Re: [PATCH 3/3] arm64: mm: log potential KASAN shadow alias

[Andrey Konovalov]

On Thu, Dec 2, 2021 at 12:27 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> When the kernel is built with KASAN_GENERIC or KASAN_SW_TAGS, shadow
> memory is allocated and mapped for all legitimate kernel addresses, and
> prior to a regular memory access instrumentation will read from the
> corresponding shadow address.
>
> Due to the way memory addresses are converted to shadow addresses,
> bogus pointers (e.g. NULL) can generate shadow addresses out of the
> bounds of allocated shadow memory. For example, with KASAN_GENERIC and
> 48-bit VAs, NULL would have a shadow address of dfff800000000000, which
> falls between the TTBR ranges.
>
> To make such cases easier to debug, this patch makes die_kernel_fault()
> recover dump the real memory address range for any potential KASAN
> shadow access. Since we can't reliably distinguish shadow accesses from
> regular accesses, we always dump this information when shadow memory is
> in use.
>
> This makes it much easier to identify such cases, e.g.
>
> | Unable to handle kernel paging request at virtual address dfff800000000001
> | Possible KASAN shadow access for range [0000000000000008..000000000000000f]
> | Mem abort info:
> |   ESR = 0x96000004
> |   EC = 0x25: DABT (current EL), IL = 32 bits
> |   SET = 0, FnV = 0
> |   EA = 0, S1PTW = 0
> |   FSC = 0x04: level 0 translation fault
> | Data abort info:
> |   ISV = 0, ISS = 0x00000004
> |   CM = 0, WnR = 0
> | [dfff800000000001] address between user and kernel address ranges
> | Internal error: Oops: 96000004 [#1] PREEMPT SMP
> | CPU: 1 PID: 285 Comm: kworker/1:3 Not tainted 5.16.0-rc3-00005-g24a22db61d64 #3
> | Hardware name: linux,dummy-virt (DT)
> | Workqueue: events netlink_sock_destruct_work
> | pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : klist_iter_exit+0x2c/0x90
> | lr : klist_iter_exit+0x20/0x90
> | sp : ffff800011fd7a00
> | x29: ffff800011fd7a00 x28: 1fffe8d1812f1e03 x27: ffff468c0a124d40
> | x26: ffffa4783a9a4000 x25: ffff468c17666620 x24: 1fffe8d182ecccc4
> | x23: ffff468c17666608 x22: 0000000000000008 x21: ffffa4783f9754a0
> | x20: 0000000000000001 x19: 0000000000000000 x18: 0000000000000000
> | x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
> | x14: 1ffff000023faeea x13: ffff7000023faf33 x12: 1ffff000023faf32
> | x11: 1ffff000023faf32 x10: ffff7000023faf32 x9 : ffffa47838735d5c
> | x8 : ffff800011fd7997 x7 : 0000000000000001 x6 : ffff7000023faf33
> | x5 : 0000000000000000 x4 : 0000000000000000 x3 : ffff468c0a124d40
> | x2 : 0000000000000000 x1 : 0000000000000000 x0 : dfff800000000000
> | Call trace:
> |  klist_iter_exit+0x2c/0x90
> |  class_dev_iter_exit+0x28/0x38
> |  nfc_genl_dump_devices_done+0x44/0x68
> |  genl_lock_done+0xa4/0x128
> |  netlink_sock_destruct+0x1d4/0x280
> |  __sk_destruct+0x58/0x6a8
> |  sk_destruct+0xc0/0xe8
> |  __sk_free+0xd4/0x350
> |  sk_free+0x78/0x120
> |  netlink_sock_destruct_work+0x28/0x38
> |  process_one_work+0x8ac/0x1bd8
> |  worker_thread+0x3f0/0xc48
> |  kthread+0x3b4/0x460
> |  ret_from_fork+0x10/0x20
> | Code: 969c54cb d2d00000 d343fed4 f2fbffe0 (38e06a80)
> | ---[ end trace 78cc63aab52d9b7b ]---
>
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> ---
>  arch/arm64/mm/fault.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index b7b9caa41bc7..3ae84ab9f0fa 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -297,6 +297,12 @@ static void die_kernel_fault(const char *msg, unsigned long addr,
>         pr_alert("Unable to handle kernel %s at virtual address %016lx\n", msg,
>                  addr);
>
> +#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
> +       pr_alert("Possible KASAN shadow access for range [%016lx..%016lx]\n",
> +                (unsigned long)kasan_shadow_to_mem((void *)addr),
> +                (unsigned long)kasan_shadow_to_mem((void *)addr + 1) - 1);
> +#endif

Hi Mark,

There's the kasan_non_canonical_hook() function that's used on x86 for
the same purpose: adding clarity to GPF faults caused by KASAN shadow
accesses. Would it possible to reuse it for arm64?

Thanks!


> +
>         mem_abort_decode(esr);
>
>         show_pte(addr);
> --
> 2.30.2
>