Subject: Re: [patch net] dpll: fix possible deadlock during netlink dump operation

From: Vadim Fedorenko <[email-masked]>
Date: Tue, 6 Feb 2024 14:10:05 +0000

On 06/02/2024 12:51, Jiri Pirko wrote:
> From: Jiri Pirko <[email-masked]>
> 
> Recently, I've been hitting following deadlock warning during dpll pin
> dump:
> 
> [52804.637962] ======================================================
> [52804.638536] WARNING: possible circular locking dependency detected
> [52804.639111] 6.8.0-rc2jiri+ #1 Not tainted
> [52804.639529] ------------------------------------------------------
> [52804.640104] python3/2984 is trying to acquire lock:
> [52804.640581] ffff88810e642678 (nlk_cb_mutex-GENERIC){+.+.}-{3:3}, at: netlink_dump+0xb3/0x780
> [52804.641417]
>                 but task is already holding lock:
> [52804.642010] ffffffff83bde4c8 (dpll_lock){+.+.}-{3:3}, at: dpll_lock_dumpit+0x13/0x20
> [52804.642747]
>                 which lock already depends on the new lock.
> 
> [52804.643551]
>                 the existing dependency chain (in reverse order) is:
> [52804.644259]
>                 -> #1 (dpll_lock){+.+.}-{3:3}:
> [52804.644836]        lock_acquire+0x174/0x3e0
> [52804.645271]        __mutex_lock+0x119/0x1150
> [52804.645723]        dpll_lock_dumpit+0x13/0x20
> [52804.646169]        genl_start+0x266/0x320
> [52804.646578]        __netlink_dump_start+0x321/0x450
> [52804.647056]        genl_family_rcv_msg_dumpit+0x155/0x1e0
> [52804.647575]        genl_rcv_msg+0x1ed/0x3b0
> [52804.648001]        netlink_rcv_skb+0xdc/0x210
> [52804.648440]        genl_rcv+0x24/0x40
> [52804.648831]        netlink_unicast+0x2f1/0x490
> [52804.649290]        netlink_sendmsg+0x36d/0x660
> [52804.649742]        __sock_sendmsg+0x73/0xc0
> [52804.650165]        __sys_sendto+0x184/0x210
> [52804.650597]        __x64_sys_sendto+0x72/0x80
> [52804.651045]        do_syscall_64+0x6f/0x140
> [52804.651474]        entry_SYSCALL_64_after_hwframe+0x46/0x4e
> [52804.652001]
>                 -> #0 (nlk_cb_mutex-GENERIC){+.+.}-{3:3}:
> [52804.652650]        check_prev_add+0x1ae/0x1280
> [52804.653107]        __lock_acquire+0x1ed3/0x29a0
> [52804.653559]        lock_acquire+0x174/0x3e0
> [52804.653984]        __mutex_lock+0x119/0x1150
> [52804.654423]        netlink_dump+0xb3/0x780
> [52804.654845]        __netlink_dump_start+0x389/0x450
> [52804.655321]        genl_family_rcv_msg_dumpit+0x155/0x1e0
> [52804.655842]        genl_rcv_msg+0x1ed/0x3b0
> [52804.656272]        netlink_rcv_skb+0xdc/0x210
> [52804.656721]        genl_rcv+0x24/0x40
> [52804.657119]        netlink_unicast+0x2f1/0x490
> [52804.657570]        netlink_sendmsg+0x36d/0x660
> [52804.658022]        __sock_sendmsg+0x73/0xc0
> [52804.658450]        __sys_sendto+0x184/0x210
> [52804.658877]        __x64_sys_sendto+0x72/0x80
> [52804.659322]        do_syscall_64+0x6f/0x140
> [52804.659752]        entry_SYSCALL_64_after_hwframe+0x46/0x4e
> [52804.660281]
>                 other info that might help us debug this:
> 
> [52804.661077]  Possible unsafe locking scenario:
> 
> [52804.661671]        CPU0                    CPU1
> [52804.662129]        ----                    ----
> [52804.662577]   lock(dpll_lock);
> [52804.662924]                                lock(nlk_cb_mutex-GENERIC);
> [52804.663538]                                lock(dpll_lock);
> [52804.664073]   lock(nlk_cb_mutex-GENERIC);
> [52804.664490]
> 
> The issue as follows: __netlink_dump_start() calls control->start(cb)
> with nlk->cb_mutex held. In control->start(cb) the dpll_lock is taken.
> Then nlk->cb_mutex is released and taken again in netlink_dump(), while
> dpll_lock still being held. That leads to ABBA deadlock when another
> CPU races with the same operation.
> 
> Fix this by moving dpll_lock taking into dumpit() callback which ensures
> correct lock taking order.
> 
> Fixes: 9d71b54b65b1 ("dpll: netlink: Add DPLL framework base functions")
> Signed-off-by: Jiri Pirko <[email-masked]>

Good catch, thanks!

Reviewed-by: Vadim Fedorenko <[email-masked]>

> ---
>   drivers/dpll/dpll_netlink.c | 20 ++++++--------------
>   drivers/dpll/dpll_nl.c      |  4 ----
>   2 files changed, 6 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/dpll/dpll_netlink.c b/drivers/dpll/dpll_netlink.c
> index 314bb3775465..4ca9ad16cd95 100644
> --- a/drivers/dpll/dpll_netlink.c
> +++ b/drivers/dpll/dpll_netlink.c
> @@ -1199,6 +1199,7 @@ int dpll_nl_pin_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
>   	unsigned long i;
>   	int ret = 0;
>   
> +	mutex_lock(&dpll_lock);
>   	xa_for_each_marked_start(&dpll_pin_xa, i, pin, DPLL_REGISTERED,
>   				 ctx->idx) {
>   		if (!dpll_pin_available(pin))
> @@ -1218,6 +1219,8 @@ int dpll_nl_pin_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
>   		}
>   		genlmsg_end(skb, hdr);
>   	}
> +	mutex_unlock(&dpll_lock);
> +
>   	if (ret == -EMSGSIZE) {
>   		ctx->idx = i;
>   		return skb->len;
> @@ -1373,6 +1376,7 @@ int dpll_nl_device_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
>   	unsigned long i;
>   	int ret = 0;
>   
> +	mutex_lock(&dpll_lock);
>   	xa_for_each_marked_start(&dpll_device_xa, i, dpll, DPLL_REGISTERED,
>   				 ctx->idx) {
>   		hdr = genlmsg_put(skb, NETLINK_CB(cb->skb).portid,
> @@ -1389,6 +1393,8 @@ int dpll_nl_device_get_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
>   		}
>   		genlmsg_end(skb, hdr);
>   	}
> +	mutex_unlock(&dpll_lock);
> +
>   	if (ret == -EMSGSIZE) {
>   		ctx->idx = i;
>   		return skb->len;
> @@ -1439,20 +1445,6 @@ dpll_unlock_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
>   	mutex_unlock(&dpll_lock);
>   }
>   
> -int dpll_lock_dumpit(struct netlink_callback *cb)
> -{
> -	mutex_lock(&dpll_lock);
> -
> -	return 0;
> -}
> -
> -int dpll_unlock_dumpit(struct netlink_callback *cb)
> -{
> -	mutex_unlock(&dpll_lock);
> -
> -	return 0;
> -}
> -
>   int dpll_pin_pre_doit(const struct genl_split_ops *ops, struct sk_buff *skb,
>   		      struct genl_info *info)
>   {
> diff --git a/drivers/dpll/dpll_nl.c b/drivers/dpll/dpll_nl.c
> index eaee5be7aa64..1e95f5397cfc 100644
> --- a/drivers/dpll/dpll_nl.c
> +++ b/drivers/dpll/dpll_nl.c
> @@ -95,9 +95,7 @@ static const struct genl_split_ops dpll_nl_ops[] = {
>   	},
>   	{
>   		.cmd	= DPLL_CMD_DEVICE_GET,
> -		.start	= dpll_lock_dumpit,
>   		.dumpit	= dpll_nl_device_get_dumpit,
> -		.done	= dpll_unlock_dumpit,
>   		.flags	= GENL_ADMIN_PERM | GENL_CMD_CAP_DUMP,
>   	},
>   	{
> @@ -129,9 +127,7 @@ static const struct genl_split_ops dpll_nl_ops[] = {
>   	},
>   	{
>   		.cmd		= DPLL_CMD_PIN_GET,
> -		.start		= dpll_lock_dumpit,
>   		.dumpit		= dpll_nl_pin_get_dumpit,
> -		.done		= dpll_unlock_dumpit,
>   		.policy		= dpll_pin_get_dump_nl_policy,
>   		.maxattr	= DPLL_A_PIN_ID,
>   		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DUMP,