[PATCH] net/l2tp: Fix reference count leak in l2tp_udp_recv_core

The reference count leak issue may take place in an error handling
path. If both conditions of tunnel->version == L2TP_HDR_VER_3 and the
return value of l2tp_v3_ensure_opt_in_linear is nonzero, the function
would directly jump to label invalid, without decrementing the reference
count of the l2tp_session object session increased earlier by
l2tp_tunnel_get_session(). This may result in refcount leaks.

Fix this issue by decrease the reference count before jumping to the
label invalid.

Signed-off-by: Xiyu Yang <redacted>
Signed-off-by: Xin Xiong <redacted>
Signed-off-by: Xin Tan <redacted>
---
 net/l2tp/l2tp_core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 53486b162f01..93271a2632b8 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c

@@ -869,8 +869,10 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
 	}
 
 	if (tunnel->version == L2TP_HDR_VER_3 &&
-	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
+	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr)) {
+		l2tp_session_dec_refcount(session);
 		goto invalid;
+	}
 
 	l2tp_recv_common(session, skb, ptr, optr, hdrflags, length);
 	l2tp_session_dec_refcount(session);

-- 
2.7.4

On  Thu, Sep 09, 2021 at 12:32:00 +0800, Xiyu Yang wrote:

The reference count leak issue may take place in an error handling
path. If both conditions of tunnel->version == L2TP_HDR_VER_3 and the
return value of l2tp_v3_ensure_opt_in_linear is nonzero, the function
would directly jump to label invalid, without decrementing the reference
count of the l2tp_session object session increased earlier by
l2tp_tunnel_get_session(). This may result in refcount leaks.

I agree with your analysis.  Thanks for catching this!

The error paths in l2tp_udp_recv_core are a bit convoluted because of
the check (!session || !session->recv_skb) which may or may not need
to drop a session reference if triggered.

I think it could be simplified since session->recv_skb is always set
for all the current session types in the tree, but doing that is probably
a little patch series on its own.

 
 	l2tp_recv_common(session, skb, ptr, optr, hdrflags, length);
 	l2tp_session_dec_refcount(session);
-- 
2.7.4

-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
Catalysts for your Embedded Linux software development

On  Thu, Sep 09, 2021 at 10:01:56 +0100, Tom Parkin wrote:

On  Thu, Sep 09, 2021 at 12:32:00 +0800, Xiyu Yang wrote:

quoted

The reference count leak issue may take place in an error handling
path. If both conditions of tunnel->version == L2TP_HDR_VER_3 and the
return value of l2tp_v3_ensure_opt_in_linear is nonzero, the function
would directly jump to label invalid, without decrementing the reference
count of the l2tp_session object session increased earlier by
l2tp_tunnel_get_session(). This may result in refcount leaks.

I agree with your analysis.  Thanks for catching this!

Also: I forgot to mention I think this fixes:

4522a70db7aa ("l2tp: fix reading optional fields of L2TPv3")

quoted

Fix this issue by decrease the reference count before jumping to the
label invalid.

Signed-off-by: Xiyu Yang <redacted>
Signed-off-by: Xin Xiong <redacted>
Signed-off-by: Xin Tan <redacted>
---
 net/l2tp/l2tp_core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 53486b162f01..93271a2632b8 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c

@@ -869,8 +869,10 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
 	}
 
 	if (tunnel->version == L2TP_HDR_VER_3 &&
-	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
+	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr)) {
+		l2tp_session_dec_refcount(session);
 		goto invalid;
+	}

The error paths in l2tp_udp_recv_core are a bit convoluted because of
the check (!session || !session->recv_skb) which may or may not need
to drop a session reference if triggered.

I think it could be simplified since session->recv_skb is always set
for all the current session types in the tree, but doing that is probably
a little patch series on its own.

quoted

 
 	l2tp_recv_common(session, skb, ptr, optr, hdrflags, length);
 	l2tp_session_dec_refcount(session);
-- 
2.7.4

-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
Catalysts for your Embedded Linux software development

-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
Catalysts for your Embedded Linux software development

Hello:

This patch was applied to netdev/net.git (refs/heads/master):

On Thu,  9 Sep 2021 12:32:00 +0800 you wrote:

The reference count leak issue may take place in an error handling
path. If both conditions of tunnel->version == L2TP_HDR_VER_3 and the
return value of l2tp_v3_ensure_opt_in_linear is nonzero, the function
would directly jump to label invalid, without decrementing the reference
count of the l2tp_session object session increased earlier by
l2tp_tunnel_get_session(). This may result in refcount leaks.

[...]

Here is the summary with links:
  - net/l2tp: Fix reference count leak in l2tp_udp_recv_core
    https://git.kernel.org/netdev/net/c/9b6ff7eb6664

You are awesome, thank you!
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