On Thu, Jun 22, 2017 at 04:52:51PM +1000, Michael Ellerman wrote:
> The SLB miss handler calls slb_allocate_realmode() in order to create an
> SLB entry for the faulting address. At the very start of that function
> we check that the faulting Effective Address (EA) is less than
> PGTABLE_RANGE (ignoring the region), ie. is it an address which could
> possibly fit in the virtual address space.
>
> For an EA which fails that test, we branch out of line (to label 8), but
> we still go on to create an SLB entry for the address. The SLB entry we
> create has a VSID of 0, which means it will never match anything in the
> hash table and so can't actually translate to a physical address.
>
> However that SLB entry will be inserted in the SLB, and so needs to be
> managed properly like any other SLB entry. In particular we need to
> insert the SLB entry in the SLB cache, so that it will be flushed when
> the process is descheduled.
>
> And that is where the bugs begin. The first bug is that slb_finish_load()
> uses cr7 to decide if it should insert the SLB entry into the SLB cache.
> When we come from the invalid EA case we don't set cr7, it just has some
> junk value from userspace. So we may or may not insert the SLB entry in
> the SLB cache. If we fail to insert it, we may then incorrectly leave it
> in the SLB when the process is descheduled.
>
> The second bug is that even if we do happen to add the entry to the SLB
> cache, we do not have enough bits in the SLB cache to remember the full
> ESID value for very large EAs.
>
> For example if a process branches to 0x788c545a18000000, that results in
> a 256MB SLB entry with an ESID of 0x788c545a1. But each entry in the SLB
> cache is only 32-bits, meaning we truncate the ESID to 0x88c545a1. This
> has the same effect as the first bug, we incorrectly leave the SLB entry
> in the SLB when the process is descheduled.
>
> When a process accesses an invalid EA it results in a SEGV signal being
> sent to the process, which typically results in the process being
> killed. Process death isn't instantaneous however, the process may catch
> the SEGV signal and continue somehow, or the kernel may start writing a
> core dump for the process, either of which means it's possible for the
> process to be preempted while its processing the SEGV but before it's
> been killed.
>
> If that happens, when the process is scheduled back onto the CPU we will
> allocate a new SLB entry for the NIP, which will insert a second entry
> into the SLB for the bad EA. Because we never flushed the original
> entry, due to either bug one or two, we now have two SLB entries that
> match the same EA.
>
> If another access is made to that EA, either by the process continuing
> after catching the SEGV, or by a second process accessing the same bad
> EA on the same CPU, we will trigger an SLB multi-hit machine check
> exception. This has been observed happening in the wild.
>
> The fix is when we hit the invalid EA case, we mark the SLB cache as
> being full. This causes us to not insert the truncated ESID into the SLB
> cache, and means when the process is switched out we will flush the
> entire SLB. Note that this works both for the original fault and for a
> subsequent call to slb_allocate_realmode() from switch_slb().
>
> Because we mark the SLB cache as full, it doesn't really matter what
> value is in cr7, but rather than leaving it as something random we set
> it to indicate the address was a kernel address. That also skips the
> attempt to insert it in the SLB cache which is a nice side effect.
>
> Another way to fix the bug would be to make the entries in the SLB cache
> wider, so that we don't truncate the ESID. However this would be a more
> intrusive change as it alters the size and layout of the paca.
>
> This bug was fixed in upstream by commit f0f558b131db ("powerpc/mm:
> Preserve CFAR value on SLB miss caused by access to bogus address"),
> which changed the way we handle a bad EA entirely removing this bug in
> the process.
>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Reviewed-by: Paul Mackerras <paulus@xxxxxxxxxx>
> ---
> arch/powerpc/mm/slb_low.S | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> Note this patch is not upstream. The bug fix was fixed differently in
> upstream prior to the bug being identified.
Now applied to 4.4 and 3.18-stable kernels, thanks,
greg k-h