ipv6: Fix KASAN: slab-out-of-bounds Read in fib6_nh_flush_exceptions

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

ipv6: Fix KASAN: slab-out-of-bounds Read in fib6_nh_flush_exceptions

Reported by syzbot:
HEAD commit: 90c911ad Merge tag 'fixes' of git://git.kernel.org/pub/scm..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
dashboard link: https://syzkaller.appspot.com/bug?extid=123aa35098fd3c000eb7
compiler: Debian clang version 11.0.1-2

==================================================================
BUG: KASAN: slab-out-of-bounds in fib6_nh_get_excptn_bucket net/ipv6/route.c:1604 [inline]
BUG: KASAN: slab-out-of-bounds in fib6_nh_flush_exceptions+0xbd/0x360 net/ipv6/route.c:1732
Read of size 8 at addr ffff8880145c78f8 by task syz-executor.4/17760

CPU: 0 PID: 17760 Comm: syz-executor.4 Not tainted 5.12.0-rc8-syzkaller #0
Call Trace:
<IRQ>
__dump_stack lib/dump_stack.c:79 [inline]
dump_stack+0x202/0x31e lib/dump_stack.c:120
print_address_description+0x5f/0x3b0 mm/kasan/report.c:232
__kasan_report mm/kasan/report.c:399 [inline]
kasan_report+0x15c/0x200 mm/kasan/report.c:416
fib6_nh_get_excptn_bucket net/ipv6/route.c:1604 [inline]
fib6_nh_flush_exceptions+0xbd/0x360 net/ipv6/route.c:1732
fib6_nh_release+0x9a/0x430 net/ipv6/route.c:3536
fib6_info_destroy_rcu+0xcb/0x1c0 net/ipv6/ip6_fib.c:174
rcu_do_batch kernel/rcu/tree.c:2559 [inline]
rcu_core+0x8f6/0x1450 kernel/rcu/tree.c:2794
__do_softirq+0x372/0x7a6 kernel/softirq.c:345
invoke_softirq kernel/softirq.c:221 [inline]
__irq_exit_rcu+0x22c/0x260 kernel/softirq.c:422
irq_exit_rcu+0x5/0x20 kernel/softirq.c:434
sysvec_apic_timer_interrupt+0x91/0xb0 arch/x86/kernel/apic/apic.c:1100
</IRQ>
asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:632
RIP: 0010:lock_acquire+0x1f6/0x720 kernel/locking/lockdep.c:5515
Code: f6 84 24 a1 00 00 00 02 0f 85 8d 02 00 00 f7 c3 00 02 00 00 49 bd 00 00 00 00 00 fc ff df 74 01 fb 48 c7 44 24 40 0e 36 e0 45 <4b> c7 44 3d 00 00 00 00 00 4b c7 44 3d 09 00 00 00 00 43 c7 44 3d
RSP: 0018:ffffc90009e06560 EFLAGS: 00000206
RAX: 1ffff920013c0cc0 RBX: 0000000000000246 RCX: dffffc0000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90009e066e0 R08: dffffc0000000000 R09: fffffbfff1f992b1
R10: fffffbfff1f992b1 R11: 0000000000000000 R12: 0000000000000000
R13: dffffc0000000000 R14: 0000000000000000 R15: 1ffff920013c0cb4
rcu_lock_acquire+0x2a/0x30 include/linux/rcupdate.h:267
rcu_read_lock include/linux/rcupdate.h:656 [inline]
ext4_get_group_info+0xea/0x340 fs/ext4/ext4.h:3231
ext4_mb_prefetch+0x123/0x5d0 fs/ext4/mballoc.c:2212
ext4_mb_regular_allocator+0x8a5/0x28f0 fs/ext4/mballoc.c:2379
ext4_mb_new_blocks+0xc6e/0x24f0 fs/ext4/mballoc.c:4982
ext4_ext_map_blocks+0x2be3/0x7210 fs/ext4/extents.c:4238
ext4_map_blocks+0xab3/0x1cb0 fs/ext4/inode.c:638
ext4_getblk+0x187/0x6c0 fs/ext4/inode.c:848
ext4_bread+0x2a/0x1c0 fs/ext4/inode.c:900
ext4_append+0x1a4/0x360 fs/ext4/namei.c:67
ext4_init_new_dir+0x337/0xa10 fs/ext4/namei.c:2768
ext4_mkdir+0x4b8/0xc00 fs/ext4/namei.c:2814
vfs_mkdir+0x45b/0x640 fs/namei.c:3819
ovl_do_mkdir fs/overlayfs/overlayfs.h:161 [inline]
ovl_mkdir_real+0x53/0x1a0 fs/overlayfs/dir.c:146
ovl_create_real+0x280/0x490 fs/overlayfs/dir.c:193
ovl_workdir_create+0x425/0x600 fs/overlayfs/super.c:788
ovl_make_workdir+0xed/0x1140 fs/overlayfs/super.c:1355
ovl_get_workdir fs/overlayfs/super.c:1492 [inline]
ovl_fill_super+0x39ee/0x5370 fs/overlayfs/super.c:2035
mount_nodev+0x52/0xe0 fs/super.c:1413
legacy_get_tree+0xea/0x180 fs/fs_context.c:592
vfs_get_tree+0x86/0x270 fs/super.c:1497
do_new_mount fs/namespace.c:2903 [inline]
path_mount+0x196f/0x2be0 fs/namespace.c:3233
do_mount fs/namespace.c:3246 [inline]
__do_sys_mount fs/namespace.c:3454 [inline]
__se_sys_mount+0x2f9/0x3b0 fs/namespace.c:3431
do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x4665f9
Code: ff ff c3 66 2e 0f 1f 84
---truncated---