linux-kernel - [PATCH net] fix a braino in cmsghdr_from_user_compat_to

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Date:   Mon, 27 Jul 2020 19:22:20 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     netdev@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Nick Bowler <nbowler@...conx.ca>
Subject: [PATCH net] fix a braino in cmsghdr_from_user_compat_to_kern()

	commit 547ce4cfb34c ("switch cmsghdr_from_user_compat_to_kern() to
copy_from_user()") missed one of the places where ucmlen should've been
replaced with cmsg.cmsg_len, now that we are fetching the entire struct
rather than doing it field-by-field.

	As the result, compat sendmsg() with several different-sized cmsg
attached started to fail with EINVAL.  Trivial to fix, fortunately.

Reported-by: Nick Bowler <nbowler@...conx.ca>
Tested-by: Nick Bowler <nbowler@...conx.ca>
Fixes: 547ce4cfb34c ("switch cmsghdr_from_user_compat_to_kern() to copy_from_user()")

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/net/compat.c b/net/compat.c
index 5e3041a2c37d..434838bef5f8 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -202,7 +202,7 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 
 		/* Advance. */
 		kcmsg = (struct cmsghdr *)((char *)kcmsg + tmp);
-		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, ucmlen);
+		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, cmsg.cmsg_len);
 	}
 
 	/*