powerpc/mm: Fix swapper_pg_dir size on 64-bit hash w/64K pages

[Michael Ellerman]

On Wed, 2017-04-12 at 10:11:07 UTC, Michael Ellerman wrote:
> Recently in commit f6eedbba7a26 ("powerpc/mm/hash: Increase VA range to 128TB"),
> we increased H_PGD_INDEX_SIZE to 15 when we're building with 64K pages. This
> makes it larger than RADIX_PGD_INDEX_SIZE (13), which means the logic to
> calculate MAX_PGD_INDEX_SIZE in book3s/64/pgtable.h is wrong.
> 
> The end result is that the PGD (Page Global Directory, ie top level page table)
> of the kernel (aka. swapper_pg_dir), is too small.
> 
> This generally doesn't lead to a crash, as we don't use the full range in normal
> operation. However if we try to dump the kernel pagetables we can trigger a
> crash because we walk off the end of the pgd into other memory and eventually
> try to dereference something bogus:
> 
>   $ cat /sys/kernel/debug/kernel_pagetables
>   Unable to handle kernel paging request for data at address 0xe8fece0000000000
>   Faulting instruction address: 0xc000000000072314
>   cpu 0xc: Vector: 380 (Data SLB Access) at [c0000000daa13890]
>       pc: c000000000072314: ptdump_show+0x164/0x430
>       lr: c000000000072550: ptdump_show+0x3a0/0x430
>      dar: e802cf0000000000
>   seq_read+0xf8/0x560
>   full_proxy_read+0x84/0xc0
>   __vfs_read+0x6c/0x1d0
>   vfs_read+0xbc/0x1b0
>   SyS_read+0x6c/0x110
>   system_call+0x38/0xfc
> 
> The root cause is that MAX_PGD_INDEX_SIZE isn't actually computed to be
> the max of H_PGD_INDEX_SIZE or RADIX_PGD_INDEX_SIZE. To fix that move
> the calculation into asm-offsets.c where we can do it easily using
> max().
> 
> Fixes: f6eedbba7a26 ("powerpc/mm/hash: Increase VA range to 128TB")
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/03dfee6d5f824d14e3ecb742518740

cheers