Re: [PATCH 1/2] powerpc: Fix user data corruption with P9N DD2.1 VSX CI load workaround emulation

[Michael Ellerman <mpe@ellerman.id.au>]

Michael Neuling <mikey@neuling.org> writes:
> __get_user_atomic_128_aligned() stores to kaddr using stvx which is a
> VMX store instruction, hence kaddr must be 16 byte aligned otherwise
> the store won't occur as expected.
>
> Unfortunately when we call __get_user_atomic_128_aligned() in
> p9_hmi_special_emu(), the buffer we pass as kaddr (ie. vbuf) isn't
> guaranteed to be 16B aligned. This means that the write to vbuf in
> __get_user_atomic_128_aligned() has the bottom bits of the address
> truncated. This results in other local variables being
> overwritten. Also vbuf will not contain the correct data which results
> in the userspace emulation being wrong and hence user data corruption.
>
> In the past we've been mostly lucky as vbuf has ended up aligned but
> this is fragile and isn't always true. CONFIG_STACKPROTECTOR in
> particular can change the stack arrangement enough that our luck runs
> out.

Actually I'm yet to find a kernel with CONFIG_STACKPROTECTOR=n that is
vulnerable to the bug.

Turning on STACKPROTECTOR changes the order GCC allocates locals on the
stack, from bottom-up to top-down. That in conjunction with the 8 byte
stack canary means we end up with 8 bytes of space below the locals,
which misaligns vbuf.

But obviously other things can change the stack layout too, so no
guarantees that CONFIG_STACKPROTECTOR=n makes it safe.

cheers