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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [CFI] llvm-cfi-verify can mark correctly protected vcalls FAIL_REGISTER_CLOBBERED"
href="https://bugs.llvm.org/show_bug.cgi?id=37215">37215</a>
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<th>Summary</th>
<td>[CFI] llvm-cfi-verify can mark correctly protected vcalls FAIL_REGISTER_CLOBBERED
</td>
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<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>vlad@tsyrklevich.net
</td>
</tr>
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<th>CC</th>
<td>kcc@google.com, llvm-bugs@lists.llvm.org, peter@pcc.me.uk
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<p>
<div>
<pre>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.</pre>
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