[PATCH] powerpc/kasan/book3s_64: warn when running with hash MMU

[Michael Ellerman]

Nathan Lynch <nathanl at linux.ibm.com> writes:
> kasan is known to crash at boot on book3s_64 with non-radix MMU. As
> noted in commit 41b7a347bf14 ("powerpc: Book3S 64-bit outline-only
> KASAN support"):
>
>   A kernel with CONFIG_KASAN=y will crash during boot on a machine
>   using HPT translation because not all the entry points to the
>   generic KASAN code are protected with a call to kasan_arch_is_ready().

I guess I thought there was some plan to fix that.

But maybe I'm misremembering. Looking now it's not entirely straight
forward with the way the headers are structured. So I guess I'm wrong
about that.

> Such crashes look like this:
>
>   BUG: Unable to handle kernel data access at 0xc00e00000308b100
>   Faulting instruction address: 0xc0000000006d0fcc
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
>   CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.0.0-rc5-02183-g3ab165dea2a2 #13
>   [...regs...]
>   NIP [c0000000006d0fcc] kasan_byte_accessible+0xc/0x20
>   LR [c0000000006cd9cc] __kasan_check_byte+0x2c/0xa0
>   Call Trace:
>   [c00000001688f930] [c00000001688f970] 0xc00000001688f970 (unreliable)
>   [c00000001688f970] [c0000000005f6a74] ksize+0x34/0xa0
>   [c00000001688f9a0] [c0000000024c03a8] __alloc_skb+0xd8/0x2d0
>   [c00000001688fa00] [c0000000003c48c0] audit_log_start+0x260/0x660
>   [c00000001688fb30] [c0000000003c50ec] audit_log+0x3c/0x70
>   [c00000001688fb60] [c00000000404590c] audit_init+0x188/0x1ac
>   [c00000001688fbe0] [c0000000000127e0] do_one_initcall+0xe0/0x610
>   [c00000001688fcd0] [c00000000400a1f0] kernel_init_freeable+0x4c0/0x574
>   [c00000001688fda0] [c000000000013450] kernel_init+0x30/0x1d0
>   [c00000001688fe10] [c00000000000cd54] ret_from_kernel_thread+0x5c/0x64
>
> If you look carefully enough at the full kernel output, you might
> notice this message, much earlier:
>
>   KASAN not enabled as it requires radix!
>
> But the eventual oops does not carry any indication that the real
> problem was detected early on and is a known limitation.
>
> Change init_book3s_64.c::kasan_init() to emit a warning backtrace and
> taint the kernel when not running on radix. When the kernel likely
> oopses later, the 'W' taint flag in the report should help minimize
> developer time spent trying to understand what's gone wrong.

Should we just panic() directly?

It's too earily in kasan_init(), nothing gets to the console.

But we have kasan_late_init() already, the patch below seems to work and
gives us:

  Booting Linux via __start() @ 0x0000000000400000 ...
  [    0.000000][    T0] hash-mmu: Page sizes from device-tree:
  [    0.000000][    T0] hash-mmu: base_shift=12: shift=12, sllp=0x0000, avpnm=0x00000000, tlbiel=1, penc=0
  [    0.000000][    T0] hash-mmu: base_shift=16: shift=16, sllp=0x0110, avpnm=0x00000000, tlbiel=1, penc=1
  [    0.000000][    T0] Using 1TB segments
  [    0.000000][    T0] hash-mmu: Initializing hash mmu with SLB
  [    0.000000][    T0] Linux version 6.0.0-rc2-00225-g8b68240ff7d7-dirty (michael at alpine1-p1) (gcc (Ubuntu 11.2.0-7ubuntu2) 11.2.0, GNU ld (GNU Binutils for Ubuntu) 2.37) #403 SMP Thu Oct 6 14:53:58 AEDT 2022
  [    0.000000][    T0] KASAN not enabled as it requires radix!
  ...
  [    0.000000][    T0] mem auto-init: stack:off, heap alloc:off, heap free:off
  [    0.000000][    T0] stackdepot hash table entries: 1048576 (order: 7, 8388608 bytes, linear)
  [    0.000000][    T0] Kernel panic - not syncing: KASAN enabled kernel can't boot with Hash MMU!
  [    0.000000][    T0] CPU: 0 PID: 0 Comm: swapper Not tainted 6.0.0-rc2-00225-g8b68240ff7d7-dirty #403
  [    0.000000][    T0] Hardware name: IBM pSeries (emulated by qemu) POWER8 (raw) 0x4d0200 0xf000004 of:SLOF,HEAD pSeries
  [    0.000000][    T0] Call Trace:
  [    0.000000][    T0] [c0000000031efd20] [c000000000bf5450] dump_stack_lvl+0x74/0xa8 (unreliable)
  [    0.000000][    T0] [c0000000031efd60] [c00000000017d160] panic+0x1b0/0x480
  [    0.000000][    T0] [c0000000031efe00] [c00000000201e028] kasan_late_init+0x30/0x34
  [    0.000000][    T0] [c0000000031efe60] [c000000002018c4c] mem_init+0xc8/0xec
  [    0.000000][    T0] [c0000000031efe90] [c000000002004538] mm_init+0x84/0xbc
  [    0.000000][    T0] [c0000000031eff00] [c0000000020047a0] start_kernel+0x1cc/0x4bc
  [    0.000000][    T0] [c0000000031eff90] [c00000000000d998] start_here_common+0x1c/0x84

cheers


diff --git a/arch/powerpc/mm/kasan/init_book3s_64.c b/arch/powerpc/mm/kasan/init_book3s_64.c
index 9300d641cf9a..cc1b4ee8be53 100644
--- a/arch/powerpc/mm/kasan/init_book3s_64.c
+++ b/arch/powerpc/mm/kasan/init_book3s_64.c
@@ -101,4 +101,8 @@ void __init kasan_init(void)
 
 void __init kasan_early_init(void) { }
 
-void __init kasan_late_init(void) { }
+void __init kasan_late_init(void)
+{
+	if (!radix_enabled())
+		panic("KASAN enabled kernel can't boot with Hash MMU!");
+}