[llvm-bugs] [Bug 37215] New: [CFI] llvm-cfi-verify can mark correctly protected vcalls FAIL_REGISTER_CLOBBERED

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37215

            Bug ID: 37215
           Summary: [CFI] llvm-cfi-verify can mark correctly protected
                    vcalls FAIL_REGISTER_CLOBBERED
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: vlad at tsyrklevich.net
                CC: kcc at google.com, llvm-bugs at lists.llvm.org,
                    peter at pcc.me.uk

Currently, llvm-cfi-verify's logic for determining whether a branch is
protected is fairly simple: it looks backwards from an indirect call and
verifies that every code path leading to that call has a branch to a ud2
instruction. One of the sanity checks that llvm-cfi-verify puts in place is
that it checks that the register could not have been modified between the
branch to ud2 and the call; however, depending on instruction ordering this can
happen when loading a pointer from a vtable. For example, the following is
fine:
$ cat bug.S
.global main

main:
  mov (%rax), %rbx
  ja fail

  call *%rbx

fail:
  ud2

$ clang -g -o bug bug.S && llvm-cfi-verify bug
-----------------------------------------------------
Instruction: 0x2010dd (PROTECTED):      callq   *%rbx 
  0x2010dd = bug.S:7:0 (frame_dummy)

Total Indirect CF Instructions: 1
Expected Protected: 1 (100.00%)
Unexpected Protected: 0 (0.00%)
Expected Unprotected: 0 (0.00%)
Unexpected Unprotected (BAD): 0 (0.00%)



However, the following fails:
$ cat bug.S
.global main

main:
  ja fail

  mov (%rax), %rbx
  call *%rbx

fail:
  ud2

$ clang -g -o bug bug.S && llvm-cfi-verify bug
-----------------------------------------------------
Instruction: 0x2010dd (FAIL_REGISTER_CLOBBERED):        callq   *%rbx 
  0x2010dd = bug.S:7:0 (frame_dummy)

Total Indirect CF Instructions: 1
Expected Protected: 0 (0.00%)
Unexpected Protected: 0 (0.00%)
Expected Unprotected: 0 (0.00%)
Unexpected Unprotected (BAD): 1 (100.00%)


CLOBBERED failures account for a quarter of Unexpected Unprotected failures in
Chrome. It's unclear to me if there's a simple way to fix this, to do it
correctly we would need to look backwards at the whole comparison logic and
ensure that the only allowed CLOBBER is derived from a memory load from
register checked in the comparison. As a simpler compromise, we could fix it by
 ensuring that there is only a single allowed clobber, and that it is an
explicit load from memory using a register as the base address.

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