Commit 6d8c50dc authored Jun 07, 2018 by Cong Wang Committed by David S. Miller Jun 10, 2018

socket: close race condition between sock_close() and sockfs_setattr()



fchownat() doesn't even hold refcnt of fd until it figures out
fd is really needed (otherwise is ignored) and releases it after
it resolves the path. This means sock_close() could race with
sockfs_setattr(), which leads to a NULL pointer dereference
since typically we set sock->sk to NULL in ->release().

As pointed out by Al, this is unique to sockfs. So we can fix this
in socket layer by acquiring inode_lock in sock_close() and
checking against NULL in sockfs_setattr().

sock_release() is called in many places, only the sock_close()
path matters here. And fortunately, this should not affect normal
sock_close() as it is only called when the last fd refcnt is gone.
It only affects sock_close() with a parallel sockfs_setattr() in
progress, which is not common.

Fixes: 86741ec2 ("net: core: Add a UID field to struct sock.")
Reported-by: shankarapailoor <shankarapailoor@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 873aca2e

net/socket.c

+15 −3

Original line number	Diff line number	Diff line
		@@ -541,7 +541,10 @@ static int sockfs_setattr(struct dentry dentry, struct iattr iattr)
		if (!err && (iattr->ia_valid & ATTR_UID)) {
		struct socket *sock = SOCKET_I(d_inode(dentry));

		if (sock->sk)
		sock->sk->sk_uid = iattr->ia_uid;
		else
		err = -ENOENT;
		}

		return err;
		@@ -590,12 +593,16 @@ EXPORT_SYMBOL(sock_alloc);
		* an inode not a file.
		*/

		void sock_release(struct socket *sock)
		static void __sock_release(struct socket sock, struct inode inode)
		{
		if (sock->ops) {
		struct module *owner = sock->ops->owner;

		if (inode)
		inode_lock(inode);
		sock->ops->release(sock);
		if (inode)
		inode_unlock(inode);
		sock->ops = NULL;
		module_put(owner);
		}
		@@ -609,6 +616,11 @@ void sock_release(struct socket *sock)
		}
		sock->file = NULL;
		}

		void sock_release(struct socket *sock)
		{
		__sock_release(sock, NULL);
		}
		EXPORT_SYMBOL(sock_release);

		void __sock_tx_timestamp(__u16 tsflags, __u8 *tx_flags)
		@@ -1171,7 +1183,7 @@ static int sock_mmap(struct file file, struct vm_area_struct vma)

		static int sock_close(struct inode inode, struct file filp)
		{
		sock_release(SOCKET_I(inode));
		__sock_release(SOCKET_I(inode), inode);
		return 0;
		}