Re: [PATCH] kasan: Fix Oops due to missing calls to kasan_arch_is_ready()

[Andrey Konovalov]

On Thu, Jan 26, 2023 at 8:08 AM Christophe Leroy
<christophe.leroy@xxxxxxxxxx> wrote:
>
> On powerpc64, you can build a kernel with KASAN as soon as you build it
> with RADIX MMU support. However if the CPU doesn't have RADIX MMU,
> KASAN isn't enabled at init and the following Oops is encountered.
>
>   [    0.000000][    T0] KASAN not enabled as it requires radix!
>
>   [    4.484295][   T26] BUG: Unable to handle kernel data access at 0xc00e000000804a04
>   [    4.485270][   T26] Faulting instruction address: 0xc00000000062ec6c
>   [    4.485748][   T26] Oops: Kernel access of bad area, sig: 11 [#1]
>   [    4.485920][   T26] BE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
>   [    4.486259][   T26] Modules linked in:
>   [    4.486637][   T26] CPU: 0 PID: 26 Comm: kworker/u2:2 Not tainted 6.2.0-rc3-02590-gf8a023b0a805 #249
>   [    4.486907][   T26] Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1200 0xf000005 of:SLOF,HEAD pSeries
>   [    4.487445][   T26] Workqueue: eval_map_wq .tracer_init_tracefs_work_func
>   [    4.488744][   T26] NIP:  c00000000062ec6c LR: c00000000062bb84 CTR: c0000000002ebcd0
>   [    4.488867][   T26] REGS: c0000000049175c0 TRAP: 0380   Not tainted  (6.2.0-rc3-02590-gf8a023b0a805)
>   [    4.489028][   T26] MSR:  8000000002009032 <SF,VEC,EE,ME,IR,DR,RI>  CR: 44002808  XER: 00000000
>   [    4.489584][   T26] CFAR: c00000000062bb80 IRQMASK: 0
>   [    4.489584][   T26] GPR00: c0000000005624d4 c000000004917860 c000000001cfc000 1800000000804a04
>   [    4.489584][   T26] GPR04: c0000000003a2650 0000000000000cc0 c00000000000d3d8 c00000000000d3d8
>   [    4.489584][   T26] GPR08: c0000000049175b0 a80e000000000000 0000000000000000 0000000017d78400
>   [    4.489584][   T26] GPR12: 0000000044002204 c000000003790000 c00000000435003c c0000000043f1c40
>   [    4.489584][   T26] GPR16: c0000000043f1c68 c0000000043501a0 c000000002106138 c0000000043f1c08
>   [    4.489584][   T26] GPR20: c0000000043f1c10 c0000000043f1c20 c000000004146c40 c000000002fdb7f8
>   [    4.489584][   T26] GPR24: c000000002fdb834 c000000003685e00 c000000004025030 c000000003522e90
>   [    4.489584][   T26] GPR28: 0000000000000cc0 c0000000003a2650 c000000004025020 c000000004025020
>   [    4.491201][   T26] NIP [c00000000062ec6c] .kasan_byte_accessible+0xc/0x20
>   [    4.491430][   T26] LR [c00000000062bb84] .__kasan_check_byte+0x24/0x90
>   [    4.491767][   T26] Call Trace:
>   [    4.491941][   T26] [c000000004917860] [c00000000062ae70] .__kasan_kmalloc+0xc0/0x110 (unreliable)
>   [    4.492270][   T26] [c0000000049178f0] [c0000000005624d4] .krealloc+0x54/0x1c0
>   [    4.492453][   T26] [c000000004917990] [c0000000003a2650] .create_trace_option_files+0x280/0x530
>   [    4.492613][   T26] [c000000004917a90] [c000000002050d90] .tracer_init_tracefs_work_func+0x274/0x2c0
>   [    4.492771][   T26] [c000000004917b40] [c0000000001f9948] .process_one_work+0x578/0x9f0
>   [    4.492927][   T26] [c000000004917c30] [c0000000001f9ebc] .worker_thread+0xfc/0x950
>   [    4.493084][   T26] [c000000004917d60] [c00000000020be84] .kthread+0x1a4/0x1b0
>   [    4.493232][   T26] [c000000004917e10] [c00000000000d3d8] .ret_from_kernel_thread+0x58/0x60
>   [    4.495642][   T26] Code: 60000000 7cc802a6 38a00000 4bfffc78 60000000 7cc802a6 38a00001 4bfffc68 60000000 3d20a80e 7863e8c2 792907c6 <7c6348ae> 20630007 78630fe0 68630001
>   [    4.496704][   T26] ---[ end trace 0000000000000000 ]---
>
> The Oops is due to kasan_byte_accessible() not checking the readiness
> of KASAN. Add missing call to kasan_arch_is_ready() and bail out when
> not ready. The same problem is observed with ____kasan_kfree_large()
> so fix it the same.
>
> Also, as KASAN is not available and no shadow area is allocated for
> linear memory mapping, there is no point in allocating shadow mem for
> vmalloc memory as shown below in /sys/kernel/debug/kernel_page_tables
>
>   ---[ kasan shadow mem start ]---
>   0xc00f000000000000-0xc00f00000006ffff  0x00000000040f0000       448K         r  w       pte  valid  present        dirty  accessed
>   0xc00f000000860000-0xc00f00000086ffff  0x000000000ac10000        64K         r  w       pte  valid  present        dirty  accessed
>   0xc00f3ffffffe0000-0xc00f3fffffffffff  0x0000000004d10000       128K         r  w       pte  valid  present        dirty  accessed
>   ---[ kasan shadow mem end ]---
>
> So, also verify KASAN readiness before allocating and poisoning
> shadow mem for VMAs.

Hi Cristophe,

Would it possible to unify kasan_arch_is_ready with the already
existing kasan_enabled check?

Both functions seem to be serving a similar purpose: for example this
patch adds kasan_arch_is_ready into __kasan_poison_vmalloc, which is
called by kasan_poison_vmalloc when kasan_enabled returns true.

The kasan_enabled is only implemented for HW_TAGS right now, but it
should be easy enough to make it work other cases by
kasan_flag_enabled into common.c and adding __wrappers for
shadow-related functions into include/linux/kasan.h. This way
architectures won't need to define their own static key and duplicate
the functionality.

I don't mind having this patch applied as is, considering that it's a
fix. However, if the unification that I mentioned is possible, that
would be a nice improvement.

Thanks!