Comment 4 for bug 1883962

After a few hours with the reproducer running on the original kernel,
the kernel errors about the reference count are observed:

Focal:
-----

$ uname -rv
5.4.0-38-generic #42-Ubuntu SMP Mon Jun 8 14:14:24 UTC 2020

$ ./aa-refcnt-af_alg
<a few hours later>

[ 9581.048189] ------------[ cut here ]------------
[ 9581.049497] refcount_t overflow at apparmor_sk_clone_security+0x35/0x70 in aa-refcnt-af_al[1023], uid/euid: 1000/1000
[ 9581.052125] WARNING: CPU: 1 PID: 1023 at kernel/panic.c:677 refcount_error_report+0x9b/0xab
[ 9581.054428] Modules linked in: ...
[ 9581.063137] CPU: 1 PID: 1023 Comm: aa-refcnt-af_al Tainted: G OE 5.4.0-38-generic #42-Ubuntu
[ 9581.065494] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 9581.067693] RIP: 0010:refcount_error_report+0x9b/0xab
...
[ 9581.088358] Call Trace:
[ 9581.089083] ex_handler_refcount+0x50/0x70
[ 9581.090147] fixup_exception+0x4a/0x61
[ 9581.091142] do_trap+0x4e/0xf0
[ 9581.091998] do_error_trap+0x7c/0xc0
[ 9581.092958] ? csum_partial_copy_generic+0x1687/0x3a10
[ 9581.094250] do_invalid_op+0x3c/0x50
[ 9581.095210] ? csum_partial_copy_generic+0x1687/0x3a10
[ 9581.096505] invalid_op+0x1e/0x30
[ 9581.097413] RIP: 0010:apparmor_sk_clone_security+0x35/0x70
...
[ 9581.113048] security_sk_clone+0x2f/0x40
[ 9581.114078] af_alg_accept+0x7e/0x190 [af_alg]
[ 9581.115456] alg_accept+0x15/0x20 [af_alg]
[ 9581.116549] __sys_accept4+0x109/0x210
[ 9581.117549] ? _cond_resched+0x19/0x30
[ 9581.118545] __x64_sys_accept+0x1c/0x20
[ 9581.119573] do_syscall_64+0x57/0x190
[ 9581.120551] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 9581.121821] RIP: 0033:0x7efc1bc390a7
...

Bionic:
------

$ uname -rv
4.15.0-107-generic #108-Ubuntu SMP Mon Jun 8 17:51:33 UTC 2020

$ ./aa-refcnt-af_alg
<a few hours later>

[ 8460.359291] ------------[ cut here ]------------
[ 8460.360638] refcount_t overflow at apparmor_sk_clone_security+0x37/0x70 in aa-refcnt-af_al[1243], uid/euid: 1000/1000
[ 8460.363332] WARNING: CPU: 1 PID: 1243 at /build/linux-oHXYZI/linux-4.15.0/kernel/panic.c:662 refcount_error_report+0x9c/0xac
[ 8460.366556] Modules linked in: ...
[ 8460.375936] CPU: 1 PID: 1243 Comm: aa-refcnt-af_al Tainted: G OE 4.15.0-107-generic #108-Ubuntu
[ 8460.378352] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 8460.380598] RIP: 0010:refcount_error_report+0x9c/0xac
...
[ 8460.397294] Call Trace:
[ 8460.398331] ex_handler_refcount+0x52/0x80
[ 8460.399432] fixup_exception+0x3a/0x50
[ 8460.400462] do_trap+0x8a/0x140
[ 8460.401346] do_error_trap+0xa6/0x140
[ 8460.402355] ? csum_partial_copy_generic+0xcfb/0x27a0
[ 8460.403671] ? ___slab_alloc+0x204/0x4f0
[ 8460.404730] ? ___slab_alloc+0x204/0x4f0
[ 8460.405786] ? get_empty_filp+0x5c/0x1c0
[ 8460.406840] do_invalid_op+0x20/0x30
[ 8460.407830] invalid_op+0x1b/0x40
[ 8460.408755] RIP: 0010:apparmor_sk_clone_security+0x37/0x70
...
[ 8460.420262] security_sk_clone+0x33/0x50
[ 8460.421314] af_alg_accept+0x81/0x1c0 [af_alg]
[ 8460.422484] ? aa_sock_accept_perm+0x25/0x30
[ 8460.423623] alg_accept+0x15/0x20 [af_alg]
[ 8460.424725] SYSC_accept4+0xff/0x210
[ 8460.425706] ? mntput+0x24/0x40
[ 8460.426598] ? __fput+0x193/0x220
[ 8460.427536] ? _cond_resched+0x19/0x40
[ 8460.428561] ? task_work_run+0x46/0xc0
[ 8460.429586] SyS_accept+0x10/0x20
[ 8460.430518] do_syscall_64+0x73/0x130
[ 8460.431522] entry_SYSCALL_64_after_hwframe+0x41/0xa6
[ 8460.432830] RIP: 0033:0x7f0ecc0c87e4
...