arm64 KASAN_HW_TAGS panic on non-MTE hardware on 5.12-rc1

[Catalin Marinas]

Hi Andrey,

Enabling CONFIG_KASAN_HW_TAGS and running the resulting kernel on
non-MTE hardware panics with an undefined STG instruction from
mte_set_mem_tag_range():

./scripts/faddr2line vmlinux kasan_unpoison_task_stack+0x18/0x40
kasan_unpoison_task_stack+0x18/0x40:
mte_set_mem_tag_range at arch/arm64/include/asm/mte-kasan.h:71
(inlined by) mte_set_mem_tag_range at arch/arm64/include/asm/mte-kasan.h:56
(inlined by) kasan_unpoison at mm/kasan/kasan.h:363
(inlined by) kasan_unpoison_task_stack at mm/kasan/common.c:72

The full trace:

------------[ cut here ]------------
kernel BUG at arch/arm64/kernel/traps.c:406!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 5.12.0-rc1-00002-ge76afd1d69f3-dirty #2
pstate: 00000085 (nzcv daIf -PAN -UAO -TCO BTYPE=--)
pc : do_undefinstr+0x2c8/0x2e8
lr : do_undefinstr+0x2d4/0x2e8
sp : ffffc07baeaa3cf0
x29: ffffc07baeaa3cf0 x28: ffffc07baeab3280 
x27: ffffc07baeaa9a00 x26: ffffc07baeaa7000 
x25: ffffc07baeab3964 x24: ffffc07baeaa9c00 
x23: 0000000040000085 x22: ffffc07baed7f0e0 
x21: 00000000d9200800 x20: ffffc07baeab3280 
x19: ffffc07baeaa3d80 x18: 0000000000000200 
x17: 000000000000000b x16: 0000000000007fff 
x15: 00000000ffffffff x14: 0000000000000000 
x13: 0000000000000048 x12: ffffc07baeab3280 
x11: ffff64d0ffc00294 x10: 0000000000000000 
x9 : 0000000000000000 x8 : 00000000389fd980 
x7 : ffff64d0ffbde5b8 x6 : 0000000000000000 
x5 : ffff64d0ffb99880 x4 : ffffc07baeab5710 
x3 : ffffc07baed7f0f0 x2 : 0000000000000000 
x1 : ffffc07baeab3280 x0 : 0000000040000085 
Call trace:
 do_undefinstr+0x2c8/0x2e8
 el1_undef+0x30/0x50
 el1_sync_handler+0x8c/0xc8
 el1_sync+0x70/0x100
 kasan_unpoison_task_stack+0x18/0x40
 sched_init+0x390/0x3f0
 start_kernel+0x2cc/0x540
 0x0
Code: 17ffff8a f9401bf7 17ffffc8 f9001bf7 (d4210000) 
random: get_random_bytes called from print_oops_end_marker+0x2c/0x68 with crng_init=0
---[ end trace c881f708bdfe36c8 ]---

If MTE is not available, I thought we should not end up calling the MTE
backend but it seems that kasan expects the backend to skip the
undefined instructions.

Does kasan fall back to sw_tags if hw_tags are not available or it just
disables kasan altogether?

-- 
Catalin