List corruption from ipv6_route_seq_start

Hi everyone,

We've had a list corruption reported to us when using the 
/proc/net/ipv6_route file to read the routing information on the system 
on the 5.4.61 kernel.
 From the list pointers, it seems that the list_head in the fib6_walker 
has been reinitialized with INIT_LIST_HEAD() in 
ipv6_route_seq_setup_walk() while the walker was still on the 
fib6_walker list.

net->ipv6.fib6_walkers : 0xFFFFFFC013114DB0
next : 0xFFFFFF81E0899C88
prev : 0xFFFFFF81E0899C88

w->lh : 0xFFFFFF81E0899C88
next : 0xFFFFFF81E0899C88 // should be 0xFFFFFFC013114DB0
prev : 0xFFFFFF81E0899C88 // should be 0xFFFFFFC013114DB0

Looking over the seq_file operations for this, the only way I can see 
ipv6_route_seq_setup_walk() being called on a walker that has not been 
removed from the list with fib6_walker_unlink() is if 
ipv6_route_iter_active() returns false during ipv6_route_seq_stop(). As 
far as I can tell, this check is trying to assess if the walker has 
reached the end of the tree, and therefore no longer placed back on the 
fib6_walker list by ipv6_route_seq_next(), to avoid trying to double 
unlink the entry. This check seems to only be needed since 
fib6_unlink_walker() uses a plain list_del() call instead of 
list_del_init(), so simply checking list_empty(&w->lh) wouldn't 
correctly indicate if the walker had been unlinked previously.

At least from the semantics of the seq_file operations being used in 
seq_read(), it seems that there wouldn't be a reason to keep the walker 
around in the list after the completion of ipv6_route_seq_stop(), so I'm 
wondering if switching to this more direct check would be appropriate, 
as there seems to be some way that the current roundabout method is 
failing.

Call trace:
   __list_add_valid+0x74/0x90
   fib6_walker_link+0x78/0xb8
   ipv6_route_seq_start+0xec/0x138
   seq_read+0x18c/0x5b8
   proc_reg_read+0xa4/0x188
   __vfs_read+0x60/0x204
   vfs_read+0xa4/0x144

Thanks,
Sean