Recently in commit f6eedbba7a ("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: f6eedbba7a ("powerpc/mm/hash: Increase VA range to 128TB")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
03dfee6d5f