Borislav Petkov reported the following unwinder warning:

  WARNING: kernel stack regs at ffffc9000024fea8 in udevadm:92 has bad 'bp' value 00007fffc4614d30
  unwind stack type:0 next_sp:          (null) mask:0x6 graph_idx:0
  ffffc9000024fea8: 000055a6100e9b38 (0x55a6100e9b38)
  ffffc9000024feb0: 000055a6100e9b35 (0x55a6100e9b35)
  ffffc9000024feb8: 000055a6100e9f68 (0x55a6100e9f68)
  ffffc9000024fec0: 000055a6100e9f50 (0x55a6100e9f50)
  ffffc9000024fec8: 00007fffc4614d30 (0x7fffc4614d30)
  ffffc9000024fed0: 000055a6100eaf50 (0x55a6100eaf50)
  ffffc9000024fed8: 0000000000000000 ...
  ffffc9000024fee0: 0000000000000100 (0x100)
  ffffc9000024fee8: ffff8801187df488 (0xffff8801187df488)
  ffffc9000024fef0: 00007ffffffff000 (0x7ffffffff000)
  ffffc9000024fef8: 0000000000000000 ...
  ffffc9000024ff10: ffffc9000024fe98 (0xffffc9000024fe98)
  ffffc9000024ff18: 00007fffc4614d00 (0x7fffc4614d00)
  ffffc9000024ff20: ffffffffffffff10 (0xffffffffffffff10)
  ffffc9000024ff28: ffffffff811c6c1f (SyS_newlstat+0xf/0x10)
  ffffc9000024ff30: 0000000000000010 (0x10)
  ffffc9000024ff38: 0000000000000296 (0x296)
  ffffc9000024ff40: ffffc9000024ff50 (0xffffc9000024ff50)
  ffffc9000024ff48: 0000000000000018 (0x18)
  ffffc9000024ff50: ffffffff816b2e6a (entry_SYSCALL_64_fastpath+0x18/0xa8)
  ...

It unwinded from an interrupt which came in right after entry code
called into a C syscall handler, before it had a chance to set up the
frame pointer, so regs->bp still had its user space value.

Add a check to silence warnings in such a case, where an interrupt
has occurred and regs->sp is almost at the end of the stack.

Reported-by: Borislav Petkov <bp@suse.de>
Tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: c32c47c6 ("x86/unwind: Warn on bad frame pointer")
Link: http://lkml.kernel.org/r/c695f0d0d4c2cfe6542b90e2d0520e11eb901eb5.1493171120.git.jpoimboe@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>