Add WRITE_OPD to check that you can't modify function
descriptors.
Gives the following result when function descriptors are
not protected:
lkdtm: Performing direct entry WRITE_OPD
lkdtm: attempting bad 16 bytes write at c00000000269b358
lkdtm: FAIL: survived bad write
lkdtm: do_nothing was hijacked!
Looks like a standard compiler barrier() is not enough to force
GCC to use the modified function descriptor. Had to add a fake empty
inline assembly to force GCC to reload the function descriptor.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7eeba50d16a35e9d799820e43304150225f20197.1644928018.git.christophe.leroy@csgroup.eu
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