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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - vtables are writable on Mac which affects the security of -fsanitize=cfi*"
href="https://llvm.org/bugs/show_bug.cgi?id=24782">24782</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>vtables are writable on Mac which affects the security of -fsanitize=cfi*
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
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<th>OS</th>
<td>MacOS X
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>peter@pcc.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>-fsanitize=cfi relies on vtables being read-only for its protection to work. We
should figure out why vtables are being mapped writable on Mac and either fix
it or create a workaround.
$ cat vt.cc
#include <stdio.h>
struct A {
A();
virtual void f();
};
A::A() {}
void A::f() {}
void call(A *a) {
a->f();
}
void takeover(void *) {
puts("hahaha");
}
int main() {
A a;
void **vt = *(void ***)&a;
printf("vt = %p\n", vt);
printf("main = %p\n", main);
*vt = (void*)takeover;
call(&a);
}
$ env DYLD_LIBRARY_PATH=~/src/llvm-build-rel/lib
~/src/llvm-build-rel/bin/clang++ vt.cc -isysroot $(xcrun -show-sdk-path)
-fsanitize=cfi -flto -Wl,-mllvm,-O1
$ ./a.out
vt = 0x104603030
main = 0x104602ee0
hahaha</pre>
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