Disable single stepping once we've stepped over the breakpoint (fix #223)

[0vercl0k]

This PR fixes a regression in the KVM backend that was introduced in v0.5.5 when implementing RIP traces.

The issue happens when the user has a breakpoint set-up that won't move execution; in that case wtf needs to step-over that breakpoint to carry on execution. To do that, it temporarily removes the breakpoint off of memory and will single-step this instruction. After the single-step, we receive a fault and we can figure out that the reason why we're getting this fault is because we were single-stepping over a breakpoint in which case we need to re-enable it, etc.

Because that single-step bit wasn't properly unset in that case, execution would carry on and re-enter with another single step instruction but this time the state didn't indicate that it was because we were performing a step-over, so wtf aborts.

Here is an illustration of the bug with the HEVD/KVM with logging on:

bubuntu:~/wtf/targets/hevd$ sudo ../../src/build/wtf run --name hevd --input ./inputs/A --backend kvm
...
Running ./inputs/A
kvm: exit_reason = KVM_EXIT_DEBUG @ 0x7ff6f5bb111e
kvm: Handling bp @ 0x7ff6f5bb111e
Hevd: Hello!
kvm: Disarming bp and turning on RFLAGS.TF (rip=0x7ff6f5bb111e)
kvm: Turning on RFLAGS.TF
kvm: exit_reason = KVM_EXIT_DEBUG @ 0x7ff83e2e6360
kvm: Received debug trap @ 0x7ff83e2e6360 <------------------------ first, expected trap
kvm: Resetting breakpoint @ 0xd37411e
kvm: Turning off RFLAGS.TF                              <------------------------ this was actually not done
kvm: exit_reason = KVM_EXIT_DEBUG @ 0x7ff83e2e6365
kvm: Received debug trap @ 0x7ff83e2e6365 <------------------------- second trap which is unexpected
Got into OnDebugTrap with LastBreakpointGpa_ = none
--------------------------------------------------
Run stats:
          Dirty pages: 53248 bytes, 13 pages, 0 MB
            UffdPages: 90112 bytes, 22 pages, 0 MB
              VMExits: 3
Instructions executed: 2
#1 cov: 0 exec/s: 0.0 lastcov: 0.0s crash: 0 timeout: 0 cr3: 0 uptime: 0.0s

Here is the expected output / the fixed version:

bubuntu:~/wtf/targets/hevd$ sudo ../../src/build/wtf run --name hevd --input ./inputs/A --backend kvm
...
Running ./inputs/A
kvm: exit_reason = KVM_EXIT_DEBUG @ 0x7ff6f5bb111e
kvm: Handling bp @ 0x7ff6f5bb111e
Hevd: This is a breakpoint executed before the first instruction :)
kvm: Disarming bp and will turn on single step (rip=0x7ff6f5bb111e)
kvm: Turning on SINGLESTEP
kvm: exit_reason = KVM_EXIT_DEBUG @ 0x7ff83e2e6360
kvm: Received debug trap @ 0x7ff83e2e6360
kvm: Resetting breakpoint @ 0xd37411e
kvm: Turning off SINGLESTEP
kvm: exit_reason = KVM_EXIT_DEBUG @ 0xfffff8046f122bb0
kvm: Handling bp @ 0xfffff8046f122bb0
Hevd: DbgPrintEx: [-] Invalid IOCTL Code: 0x%X
kvm: The bp handler ended up moving @rip from 0xfffff8046f122bb0 to 0xfffff8046ca955ec so no need to do the step-over dance
kvm: exit_reason = KVM_EXIT_DEBUG @ 0x7ff6f5bb1124
kvm: Handling bp @ 0x7ff6f5bb1124
Hevd: Back from kernel!
kvm: The bp handler asked us to stop so no need to do the step-over dance
--------------------------------------------------
Run stats:
          Dirty pages: 663552 bytes, 162 pages, 0 MB
            UffdPages: 684032 bytes, 167 pages, 0 MB
              VMExits: 4
Instructions executed: 6400
#1 cov: 0 exec/s: 0.0 lastcov: 0.0s crash: 0 timeout: 0 cr3: 0 uptime: 0.0s