Re: [PATCH] powerpc/powernv/idle: Restore IAMR after idle

[Thiago Jung Bauermann <bauerman@linux.ibm.com>]

Russell Currey <ruscur@russell.cc> writes:

> On Thu, 2019-02-07 at 15:08 +1000, Nicholas Piggin wrote:
>> Russell Currey's on February 6, 2019 4:28 pm:
>> > Without restoring the IAMR after idle, execution prevention on
>> > POWER9
>> > with Radix MMU is overwritten and the kernel can freely execute
>> > userspace without
>> > faulting.
>> >
>> > This is necessary when returning from any stop state that modifies
>> > user
>> > state, as well as hypervisor state.
>> >
>> > To test how this fails without this patch, load the lkdtm driver
>> > and
>> > do the following:
>> >
>> >    echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT
>> >
>> > which won't fault, then boot the kernel with powersave=off, where
>> > it
>> > will fault.  Applying this patch will fix this.
>> >
>> > Fixes: 3b10d0095a1e ("powerpc/mm/radix: Prevent kernel execution of
>> > user
>> > space")
>> > Cc: <stable@vger.kernel.org>
>> > Signed-off-by: Russell Currey <ruscur@russell.cc>
>>
>> Good catch and debugging. This really should be a quirk, we don't
>> want
>> to have to restore this thing on a thread switch.
>>
>> Can we put it under a CONFIG option if we're not using IAMR?
>
> I don't exactly know when we do or don't use the IAMR (since the only
> thing I've used it for is radix).  When wouldn't we care about
> restoring it on hash?

On hash it's used for memory protection keys (code is in
arch/powerpc/mm/pkeys.c). The kernel doesn't use protection keys, but
userspace apps may use it explicitly via specific syscalls
(pkey_alloc(), pkey_mprotect, pkey_free()).

Also, the kernel may use a protection key if the process does an
mmap(PROT_EXEC).

--
Thiago Jung Bauermann
IBM Linux Technology Center