riscv: Use READ_ONCE_NOCHECK in imprecise unwinding stack mode

When CONFIG_FRAME_POINTER is unset, the stack unwinding function
walk_stackframe randomly reads the stack and then, when KASAN is enabled,
it can lead to the following backtrace:

[    0.000000] ==================================================================
[    0.000000] BUG: KASAN: stack-out-of-bounds in walk_stackframe+0xa6/0x11a
[    0.000000] Read of size 8 at addr ffffffff81807c40 by task swapper/0
[    0.000000]
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.2.0-12919-g24203e6db61f #43
[    0.000000] Hardware name: riscv-virtio,qemu (DT)
[    0.000000] Call Trace:
[    0.000000] [<ffffffff80007ba8>] walk_stackframe+0x0/0x11a
[    0.000000] [<ffffffff80099ecc>] init_param_lock+0x26/0x2a
[    0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[    0.000000] [<ffffffff80c49c80>] dump_stack_lvl+0x22/0x36
[    0.000000] [<ffffffff80c3783e>] print_report+0x198/0x4a8
[    0.000000] [<ffffffff80099ecc>] init_param_lock+0x26/0x2a
[    0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[    0.000000] [<ffffffff8015f68a>] kasan_report+0x9a/0xc8
[    0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[    0.000000] [<ffffffff80007c4a>] walk_stackframe+0xa2/0x11a
[    0.000000] [<ffffffff8006e99c>] desc_make_final+0x80/0x84
[    0.000000] [<ffffffff8009a04e>] stack_trace_save+0x88/0xa6
[    0.000000] [<ffffffff80099fc2>] filter_irq_stacks+0x72/0x76
[    0.000000] [<ffffffff8006b95e>] devkmsg_read+0x32a/0x32e
[    0.000000] [<ffffffff8015ec16>] kasan_save_stack+0x28/0x52
[    0.000000] [<ffffffff8006e998>] desc_make_final+0x7c/0x84
[    0.000000] [<ffffffff8009a04a>] stack_trace_save+0x84/0xa6
[    0.000000] [<ffffffff8015ec52>] kasan_set_track+0x12/0x20
[    0.000000] [<ffffffff8015f22e>] __kasan_slab_alloc+0x58/0x5e
[    0.000000] [<ffffffff8015e7ea>] __kmem_cache_create+0x21e/0x39a
[    0.000000] [<ffffffff80e133ac>] create_boot_cache+0x70/0x9c
[    0.000000] [<ffffffff80e17ab2>] kmem_cache_init+0x6c/0x11e
[    0.000000] [<ffffffff80e00fd6>] mm_init+0xd8/0xfe
[    0.000000] [<ffffffff80e011d8>] start_kernel+0x190/0x3ca
[    0.000000]
[    0.000000] The buggy address belongs to stack of task swapper/0
[    0.000000]  and is located at offset 0 in frame:
[    0.000000]  stack_trace_save+0x0/0xa6
[    0.000000]
[    0.000000] This frame has 1 object:
[    0.000000]  [32, 56) 'c'
[    0.000000]
[    0.000000] The buggy address belongs to the physical page:
[    0.000000] page:(____ptrval____) refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x81a07
[    0.000000] flags: 0x1000(reserved|zone=0)
[    0.000000] raw: 0000000000001000 ff600003f1e3d150 ff600003f1e3d150 0000000000000000
[    0.000000] raw: 0000000000000000 0000000000000000 00000001ffffffff
[    0.000000] page dumped because: kasan: bad access detected
[    0.000000]
[    0.000000] Memory state around the buggy address:
[    0.000000]  ffffffff81807b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    0.000000]  ffffffff81807b80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    0.000000] >ffffffff81807c00: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 00 f3
[    0.000000]                                            ^
[    0.000000]  ffffffff81807c80: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
[    0.000000]  ffffffff81807d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    0.000000] ==================================================================

Fix that by using READ_ONCE_NOCHECK when reading the stack in imprecise
mode.

Fixes: 5d8544e2 ("RISC-V: Generic library routines and assembly")
Reported-by: Chathura Rajapaksha <chathura.abeyrathne.lk@gmail.com>
Link: https://lore.kernel.org/all/CAD7mqryDQCYyJ1gAmtMm8SASMWAQ4i103ptTb0f6Oda=tPY2=A@mail.gmail.com/Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230308091639.602024-1-alexghiti@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/stacktrace.c

@@ -101,7 +101,7 @@ void notrace walk_stackframe(struct task_struct *task,
 	while (!kstack_end(ksp)) {
 		if (__kernel_text_address(pc) && unlikely(!fn(arg, pc)))
 			break;
-		pc = (*ksp++) - 0x4;
+		pc = READ_ONCE_NOCHECK(*ksp++) - 0x4;
 	}
 }