assertion failed: pte_is_unmapped(pte)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 7a1e025a vmm: reimplement the x86 instruction decoder
git tree: akaros
console output: https://syzkaller.appspot.com/x/log.txt?x=1382272f600000
kernel config: https://syzkaller.appspot.com/x/.config?x=9b018fab5edd31b3
dashboard link: https://syzkaller.appspot.com/bug?extid=28ec6ca66d7b660fbf4d

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+28ec6c...@syzkaller.appspotmail.com

kernel panic at kern/src/process.c:504, from core 0: assertion failed:
pte_is_unmapped(pte)
Stack Backtrace on Core 0:
#01 [<0xffffffffc200a7fc>] in backtrace at src/kdebug.c:235
#02 [<0xffffffffc2009f95>] in _panic at src/init.c:275
#03 [<0xffffffffc20498f7>] in __cb_assert_no_pg at src/process.c:504
#04 [<0xffffffffc20b16c0>] in trampoline_cb.9642 at arch/x86/pmap64.c:539
#05 [<0xffffffffc20b1118>] in __pml_for_each at arch/x86/pmap64.c:349
#06 [<0xffffffffc20b10f3>] in __pml_for_each at arch/x86/pmap64.c:339
#07 [<0xffffffffc20b10f3>] in __pml_for_each at arch/x86/pmap64.c:339
#08 [<0xffffffffc20b10f3>] in __pml_for_each at arch/x86/pmap64.c:339
#09 [< [inline] >] in pml_for_each at arch/x86/pmap64.c:359
#09 [<0xffffffffc20b1d41>] in env_user_mem_walk at arch/x86/pmap64.c:546
#10 [<0xffffffffc2049801>] in __proc_free at src/process.c:551
#11 [<0xffffffffc204968b>] in kref_put at include/kref.h:70
#12 [<0xffffffffc204b870>] in proc_decref at src/process.c:587
#13 [<0xffffffffc205a1b8>] in sys_fork at src/syscall.c:916
#14 [<0xffffffffc2059f29>] in syscall at src/syscall.c:2580
#15 [<0xffffffffc205aad8>] in run_local_syscall at src/syscall.c:2617
#16 [<0xffffffffc205b019>] in prep_syscalls at src/syscall.c:2637
#17 [<0xffffffffc20b6282>] in sysenter_callwrapper at arch/x86/trap.c:926


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