efi/arm*: Only register page tables when they exist

Currently the arm/arm64 runtime code registers the runtime servies
pagetables with ptdump regardless of whether runtime services page
tables have been created.

As efi_mm.pgd is NULL in these cases, attempting to dump the efi page
tables results in a NULL pointer dereference in the ptdump code:

/sys/kernel/debug# cat efi_page_tables
[  479.522600] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[  479.522715] Mem abort info:
[  479.522764]   ESR = 0x96000006
[  479.522850]   Exception class = DABT (current EL), IL = 32 bits
[  479.522899]   SET = 0, FnV = 0
[  479.522937]   EA = 0, S1PTW = 0
[  479.528200] Data abort info:
[  479.528230]   ISV = 0, ISS = 0x00000006
[  479.528317]   CM = 0, WnR = 0
[  479.528317] user pgtable: 4k pages, 48-bit VAs, pgd = 0000000064ab0cb0
[  479.528449] [0000000000000000] *pgd=00000000fbbe4003, *pud=00000000fb66e003, *pmd=0000000000000000
[  479.528600] Internal error: Oops: 96000006 [#1] PREEMPT SMP
[  479.528664] Modules linked in:
[  479.528699] CPU: 0 PID: 2457 Comm: cat Not tainted 4.15.0-rc3-00065-g2ad2ee7ecb5c-dirty #7
[  479.528799] Hardware name: FVP Base (DT)
[  479.528899] pstate: 00400009 (nzcv daif +PAN -UAO)
[  479.528941] pc : walk_pgd.isra.1+0x20/0x1d0
[  479.529011] lr : ptdump_walk_pgd+0x30/0x50
[  479.529105] sp : ffff00000bf4bc20
[  479.529185] x29: ffff00000bf4bc20 x28: 0000ffff9d22e000
[  479.529271] x27: 0000000000020000 x26: ffff80007b4c63c0
[  479.529358] x25: 00000000014000c0 x24: ffff80007c098900
[  479.529445] x23: ffff00000bf4beb8 x22: 0000000000000000
[  479.529532] x21: ffff00000bf4bd70 x20: 0000000000000001
[  479.529618] x19: ffff00000bf4bcb0 x18: 0000000000000000
[  479.529760] x17: 000000000041a1c8 x16: ffff0000082139d8
[  479.529800] x15: 0000ffff9d3c6030 x14: 0000ffff9d2527f4
[  479.529924] x13: 00000000000003f3 x12: 0000000000000038
[  479.530000] x11: 0000000000000003 x10: 0101010101010101
[  479.530099] x9 : 0000000017e94050 x8 : 000000000000003f
[  479.530226] x7 : 0000000000000000 x6 : 0000000000000000
[  479.530313] x5 : 0000000000000001 x4 : 0000000000000000
[  479.530416] x3 : ffff000009069fd8 x2 : 0000000000000000
[  479.530500] x1 : 0000000000000000 x0 : 0000000000000000
[  479.530599] Process cat (pid: 2457, stack limit = 0x000000005d1b0e6f)
[  479.530660] Call trace:
[  479.530746]  walk_pgd.isra.1+0x20/0x1d0
[  479.530833]  ptdump_walk_pgd+0x30/0x50
[  479.530907]  ptdump_show+0x10/0x20
[  479.530920]  seq_read+0xc8/0x470
[  479.531023]  full_proxy_read+0x60/0x90
[  479.531100]  __vfs_read+0x18/0x100
[  479.531180]  vfs_read+0x88/0x160
[  479.531267]  SyS_read+0x48/0xb0
[  479.531299]  el0_svc_naked+0x20/0x24
[  479.531400] Code: 91400420 f90033a0 a90707a2 f9403fa0 (f9400000)
[  479.531499] ---[ end trace bfe8e28d8acb2b67 ]---
Segmentation fault

Let's avoid this problem by only registering the tables after their
successful creation, which is also less confusing when EFI runtime
services are not in use.

Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20180308080020.22828-2-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/arm-runtime.c

@@ -54,6 +54,9 @@ static struct ptdump_info efi_ptdump_info = {
 
 static int __init ptdump_init(void)
 {
+	if (!efi_enabled(EFI_RUNTIME_SERVICES))
+		return 0;
+
 	return ptdump_debugfs_register(&efi_ptdump_info, "efi_page_tables");
 }
 device_initcall(ptdump_init);