strange kernel crash

Unread portion of the kernel message buffer:

Fatal trap 1: privileged instruction fault while in kernel mode
cpuid = 0; apic id = 00
instruction pointer     = 0x20:0xffffffff80619a1e
stack pointer           = 0x28:0xfffffe04f57856f0
frame pointer           = 0x28:0xfffffe04f57857b0
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 2658 (firefox)
trap number             = 1
panic: privileged instruction fault
cpuid = 0
curthread: 0xfffff803270b6000
stack: 0xfffffe04f5782000 - 0xfffffe04f5786000
stack pointer: 0xfffffe04f5785320
KDB: stack backtrace:
db_trace_self_wrapper() at 0xffffffff8041e86b = db_trace_self_wrapper+0x2b/frame
0xfffffe04f5785250
kdb_backtrace() at 0xffffffff80669f39 = kdb_backtrace+0x39/frame 0xfffffe04f5785300
vpanic() at 0xffffffff8063531c = vpanic+0x14c/frame 0xfffffe04f5785340
panic() at 0xffffffff80635063 = panic+0x43/frame 0xfffffe04f57853a0
trap_fatal() at 0xffffffff8081fc0f = trap_fatal+0x33f/frame 0xfffffe04f5785400
trap() at 0xffffffff8081f872 = trap+0x7d2/frame 0xfffffe04f5785610
trap_check() at 0xffffffff8081ff2a = trap_check+0x2a/frame 0xfffffe04f5785630
calltrap() at 0xffffffff80807ea0 = calltrap+0x8/frame 0xfffffe04f5785630
--- trap 0x1, rip = 0xffffffff80619a1e, rsp = 0xfffffe04f5785700, rbp =
0xfffffe04f57857b0 ---
__mtx_lock_flags() at 0xffffffff80619a1e = __mtx_lock_flags+0x2ee/frame
0xfffffe04f57857b0
uma_dbg_getslab() at 0xffffffff807df15c = uma_dbg_getslab+0x3c/frame
0xfffffe04f57857d0
uma_dbg_alloc() at 0xffffffff807df08d = uma_dbg_alloc+0x2d/frame 0xfffffe04f5785800
uma_zalloc_arg() at 0xffffffff807dacf1 = uma_zalloc_arg+0x4b1/frame
0xfffffe04f5785890
uma_zalloc() at 0xffffffff8068b040 = uma_zalloc+0x10/frame 0xfffffe04f57858a0
selfdalloc() at 0xffffffff8068aa12 = selfdalloc+0x22/frame 0xfffffe04f57858c0
pollscan() at 0xffffffff8068a615 = pollscan+0x95/frame 0xfffffe04f5785910
kern_poll() at 0xffffffff8068a4b1 = kern_poll+0x1f1/frame 0xfffffe04f5785a70
sys_poll() at 0xffffffff8068a2b9 = sys_poll+0x79/frame 0xfffffe04f5785a90
syscallenter() at 0xffffffff80820560 = syscallenter+0x320/frame 0xfffffe04f5785b00
amd64_syscall() at 0xffffffff8082012f = amd64_syscall+0x1f/frame 0xfffffe04f5785bf0
Xfast_syscall() at 0xffffffff8080818b = Xfast_syscall+0xfb/frame 0xfffffe04f5785bf0
--- syscall (209, FreeBSD ELF64, sys_poll), rip = 0x80146342a, rsp =
0x7fffffffd8e8, rbp = 0x7fffffffd920 ---
Uptime: 1d12h57m32s


Now the strange part:

   0xffffffff80619a18 <+744>:   jne    0xffffffff80619a61 <__mtx_lock_flags+817>
   0xffffffff80619a1a <+746>:   mov    %rbx,(%rsp)
=> 0xffffffff80619a1e <+750>:   movq   $0x0,0x18(%rsp)
   0xffffffff80619a27 <+759>:   movq   $0x0,0x10(%rsp)
   0xffffffff80619a30 <+768>:   movq   $0x0,0x8(%rsp)

RSP value seems to be sane and consistent with the stack information above:
(kgdb) i reg
rax            0x4      4
rbx            0xfffff80126ea54f0       -8791145163536
rcx            0xffffffff8099a600       -2137414144
rdx            0xfffff803270b6000       -8782553063424
rsi            0x4      4
rdi            0xfffff80027f41318       -8795422715112
rbp            0xfffffe04f57857b0       0xfffffe04f57857b0
rsp            0xfffffe04f5785700       0xfffffe04f5785700
r8             0xffffffff809a7727       -2137360601
r9             0xfffff80126ea54f0       -8791145163536
r10            0x3e8    1000
r11            0xfffffe04f5785cc0       -2177725080384
r12            0x1      1
r13            0xfffff803270b6000       -8782553063424
r14            0xfffff80027f41318       -8795422715112
r15            0x0      0
rip            0xffffffff80619a1e       0xffffffff80619a1e <__mtx_lock_flags+750>
eflags         0x10246  [ PF ZF IF RF ]
cs             0x20     32
ss             0x28     40
ds             <unavailable>
es             <unavailable>
fs             <unavailable>
gs             <unavailable>

(kgdb) x/a $rsp
0xfffffe04f5785700:     0xfffff80126ea54f0
(kgdb) x/a $rsp + 0x18
0xfffffe04f5785718:     0x0

I have no idea what could have caused the #GP.  This is certainly not a stack
overflow.

-- 
Andriy Gapon