[PATCH] Fix KASAN splats with DEBUG_WX

[Mark Rutland]

Booting a kernel built with both CONFIG_KASAN and CONFIG_DEBUG_WX
results in a stream of KASAN splats for stack-out-of-bounds accesses,
e.g.

==================================================================
BUG: KASAN: stack-out-of-bounds in note_page+0xd8/0x650 at addr ffff8009364ebdd0
Read of size 8 by task swapper/0/1
page:ffff7e0024d93ac0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x4000000000000000()
page dumped because: kasan: bad access detected
CPU: 2 PID: 1 Comm: swapper/0 Not tainted 4.9.0-rc3-00004-g25f7267 #77
Hardware name: ARM Juno development board (r1) (DT)
Call trace:
[<ffff20000808b2d8>] dump_backtrace+0x0/0x278
[<ffff20000808b564>] show_stack+0x14/0x20
[<ffff2000084e4e4c>] dump_stack+0xa4/0xc8
[<ffff200008256ee0>] kasan_report_error+0x4a8/0x4d0
[<ffff200008257330>] kasan_report+0x40/0x48
[<ffff200008255894>] __asan_load8+0x84/0x98
[<ffff2000080a0928>] note_page+0xd8/0x650
[<ffff2000080a0fb4>] walk_pgd.isra.1+0x114/0x220
[<ffff2000080a1248>] ptdump_check_wx+0xa8/0x118
[<ffff20000809ed40>] mark_rodata_ro+0x90/0xd0
[<ffff200008cafb88>] kernel_init+0x28/0x110
[<ffff200008083680>] ret_from_fork+0x10/0x50
Memory state around the buggy address:
 ffff8009364ebc80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8009364ebd00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8009364ebd80: 00 00 00 00 f1 f1 f1 f1 00 00 f4 f4 f2 f2 f2 f2
                                                 ^
 ffff8009364ebe00: 00 00 00 00 00 00 00 00 f3 f3 f3 f3 00 00 00 00
 ffff8009364ebe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

... this happens because note_page assumes that the marker array has at
least two elements (the latter of which may be the terminator), but the
marker array for ptdump_check_wx only contains one element. Thus we
dereference some garbage on the stack when looking at
marker[1].start_address.

Given we don't need the markers for the WX checks, we could modify
note_page to allow for a NULL marker array, but for now it's simpler to
add an entry to the ptdump_check_wx marker array, so let's do that. As
it's somewhat confusing to have two identical entries, we add an initial
entry with a start address of zero.

Reported-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
---
 arch/arm64/mm/dump.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/mm/dump.c b/arch/arm64/mm/dump.c
index ef8aca8..ca74a2a 100644
--- a/arch/arm64/mm/dump.c
+++ b/arch/arm64/mm/dump.c
@@ -383,6 +383,7 @@ void ptdump_check_wx(void)
 	struct pg_state st = {
 		.seq = NULL,
 		.marker = (struct addr_marker[]) {
+			{ 0, NULL},
 			{ -1, NULL},
 		},
 		.check_wx = true,
-- 
1.9.1