On Fri, Aug 07, 2020 at 06:06:27PM +0200, Marco Elver 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]
That really looks like more fun from my moving the freelist pointer...
> R13: cccccca41caea160 R14: ffffe7c6a072ba80 R15: ffffa3a41c96d540
Except that it's all cccc at the start, which doesn't look like "data"
nor the hardened freelist obfuscation.
> 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();
>
Which kernel version? Can you send your CONFIG too?
--
Kees Cook
