codetag: debug: handle existing CODETAG_EMPTY in mark_objexts_empty for slabobj_ext

In the Linux kernel, the following vulnerability has been resolved:

codetag: debug: handle existing CODETAG_EMPTY in mark_objexts_empty for slabobj_ext

When alloc_slab_obj_exts() fails and then later succeeds in allocating a
slab extension vector, it calls handle_failed_objexts_alloc() to mark all
objects in the vector as empty. As a result all objects in this slab
(slabA) will have their extensions set to CODETAG_EMPTY.

Later on if this slabA is used to allocate a slabobj_ext vector for
another slab (slabB), we end up with the slabB->obj_exts pointing to a
slabobj_ext vector that itself has a non-NULL slabobj_ext equal to
CODETAG_EMPTY. When slabB gets freed, free_slab_obj_exts() is called to
free slabB->obj_exts vector.

free_slab_obj_exts() calls mark_objexts_empty(slabB->obj_exts) which will
generate a warning because it expects slabobj_ext vectors to have a NULL
obj_ext, not CODETAG_EMPTY.

Modify mark_objexts_empty() to skip the warning and setting the obj_ext
value if it's already set to CODETAG_EMPTY.

To quickly detect this WARN, I modified the code from
WARN_ON(slab_exts[offs].ref.ct) to BUG_ON(slab_exts[offs].ref.ct == 1);

We then obtained this message:

[21630.898561] ------------[ cut here ]------------
[21630.898596] kernel BUG at mm/slub.c:2050!
[21630.898611] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
[21630.900372] Modules linked in: squashfs isofs vfio_iommu_type1
vhost_vsock vfio vhost_net vmw_vsock_virtio_transport_common vhost tap
vhost_iotlb iommufd vsock binfmt_misc nfsv3 nfs_acl nfs lockd grace
netfs tls rds dns_resolver tun brd overlay ntfs3 exfat btrfs
blake2b_generic xor xor_neon raid6_pq loop sctp ip6_udp_tunnel
udp_tunnel nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib
nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct
nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
nf_tables rfkill ip_set sunrpc vfat fat joydev sg sch_fq_codel nfnetlink
virtio_gpu sr_mod cdrom drm_client_lib virtio_dma_buf drm_shmem_helper
drm_kms_helper drm ghash_ce backlight virtio_net virtio_blk virtio_scsi
net_failover virtio_console failover virtio_mmio dm_mirror
dm_region_hash dm_log dm_multipath dm_mod fuse i2c_dev virtio_pci
virtio_pci_legacy_dev virtio_pci_modern_dev virtio virtio_ring autofs4
aes_neon_bs aes_ce_blk [last unloaded: hwpoison_inject]
[21630.909177] CPU: 3 UID: 0 PID: 3787 Comm: kylin-process-m Kdump:
loaded Tainted: G        W           6.18.0-rc1+ #74 PREEMPT(voluntary)
[21630.910495] Tainted: [W]=WARN
[21630.910867] Hardware name: QEMU KVM Virtual Machine, BIOS unknown
2/2/2022
[21630.911625] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS
BTYPE=--)
[21630.912392] pc : __free_slab+0x228/0x250
[21630.912868] lr : __free_slab+0x18c/0x250[21630.913334] sp :
ffff8000a02f73e0
[21630.913830] x29: ffff8000a02f73e0 x28: fffffdffc43fc800 x27:
ffff0000c0011c40
[21630.914677] x26: ffff0000c000cac0 x25: ffff00010fe5e5f0 x24:
ffff000102199b40
[21630.915469] x23: 0000000000000003 x22: 0000000000000003 x21:
ffff0000c0011c40
[21630.916259] x20: fffffdffc4086600 x19: fffffdffc43fc800 x18:
0000000000000000
[21630.917048] x17: 0000000000000000 x16: 0000000000000000 x15:
0000000000000000
[21630.917837] x14: 0000000000000000 x13: 0000000000000000 x12:
ffff70001405ee66
[21630.918640] x11: 1ffff0001405ee65 x10: ffff70001405ee65 x9 :
ffff800080a295dc
[21630.919442] x8 : ffff8000a02f7330 x7 : 0000000000000000 x6 :
0000000000003000
[21630.920232] x5 : 0000000024924925 x4 : 0000000000000001 x3 :
0000000000000007
[21630.921021] x2 : 0000000000001b40 x1 : 000000000000001f x0 :
0000000000000001
[21630.921810] Call trace:
[21630.922130]  __free_slab+0x228/0x250 (P)
[21630.922669]  free_slab+0x38/0x118
[21630.923079]  free_to_partial_list+0x1d4/0x340
[21630.923591]  __slab_free+0x24c/0x348
[21630.924024]  ___cache_free+0xf0/0x110
[21630.924468]  qlist_free_all+0x78/0x130
[21630.924922]  kasan_quarantine_reduce+0x11
---truncated---