On 05/12/2016 02:53 PM, Ralf Baechle wrote:
On Thu, May 12, 2016 at 12:46:37PM +0200, Florian Weimer wrote:
The GCC compile farm has a big-endian 64-bit MIPS box. The kernel is:
Linux erpro8-fsf1 3.14.10-er8mod-00013-ge0fe977 #1 SMP PREEMPT Wed Jan
14 12:33:22 PST 2015 mips64 GNU/Linux
Which is a vendor kernel for the EdgeRouter Pro-8.
/proc/cpuinfo reports:
system type : UBNT_E200 (CN6120p1.1-1000-NSP)
machine : Unknown
processor : 0
cpu model : Cavium Octeon II V0.1
While testing W^X (execmod, DEP, NX) stack enforcement, I noticed that once
I try to execute code off a non-executable page, I do not get a signal, but
the code appears to enter an infinite loop. The generated function starts
with a jump instruction to return to the caller, but instead, the program
counter does not seem to change at all.
“si” in GDB also hangs (but can be interrupted with ^C).
My test code is here:
https://pagure.io/execmod-tests
Is this a kernel bug or an issue with the silicon?
I see the test case uses mprotect to add PROT_EXEC after writing the code
to memory. I don't think mprotect however gives any guarantee that this
will make the I-cache coherent with the D-cache, that is that the CPU will
actually fetch and execute the instruction that were just written to memory.
For that you have to do something architecture specific such as dancing
around a fire waving a dead chicken. Or on MIPS call cacheflush(), see
the man page for details.
There is a fork between the write and the execute. It is somewhat
unlikely that that's not a barrier, but it did happen on POWER.
However, I can successfully execute code without the barrier, so this
whole thing goes in the wrong direction. :)
I have added it, just to be on the safe side.
For portability sake to some broken processors you should also ensure
that a 32 byte cache line is entirely filled with valid instructions by
padding the two test instructions with another six no-op (opcode 0).
Added as well.
The test case as it is guarantees this implicitly by using a freshly
allocated page but I thought I should mention it.
There are some tests that don't (the stack variable might be clobbered,
for example).
Anyway, neither change fixed things for me. Given the peculiar “si”
behavior in GDB, that's not entirely unexpected ...
Thanks,
Florian
