000000000207 R12: 0000000000400440 R13: 00007ffe14fc16b0 R14: 0000000000000000 R15: 0000000000000000

Cc: stable@vger.kernel.org Signed-off-by: Greg Hackmann ghackmann@google.com --- net/xfrm/xfrm_ipcomp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c index ccfdc7115a83..a00ec715aa46 100644 --- a/net/xfrm/xfrm_ipcomp.c +++ b/net/xfrm/xfrm_ipcomp.c @@ -283,7 +283,7 @@ static struct crypto_comp * __percpu *ipcomp_alloc_tfms(const char *alg_name) struct crypto_comp *tfm;

/* This can be any valid CPU ID so we don't need locking. */ - tfm = __this_cpu_read(*pos->tfms); + tfm = this_cpu_read(*pos->tfms);

if (!strcmp(crypto_comp_name(tfm), alg_name)) { pos->users++;