There is a missing address check in both show_opcodes() callers.
show_opcodes() is mostly used by the kernel to print the raw
instruction bytes surrounding an instruction that generated an
unexpected exception; however, sometimes it is also used to
print userspace instructions.
Because the userspace address isn't checked against TASK_SIZE_MAX, if userspace
faults on a kernel address, the kernel can dump data from a user-controlled
address into dmesg.

show_opcodes() has two callers:
 - since commit ba54d856a9d8 (first in 4.18): show_signal_msg() shows userspace
   instructions when userspace e.g. segfaults
 - show_ip() is used when the kernel detects some sort of bug; this means that
   to trigger it, you need some way to at least trigger a WARN() or so

Repro for the first variant:

=========================
user@debian:~/segfault$ sudo grep core_pattern /proc/kallsyms
ffffffff9ae34180 D core_pattern
ffffffff9be99500 t _GLOBAL__sub_I_65535_1_core_pattern
ffffffff9bff2860 t _GLOBAL__sub_D_65535_0_core_pattern
user@debian:~/segfault$ cat segfault.c
int main(void) {
	void (*fn)(void) = (void*)0xffffffff9ae34180;
	fn();
}
user@debian:~/segfault$ gcc -o segfault segfault.c
user@debian:~/segfault$ ./segfault 
Segmentation fault
user@debian:~/segfault$ sudo dmesg | tail -n2
[19511.957855] segfault[2622]: segfault at ffffffff9ae34180 ip ffffffff9ae34180 sp 00007ffe0adf1568 error 15
[19511.962055] Code: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <63> 6f 72 65 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=========================

Note the "<63> 6f 72 65 00" - that's "core\0".

Repro for the second variant:

Patch the kernel like this to get an easy way to trigger a WARN() in the right
context:
=========================
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index b9123c497e0a..fab40edd4c9e 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -891,6 +891,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
                tsk->thread.trap_nr     = X86_TRAP_PF;
 
                force_sig_info_fault(SIGSEGV, si_code, address, tsk, pkey, 0);
+                WARN(1, "TESTING WARN()");
 
                return;
        }
=========================

Then run the same repro code as before (with the core_pattern address fixed up).
Result:
=========================
[  125.564041] segfault[1602]: segfault at ffffffff854340c0 ip ffffffff854340c0 sp 00007ffd4cc7a568 error 15
[  125.569923] Code: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <63> 6f 72 65 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  125.576859] ------------[ cut here ]------------
[  125.578406] TESTING WARN()
[  125.578439] WARNING: CPU: 6 PID: 1602 at arch/x86/mm/fault.c:894 __bad_area_nosemaphore+0x147/0x270
[  125.582172] Modules linked in: bpfilter
[  125.583394] CPU: 6 PID: 1602 Comm: segfault Tainted: G        W         4.18.0+ #108
[  125.585811] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  125.588410] RIP: 0010:__bad_area_nosemaphore+0x147/0x270
[  125.590078] Code: 48 89 d9 48 89 ea 44 89 e6 48 c7 83 30 0b 00 00 0e 00 00 00 bf 0b 00 00 00 e8 f5 eb ff ff 48 c7 c7 00 61 66 84 e8 79 11 05 00 <0f> 0b 48 83 c4 28 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 83 c4 28 4c
[  125.595779] RSP: 0018:ffff8801cb3b7e18 EFLAGS: 00010286
[  125.597426] RAX: 0000000000000000 RBX: ffff8801cbb9e000 RCX: 0000000000000000
[  125.599605] RDX: 0000000000000001 RSI: dffffc0000000000 RDI: ffffffff86678ea0
[  125.601800] RBP: ffffffff854340c0 R08: ffffed003d873ed5 R09: ffffed003d873ed5
[  125.603935] R10: 0000000000000001 R11: ffffed003d873ed4 R12: 0000000000000001
[  125.606113] R13: 0000000000000000 R14: 0000000000000015 R15: ffff8801cb3b7f58
[  125.608250] FS:  00007fe30d518700(0000) GS:ffff8801ec380000(0000) knlGS:0000000000000000
[  125.610608] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  125.612331] CR2: ffffffff854340c0 CR3: 00000001d563e001 CR4: 00000000003606e0
[  125.614470] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  125.616607] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  125.618736] Call Trace:
[  125.619475]  __do_page_fault+0x133/0x780
[  125.620646]  ? mm_fault_error+0x1b0/0x1b0
[  125.622236]  ? async_page_fault+0x8/0x30
[  125.623388]  async_page_fault+0x1e/0x30
[  125.624526] RIP: 0033:core_pattern+0x0/0x880
[  125.625786] Code: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <63> 6f 72 65 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  125.631208] RSP: 002b:00007ffd4cc7a568 EFLAGS: 00010202
[  125.632737] RAX: ffffffff854340c0 RBX: 0000000000000000 RCX: 0000000000000000
[  125.635039] RDX: 00007ffd4cc7a678 RSI: 00007ffd4cc7a668 RDI: 0000000000000001
[  125.637088] RBP: 00007ffd4cc7a580 R08: 0000562d395106f0 R09: 00007fe30d323cb0
[  125.639153] R10: 0000000000000000 R11: 00007fe30d0d23c0 R12: 0000562d39510530
[  125.641183] R13: 00007ffd4cc7a660 R14: 0000000000000000 R15: 0000000000000000
[  125.643221] ---[ end trace fb20716f9d6369bd ]---

=========================