[PATCH net v2] netlabel: fix RCU annotation for IPv4 options on socket creation

[PATCH net v2] netlabel: fix RCU annotation for IPv4 options on socket creation

[Davide Caratti]

Xiumei reports the following splat when netlabel and TCP socket are used:

 =============================
 WARNING: suspicious RCU usage
 6.9.0-rc2+ #637 Not tainted
 -----------------------------
 net/ipv4/cipso_ipv4.c:1880 suspicious rcu_dereference_protected() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by ncat/23333:
  #0: ffffffff906030c0 (rcu_read_lock){....}-{1:2}, at: netlbl_sock_setattr+0x25/0x1b0

 stack backtrace:
 CPU: 11 PID: 23333 Comm: ncat Kdump: loaded Not tainted 6.9.0-rc2+ #637
 Hardware name: Supermicro SYS-6027R-72RF/X9DRH-7TF/7F/iTF/iF, BIOS 3.0  07/26/2013
 Call Trace:
  <TASK>
  dump_stack_lvl+0xa9/0xc0
  lockdep_rcu_suspicious+0x117/0x190
  cipso_v4_sock_setattr+0x1ab/0x1b0
  netlbl_sock_setattr+0x13e/0x1b0
  selinux_netlbl_socket_post_create+0x3f/0x80
  selinux_socket_post_create+0x1a0/0x460
  security_socket_post_create+0x42/0x60
  __sock_create+0x342/0x3a0
  __sys_socket_create.part.22+0x42/0x70
  __sys_socket+0x37/0xb0
  __x64_sys_socket+0x16/0x20
  do_syscall_64+0x96/0x180
  ? do_user_addr_fault+0x68d/0xa30
  ? exc_page_fault+0x171/0x280
  ? asm_exc_page_fault+0x22/0x30
  entry_SYSCALL_64_after_hwframe+0x71/0x79
 RIP: 0033:0x7fbc0ca3fc1b
 Code: 73 01 c3 48 8b 0d 05 f2 1b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 29 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d d5 f1 1b 00 f7 d8 64 89 01 48
 RSP: 002b:00007fff18635208 EFLAGS: 00000246 ORIG_RAX: 0000000000000029
 RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fbc0ca3fc1b
 RDX: 0000000000000006 RSI: 0000000000000001 RDI: 0000000000000002
 RBP: 000055d24f80f8a0 R08: 0000000000000003 R09: 0000000000000001

R10: 0000000000020000 R11: 0000000000000246 R12: 000055d24f80f8a0
 R13: 0000000000000000 R14: 000055d24f80fb88 R15: 0000000000000000
  </TASK>

The current implementation of cipso_v4_sock_setattr() replaces IP options
under the assumption that the caller holds the socket lock; however, such
assumption is not true, nor needed, in selinux_socket_post_create() hook.

Let all callers of cipso_v4_sock_setattr() specify the "socket lock held"
condition, except selinux_socket_post_create() _ where such condition can
safely be set as true even without holding the socket lock.
While at it: use rcu_replace_pointer() instead of open coding, and remove
useless NULL check of 'old' before kfree_rcu(old, ...).

v2:
 - pass lockdep_sock_is_held() through a boolean variable in the stack
   (thanks Eric Dumazet, Paul Moore, Casey Schaufler)
 - use rcu_replace_pointer() instead of rcu_dereference_protected() +
   rcu_assign_pointer()
 - remove NULL check of 'old' before kfree_rcu()

Fixes: f6d8bd051c39 ("inet: add RCU protection to inet->opt")
Reported-by: Xiumei Mu <xmu@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
---
 include/net/cipso_ipv4.h     |  6 ++++--
 include/net/netlabel.h       |  6 ++++--
 net/ipv4/cipso_ipv4.c        | 13 ++++++-------
 net/netlabel/netlabel_kapi.c |  9 ++++++---
 security/selinux/netlabel.c  |  5 ++++-
 security/smack/smack_lsm.c   |  3 ++-
 6 files changed, 26 insertions(+), 16 deletions(-)

diff --git a/include/net/cipso_ipv4.h b/include/net/cipso_ipv4.h
index 53dd7d988a2d..5f8193a9808c 100644
--- a/include/net/cipso_ipv4.h
+++ b/include/net/cipso_ipv4.h
@@ -183,7 +183,8 @@ int cipso_v4_getattr(const unsigned char *cipso,
 		     struct netlbl_lsm_secattr *secattr);
 int cipso_v4_sock_setattr(struct sock *sk,
 			  const struct cipso_v4_doi *doi_def,
-			  const struct netlbl_lsm_secattr *secattr);
+			  const struct netlbl_lsm_secattr *secattr,
+			  bool slock_held);
 void cipso_v4_sock_delattr(struct sock *sk);
 int cipso_v4_sock_getattr(struct sock *sk, struct netlbl_lsm_secattr *secattr);
 int cipso_v4_req_setattr(struct request_sock *req,
@@ -214,7 +215,8 @@ static inline int cipso_v4_getattr(const unsigned char *cipso,
 
 static inline int cipso_v4_sock_setattr(struct sock *sk,
 				      const struct cipso_v4_doi *doi_def,
-				      const struct netlbl_lsm_secattr *secattr)
+				      const struct netlbl_lsm_secattr *secattr,
+				      bool slock_held)
 {
 	return -ENOSYS;
 }
diff --git a/include/net/netlabel.h b/include/net/netlabel.h
index f3ab0b8a4b18..6f098b618461 100644
--- a/include/net/netlabel.h
+++ b/include/net/netlabel.h
@@ -470,7 +470,8 @@ void netlbl_bitmap_setbit(unsigned char *bitmap, u32 bit, u8 state);
 int netlbl_enabled(void);
 int netlbl_sock_setattr(struct sock *sk,
 			u16 family,
-			const struct netlbl_lsm_secattr *secattr);
+			const struct netlbl_lsm_secattr *secattr,
+			bool slock_held);
 void netlbl_sock_delattr(struct sock *sk);
 int netlbl_sock_getattr(struct sock *sk,
 			struct netlbl_lsm_secattr *secattr);
@@ -614,7 +615,8 @@ static inline int netlbl_enabled(void)
 }
 static inline int netlbl_sock_setattr(struct sock *sk,
 				      u16 family,
-				      const struct netlbl_lsm_secattr *secattr)
+				      const struct netlbl_lsm_secattr *secattr,
+				      bool slock_held)
 {
 	return -ENOSYS;
 }
diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
index 8b17d83e5fde..c4ac704cbcc2 100644
--- a/net/ipv4/cipso_ipv4.c
+++ b/net/ipv4/cipso_ipv4.c
@@ -1815,6 +1815,7 @@ static int cipso_v4_genopt(unsigned char *buf, u32 buf_len,
  * @sk: the socket
  * @doi_def: the CIPSO DOI to use
  * @secattr: the specific security attributes of the socket
+ * @slock_held: true if caller holds the socket lock
  *
  * Description:
  * Set the CIPSO option on the given socket using the DOI definition and
@@ -1826,7 +1827,8 @@ static int cipso_v4_genopt(unsigned char *buf, u32 buf_len,
  */
 int cipso_v4_sock_setattr(struct sock *sk,
 			  const struct cipso_v4_doi *doi_def,
-			  const struct netlbl_lsm_secattr *secattr)
+			  const struct netlbl_lsm_secattr *secattr,
+			  bool slock_held)
 {
 	int ret_val = -EPERM;
 	unsigned char *buf = NULL;
@@ -1876,18 +1878,15 @@ int cipso_v4_sock_setattr(struct sock *sk,
 
 	sk_inet = inet_sk(sk);
 
-	old = rcu_dereference_protected(sk_inet->inet_opt,
-					lockdep_sock_is_held(sk));
+	old = rcu_replace_pointer(sk_inet->inet_opt, opt, slock_held);
 	if (inet_test_bit(IS_ICSK, sk)) {
 		sk_conn = inet_csk(sk);
 		if (old)
 			sk_conn->icsk_ext_hdr_len -= old->opt.optlen;
-		sk_conn->icsk_ext_hdr_len += opt->opt.optlen;
+		sk_conn->icsk_ext_hdr_len += opt_len;
 		sk_conn->icsk_sync_mss(sk, sk_conn->icsk_pmtu_cookie);
 	}
-	rcu_assign_pointer(sk_inet->inet_opt, opt);
-	if (old)
-		kfree_rcu(old, rcu);
+	kfree_rcu(old, rcu);
 
 	return 0;
 
diff --git a/net/netlabel/netlabel_kapi.c b/net/netlabel/netlabel_kapi.c
index 1ba4f58e1d35..0083749705c4 100644
--- a/net/netlabel/netlabel_kapi.c
+++ b/net/netlabel/netlabel_kapi.c
@@ -965,6 +965,7 @@ int netlbl_enabled(void)
  * @sk: the socket to label
  * @family: protocol family
  * @secattr: the security attributes
+ * @slock_held: true if caller holds the socket lock
  *
  * Description:
  * Attach the correct label to the given socket using the security attributes
@@ -977,7 +978,8 @@ int netlbl_enabled(void)
  */
 int netlbl_sock_setattr(struct sock *sk,
 			u16 family,
-			const struct netlbl_lsm_secattr *secattr)
+			const struct netlbl_lsm_secattr *secattr,
+			bool slock_held)
 {
 	int ret_val;
 	struct netlbl_dom_map *dom_entry;
@@ -997,7 +999,7 @@ int netlbl_sock_setattr(struct sock *sk,
 		case NETLBL_NLTYPE_CIPSOV4:
 			ret_val = cipso_v4_sock_setattr(sk,
 							dom_entry->def.cipso,
-							secattr);
+							secattr, slock_held);
 			break;
 		case NETLBL_NLTYPE_UNLABELED:
 			ret_val = 0;
@@ -1126,7 +1128,8 @@ int netlbl_conn_setattr(struct sock *sk,
 		switch (entry->type) {
 		case NETLBL_NLTYPE_CIPSOV4:
 			ret_val = cipso_v4_sock_setattr(sk,
-							entry->cipso, secattr);
+							entry->cipso, secattr,
+							lockdep_sock_is_held(sk));
 			break;
 		case NETLBL_NLTYPE_UNLABELED:
 			/* just delete the protocols we support for right now
diff --git a/security/selinux/netlabel.c b/security/selinux/netlabel.c
index 8f182800e412..55885634e880 100644
--- a/security/selinux/netlabel.c
+++ b/security/selinux/netlabel.c
@@ -402,7 +402,10 @@ int selinux_netlbl_socket_post_create(struct sock *sk, u16 family)
 	secattr = selinux_netlbl_sock_genattr(sk);
 	if (secattr == NULL)
 		return -ENOMEM;
-	rc = netlbl_sock_setattr(sk, family, secattr);
+	/* On socket creation, replacement of IP options is safe even if
+	 * the caller does not hold the socket lock.
+	 */
+	rc = netlbl_sock_setattr(sk, family, secattr, true);
 	switch (rc) {
 	case 0:
 		sksec->nlbl_state = NLBL_LABELED;
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 146667937811..1ab2125d352d 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -2565,7 +2565,8 @@ static int smack_netlbl_add(struct sock *sk)
 	local_bh_disable();
 	bh_lock_sock_nested(sk);
 
-	rc = netlbl_sock_setattr(sk, sk->sk_family, &skp->smk_netlabel);
+	rc = netlbl_sock_setattr(sk, sk->sk_family, &skp->smk_netlabel,
+				 lockdep_sock_is_held(sk));
 	switch (rc) {
 	case 0:
 		ssp->smk_state = SMK_NETLBL_LABELED;
-- 
2.44.0

Re: [PATCH v2] netlabel: fix RCU annotation for IPv4 options on socket creation

[Paul Moore]

On Apr 24, 2024 Davide Caratti <dcaratti@redhat.com> wrote:
> 
> Xiumei reports the following splat when netlabel and TCP socket are used:
> 
>  =============================
>  WARNING: suspicious RCU usage
>  6.9.0-rc2+ #637 Not tainted
>  -----------------------------
>  net/ipv4/cipso_ipv4.c:1880 suspicious rcu_dereference_protected() usage!
> 
>  other info that might help us debug this:
> 
>  rcu_scheduler_active = 2, debug_locks = 1
>  1 lock held by ncat/23333:
>   #0: ffffffff906030c0 (rcu_read_lock){....}-{1:2}, at: netlbl_sock_setattr+0x25/0x1b0
> 
>  stack backtrace:
>  CPU: 11 PID: 23333 Comm: ncat Kdump: loaded Not tainted 6.9.0-rc2+ #637
>  Hardware name: Supermicro SYS-6027R-72RF/X9DRH-7TF/7F/iTF/iF, BIOS 3.0  07/26/2013
>  Call Trace:
>   <TASK>
>   dump_stack_lvl+0xa9/0xc0
>   lockdep_rcu_suspicious+0x117/0x190
>   cipso_v4_sock_setattr+0x1ab/0x1b0
>   netlbl_sock_setattr+0x13e/0x1b0
>   selinux_netlbl_socket_post_create+0x3f/0x80
>   selinux_socket_post_create+0x1a0/0x460
>   security_socket_post_create+0x42/0x60
>   __sock_create+0x342/0x3a0
>   __sys_socket_create.part.22+0x42/0x70
>   __sys_socket+0x37/0xb0
>   __x64_sys_socket+0x16/0x20
>   do_syscall_64+0x96/0x180
>   ? do_user_addr_fault+0x68d/0xa30
>   ? exc_page_fault+0x171/0x280
>   ? asm_exc_page_fault+0x22/0x30
>   entry_SYSCALL_64_after_hwframe+0x71/0x79
>  RIP: 0033:0x7fbc0ca3fc1b
>  Code: 73 01 c3 48 8b 0d 05 f2 1b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 29 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d d5 f1 1b 00 f7 d8 64 89 01 48
>  RSP: 002b:00007fff18635208 EFLAGS: 00000246 ORIG_RAX: 0000000000000029
>  RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fbc0ca3fc1b
>  RDX: 0000000000000006 RSI: 0000000000000001 RDI: 0000000000000002
>  RBP: 000055d24f80f8a0 R08: 0000000000000003 R09: 0000000000000001
> 
> R10: 0000000000020000 R11: 0000000000000246 R12: 000055d24f80f8a0
>  R13: 0000000000000000 R14: 000055d24f80fb88 R15: 0000000000000000
>   </TASK>
> 
> The current implementation of cipso_v4_sock_setattr() replaces IP options
> under the assumption that the caller holds the socket lock; however, such
> assumption is not true, nor needed, in selinux_socket_post_create() hook.
> 
> Let all callers of cipso_v4_sock_setattr() specify the "socket lock held"
> condition, except selinux_socket_post_create() _ where such condition can
> safely be set as true even without holding the socket lock.
> While at it: use rcu_replace_pointer() instead of open coding, and remove
> useless NULL check of 'old' before kfree_rcu(old, ...).
> 
> v2:
>  - pass lockdep_sock_is_held() through a boolean variable in the stack
>    (thanks Eric Dumazet, Paul Moore, Casey Schaufler)
>  - use rcu_replace_pointer() instead of rcu_dereference_protected() +
>    rcu_assign_pointer()
>  - remove NULL check of 'old' before kfree_rcu()
> 
> Fixes: f6d8bd051c39 ("inet: add RCU protection to inet->opt")
> Reported-by: Xiumei Mu <xmu@redhat.com>
> Signed-off-by: Davide Caratti <dcaratti@redhat.com>
> ---
>  include/net/cipso_ipv4.h     |  6 ++++--
>  include/net/netlabel.h       |  6 ++++--
>  net/ipv4/cipso_ipv4.c        | 13 ++++++-------
>  net/netlabel/netlabel_kapi.c |  9 ++++++---
>  security/selinux/netlabel.c  |  5 ++++-
>  security/smack/smack_lsm.c   |  3 ++-
>  6 files changed, 26 insertions(+), 16 deletions(-)

...

> diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
> index 8b17d83e5fde..c4ac704cbcc2 100644
> --- a/net/ipv4/cipso_ipv4.c
> +++ b/net/ipv4/cipso_ipv4.c
> @@ -1815,6 +1815,7 @@ static int cipso_v4_genopt(unsigned char *buf, u32 buf_len,
>   * @sk: the socket
>   * @doi_def: the CIPSO DOI to use
>   * @secattr: the specific security attributes of the socket
> + * @slock_held: true if caller holds the socket lock
>   *
>   * Description:
>   * Set the CIPSO option on the given socket using the DOI definition and
> @@ -1826,7 +1827,8 @@ static int cipso_v4_genopt(unsigned char *buf, u32 buf_len,
>   */
>  int cipso_v4_sock_setattr(struct sock *sk,
>  			  const struct cipso_v4_doi *doi_def,
> -			  const struct netlbl_lsm_secattr *secattr)
> +			  const struct netlbl_lsm_secattr *secattr,
> +			  bool slock_held)

This is a nitpicky bikeshedding remark, but "slock_held" sounds really
awkward to me, something like "sk_locked" sounds much better.

>  {
>  	int ret_val = -EPERM;
>  	unsigned char *buf = NULL;
> @@ -1876,18 +1878,15 @@ int cipso_v4_sock_setattr(struct sock *sk,
>  
>  	sk_inet = inet_sk(sk);
>  
> -	old = rcu_dereference_protected(sk_inet->inet_opt,
> -					lockdep_sock_is_held(sk));
> +	old = rcu_replace_pointer(sk_inet->inet_opt, opt, slock_held);
>  	if (inet_test_bit(IS_ICSK, sk)) {
>  		sk_conn = inet_csk(sk);
>  		if (old)
>  			sk_conn->icsk_ext_hdr_len -= old->opt.optlen;
> -		sk_conn->icsk_ext_hdr_len += opt->opt.optlen;
> +		sk_conn->icsk_ext_hdr_len += opt_len;
>  		sk_conn->icsk_sync_mss(sk, sk_conn->icsk_pmtu_cookie);
>  	}
> -	rcu_assign_pointer(sk_inet->inet_opt, opt);
> -	if (old)
> -		kfree_rcu(old, rcu);
> +	kfree_rcu(old, rcu);

Thanks for sticking with this and posting a v2.

These changes look okay to me, but considering the 'Fixes:' tag and the
RCU splat it is reasonable to expect that this is going to be backported
to the various stable trees.  With that in mind, I think we should try
to keep the immediate fix as simple as possible, saving the other
changes for a separate patch.  This means sticking with
rcu_dereference_protected() and omitting the opt_len optimization; both
can be done in a second patch without the 'Fixes:' marking.

Unless I missing something and those changes are somehow part of the
fix?

--
paul-moore.com

Re: [PATCH v2] netlabel: fix RCU annotation for IPv4 options on socket creation

[Davide Caratti]

hello Paul, thanks for reviewing!

On Thu, Apr 25, 2024 at 05:01:36PM -0400, Paul Moore wrote:
> On Apr 24, 2024 Davide Caratti <dcaratti@redhat.com> wrote:
> >

[...]
 
> > @@ -1826,7 +1827,8 @@ static int cipso_v4_genopt(unsigned char *buf, u32 buf_len,
> >   */
> >  int cipso_v4_sock_setattr(struct sock *sk,
> >  			  const struct cipso_v4_doi *doi_def,
> > -			  const struct netlbl_lsm_secattr *secattr)
> > +			  const struct netlbl_lsm_secattr *secattr,
> > +			  bool slock_held)
> 
> This is a nitpicky bikeshedding remark, but "slock_held" sounds really
> awkward to me, something like "sk_locked" sounds much better.

ok, will fix that in v3.

[...]

> > @@ -1876,18 +1878,15 @@ int cipso_v4_sock_setattr(struct sock *sk,
> >  
> >  	sk_inet = inet_sk(sk);
> >  
> > -	old = rcu_dereference_protected(sk_inet->inet_opt,
> > -					lockdep_sock_is_held(sk));
> > +	old = rcu_replace_pointer(sk_inet->inet_opt, opt, slock_held);
> >  	if (inet_test_bit(IS_ICSK, sk)) {
> >  		sk_conn = inet_csk(sk);
> >  		if (old)
> >  			sk_conn->icsk_ext_hdr_len -= old->opt.optlen;
> > -		sk_conn->icsk_ext_hdr_len += opt->opt.optlen;
> > +		sk_conn->icsk_ext_hdr_len += opt_len;
> >  		sk_conn->icsk_sync_mss(sk, sk_conn->icsk_pmtu_cookie);
> >  	}
> > -	rcu_assign_pointer(sk_inet->inet_opt, opt);
> > -	if (old)
> > -		kfree_rcu(old, rcu);
> > +	kfree_rcu(old, rcu);
> 
> Thanks for sticking with this and posting a v2.
> 
> These changes look okay to me, but considering the 'Fixes:' tag and the
> RCU splat it is reasonable to expect that this is going to be backported
> to the various stable trees.  With that in mind, I think we should try
> to keep the immediate fix as simple as possible, saving the other
> changes for a separate patch.  This means sticking with
> rcu_dereference_protected() and omitting the opt_len optimization; both
> can be done in a second patch without the 'Fixes:' marking.
> 
> Unless I missing something and those changes are somehow part of the
> fix?

just checked, rcu_replace_pointer() can be used also in the oldest LTS
but I'm not sure if kfree_rcu(NULL, ...) is ok. I agree to keep
rcu_dereference_protected(), and the useless NULL check - I will
follow-up with another patch (targeting net-next), after this one is
merged.

--
davide

Re: [PATCH v2] netlabel: fix RCU annotation for IPv4 options on socket creation

[Paul Moore]

On Mon, Apr 29, 2024 at 6:10 AM Davide Caratti <dcaratti@redhat.com> wrote:
> On Thu, Apr 25, 2024 at 05:01:36PM -0400, Paul Moore wrote:
> > On Apr 24, 2024 Davide Caratti <dcaratti@redhat.com> wrote:

...

> > > @@ -1876,18 +1878,15 @@ int cipso_v4_sock_setattr(struct sock *sk,
> > >
> > >     sk_inet = inet_sk(sk);
> > >
> > > -   old = rcu_dereference_protected(sk_inet->inet_opt,
> > > -                                   lockdep_sock_is_held(sk));
> > > +   old = rcu_replace_pointer(sk_inet->inet_opt, opt, slock_held);
> > >     if (inet_test_bit(IS_ICSK, sk)) {
> > >             sk_conn = inet_csk(sk);
> > >             if (old)
> > >                     sk_conn->icsk_ext_hdr_len -= old->opt.optlen;
> > > -           sk_conn->icsk_ext_hdr_len += opt->opt.optlen;
> > > +           sk_conn->icsk_ext_hdr_len += opt_len;
> > >             sk_conn->icsk_sync_mss(sk, sk_conn->icsk_pmtu_cookie);
> > >     }
> > > -   rcu_assign_pointer(sk_inet->inet_opt, opt);
> > > -   if (old)
> > > -           kfree_rcu(old, rcu);
> > > +   kfree_rcu(old, rcu);
> >
> > Thanks for sticking with this and posting a v2.
> >
> > These changes look okay to me, but considering the 'Fixes:' tag and the
> > RCU splat it is reasonable to expect that this is going to be backported
> > to the various stable trees.  With that in mind, I think we should try
> > to keep the immediate fix as simple as possible, saving the other
> > changes for a separate patch.  This means sticking with
> > rcu_dereference_protected() and omitting the opt_len optimization; both
> > can be done in a second patch without the 'Fixes:' marking.
> >
> > Unless I missing something and those changes are somehow part of the
> > fix?
>
> just checked, rcu_replace_pointer() can be used also in the oldest LTS
> but I'm not sure if kfree_rcu(NULL, ...) is ok. I agree to keep
> rcu_dereference_protected(), and the useless NULL check - I will
> follow-up with another patch (targeting net-next), after this one is
> merged.

The issue isn't so much about if a particular function is available in
an older kernel, it is more about keeping the patch focused on a
single immediate purpose both to limit any unintended behaviors and
for the simple reason that smaller patches are almost always easier to
port by hand if needed.

-- 
paul-moore.com