Re: [PATCH net-next] net: dst: fix missing initialization of rt_uncached

[David Ahern <dsahern@kernel.org>]

On Tue, Apr 18, 2023 at 06:54:26PM +0200, Maxime Bizon wrote:
> xfrm_alloc_dst() followed by xfrm4_dst_destroy(), without a
> xfrm4_fill_dst() call in between, causes the following BUG:
> 
>  BUG: spinlock bad magic on CPU#0, fbxhostapd/732
>   lock: 0x890b7668, .magic: 890b7668, .owner: <none>/-1, .owner_cpu: 0
>  CPU: 0 PID: 732 Comm: fbxhostapd Not tainted 6.3.0-rc6-next-20230414-00613-ge8de66369925-dirty #9
>  Hardware name: Marvell Kirkwood (Flattened Device Tree)
>   unwind_backtrace from show_stack+0x10/0x14
>   show_stack from dump_stack_lvl+0x28/0x30
>   dump_stack_lvl from do_raw_spin_lock+0x20/0x80
>   do_raw_spin_lock from rt_del_uncached_list+0x30/0x64
>   rt_del_uncached_list from xfrm4_dst_destroy+0x3c/0xbc
>   xfrm4_dst_destroy from dst_destroy+0x5c/0xb0
>   dst_destroy from rcu_process_callbacks+0xc4/0xec
>   rcu_process_callbacks from __do_softirq+0xb4/0x22c
>   __do_softirq from call_with_stack+0x1c/0x24
>   call_with_stack from do_softirq+0x60/0x6c
>   do_softirq from __local_bh_enable_ip+0xa0/0xcc
> 
> Patch "net: dst: Prevent false sharing vs. dst_entry:: __refcnt" moved
> rt_uncached and rt_uncached_list fields from rtable struct to dst
> struct, so they are more zeroed by memset_after(xdst, 0, u.dst) in
> xfrm_alloc_dst().
> 
> Note that rt_uncached (list_head) was never properly initialized at
> alloc time, but xfrm[46]_dst_destroy() is written in such a way that
> it was not an issue thanks to the memset:
> 
> 	if (xdst->u.rt.dst.rt_uncached_list)
> 		rt_del_uncached_list(&xdst->u.rt);
> 
> The route code does it the other way around: rt_uncached_list is
> assumed to be valid IIF rt_uncached list_head is not empty:
> 
> void rt_del_uncached_list(struct rtable *rt)
> {
>         if (!list_empty(&rt->dst.rt_uncached)) {
>                 struct uncached_list *ul = rt->dst.rt_uncached_list;
> 
>                 spin_lock_bh(&ul->lock);
>                 list_del_init(&rt->dst.rt_uncached);
>                 spin_unlock_bh(&ul->lock);
>         }
> }
> 
> This patch adds mandatory rt_uncached list_head initialization in
> generic dst_init(), and adapt xfrm[46]_dst_destroy logic to match the
> rest of the code.
> 
> Fixes: d288a162dd1c ("net: dst: Prevent false sharing vs. dst_entry:: __refcnt")
> Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
> ---
>  net/core/dst.c          | 1 +
>  net/ipv4/xfrm4_policy.c | 4 +---
>  net/ipv6/route.c        | 1 -
>  net/ipv6/xfrm6_policy.c | 4 +---
>  4 files changed, 3 insertions(+), 7 deletions(-)
> 

This fixes the bug introduced by the Fixes commit, so:

Reviewed-by: David Ahern <dsahern@kernel.org>

uncached_list is defined twice  - net/ipv{4,6}/route.c.
Since uncached_list was moved from protocol local structs to dst_entry,
the definition for uncached_list should be moved there as well.