Re: Odd-sized kmem_cache_alloc and slub_debug=Z

[Pekka Enberg]

Hi Marco,

On Fri, Aug 7, 2020 at 7:07 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> I found that the below debug-code using kmem_cache_alloc(), when using
> slub_debug=Z, results in the following crash:
>
>         general protection fault, probably for non-canonical address 0xcccccca41caea170: 0000 [#1] PREEMPT SMP PTI
>         CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.8.0+ #1
>         Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
>         RIP: 0010:freelist_dereference mm/slub.c:272 [inline]
>         RIP: 0010:get_freepointer mm/slub.c:278 [inline]
>         RIP: 0010:deactivate_slab+0x54/0x460 mm/slub.c:2111
>         Code: 8b bc c7 e0 00 00 00 48 85 d2 0f 84 00 01 00 00 49 89 d5 31 c0 48 89 44 24 08 66 66 2e 0f 1f 84 00 00 00 00 00 90 44 8b 43 20 <4b> 8b 44 05 00 48 85 c0 0f 84 1e 01 00 00 4c 89 ed 49 89 c5 8b 43
>         RSP: 0000:ffffffffa7e03e18 EFLAGS: 00010046
>         RAX: 0000000000000000 RBX: ffffa3a41c972340 RCX: 0000000000000000
>         RDX: cccccca41caea160 RSI: ffffe7c6a072ba80 RDI: ffffa3a41c972340
>         RBP: ffffa3a41caea008 R08: 0000000000000010 R09: ffffa3a41caea01d
>         R10: ffffffffa7f8dc50 R11: ffffffffa68f44c0 R12: ffffa3a41c972340
>         R13: cccccca41caea160 R14: ffffe7c6a072ba80 R15: ffffa3a41c96d540
>         FS:  0000000000000000(0000) GS:ffffa3a41fc00000(0000) knlGS:0000000000000000
>         CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>         CR2: ffffa3a051c01000 CR3: 000000045140a001 CR4: 0000000000770ef0
>         DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>         DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>         PKRU: 00000000
>         Call Trace:
>          ___slab_alloc+0x336/0x340 mm/slub.c:2690
>          __slab_alloc mm/slub.c:2714 [inline]
>          slab_alloc_node mm/slub.c:2788 [inline]
>          slab_alloc mm/slub.c:2832 [inline]
>          kmem_cache_alloc+0x135/0x200 mm/slub.c:2837
>          start_kernel+0x3d6/0x44e init/main.c:1049
>          secondary_startup_64+0xb6/0xc0 arch/x86/kernel/head_64.S:243
>
> Any ideas what might be wrong?
>
> This does not crash when redzones are not enabled.
>
> Thanks,
> -- Marco
>
> ------ >8 ------
>
> diff --git a/init/main.c b/init/main.c
> index 15bd0efff3df..f4aa5bb3f2ec 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1041,6 +1041,16 @@ asmlinkage __visible void __init start_kernel(void)
>         sfi_init_late();
>         kcsan_init();
>
> +       /* DEBUG CODE */
> +       {
> +               struct kmem_cache *c = kmem_cache_create("test", 21, 1, 0, NULL);
> +               char *buf;
> +               BUG_ON(!c);
> +               buf = kmem_cache_alloc(c, GFP_KERNEL);
> +               kmem_cache_free(c, buf);
> +               kmem_cache_destroy(c);
> +       }
> +
>         /* Do the rest non-__init'ed, we're now alive */
>         arch_call_rest_init();

Anything interesting in your .config? The fault does not reproduce
with 5.8.0 + x86-64 defconfig.

- Pekka