syzbot

Fatal trap 12: page fault while in kernel mode
cpuid = 1; apic id = 01
fault virtual address	= 0x30
fault code		= supervisor read data  , page not present
instruction pointer	= 0x20:0xffffffff8156dbda
stack pointer	        = 0x28:0xfffffe00212187a0
frame pointer	        = 0x28:0xfffffe00212187e0
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		= 759 (syz-executor3299822)
trap number		= 12
panic: page fault
cpuid = 1
time = 1552900717
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame 0xfffffe0021218400
vpanic() at vpanic+0x1e0/frame 0xfffffe0021218460
panic() at panic+0x43/frame 0xfffffe00212184c0
trap_fatal() at trap_fatal+0x4c6/frame 0xfffffe0021218540
trap_pfault() at trap_pfault+0x9f/frame 0xfffffe00212185b0
trap() at trap+0x44d/frame 0xfffffe00212186d0
calltrap() at calltrap+0x8/frame 0xfffffe00212186d0
--- trap 0xc, rip = 0xffffffff8156dbda, rsp = 0xfffffe00212187a0, rbp = 0xfffffe00212187e0 ---
vm_page_unhold_pages() at vm_page_unhold_pages+0x5a/frame 0xfffffe00212187e0
pipe_write() at pipe_write+0x16d8/frame 0xfffffe00212188b0
dofilewrite() at dofilewrite+0xfd/frame 0xfffffe0021218910
kern_writev() at kern_writev+0x66/frame 0xfffffe0021218950
sys_writev() at sys_writev+0x50/frame 0xfffffe0021218980
amd64_syscall() at amd64_syscall+0x436/frame 0xfffffe0021218ab0
fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe0021218ab0
--- syscall (0, FreeBSD ELF64, nosys), rip = 0x4575ba, rsp = 0x7fffdfffdf88, rbp = 0x6b5ac8 ---
KDB: enter: panic
[ thread pid 759 tid 100098 ]
Stopped at      kdb_enter+0x6a: movq    $0,kdb_why