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SubjectUBSAN undefined behavior in arch/x86/include/futex.h
Hi,

I encountered this bug in kernel 4.13-rc7 while fuzzing with Syzkaller:

================================================================================
UBSAN: Undefined behaviour in ./arch/x86/include/asm/futex.h:53:13
shift exponent -1 is negative
CPU: 0 PID: 8469 Comm: syz-executor2 Not tainted 4.13.0-rc7 #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
Ubuntu-1.8.2-1ubuntu1 04/01/2014
Call Trace:
__dump_stack lib/dump_stack.c:16 [inline]
dump_stack+0xf7/0x1ae lib/dump_stack.c:52
ubsan_epilogue+0x12/0x8f lib/ubsan.c:164
__ubsan_handle_shift_out_of_bounds+0x2b2/0x32c lib/ubsan.c:421
futex_atomic_op_inuser arch/x86/include/asm/futex.h:53 [inline]
futex_wake_op kernel/futex.c:1577 [inline]
do_futex+0x1a7f/0x2600 kernel/futex.c:3404
SYSC_futex kernel/futex.c:3454 [inline]
SyS_futex+0x285/0x380 kernel/futex.c:3422
entry_SYSCALL_64_fastpath+0x18/0xad
RIP: 0033:0x451e59
RSP: 002b:00007f6cbefecc08 EFLAGS: 00000216 ORIG_RAX: 00000000000000ca
RAX: ffffffffffffffda RBX: 0000000000718000 RCX: 0000000000451e59
RDX: 0000000000000000 RSI: 8000000000000005 RDI: 0000000020005000
RBP: 0000000000000046 R08: 0000000020005ffc R09: fffffffffffffffe
R10: 0000000020005ff0 R11: 0000000000000216 R12: 0000000000000000
R13: 00007ffe5d358caf R14: 00007f6cbefed9c0 R15: 0000000000000000
================================================================================


Here is the link to the repro program: https://pastebin.com/RDX3q4nM

Regards,
Shankara

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