smb: client: fix off-by-8 bounds check in check_wsl_eas()

The bounds check uses (u8 *)ea + nlen + 1 + vlen as the end of the EA
name and value, but ea_data sits at offset sizeof(struct
smb2_file_full_ea_info) = 8 from ea, not at offset 0.  The strncmp()
later reads ea->ea_data[0..nlen-1] and the value bytes follow at
ea_data[nlen+1..nlen+vlen], so the actual end is ea->ea_data + nlen + 1
+ vlen.  Isn't pointer math fun?

The earlier check (u8 *)ea > end - sizeof(*ea) only guarantees the
8-byte header is in bounds, but since the last EA is placed within 8
bytes of the end of the response, the name and value bytes are read past
the end of iov.

Fix this mess all up by using ea->ea_data as the base for the bounds
check.

An "untrusted" server can use this to leak up to 8 bytes of kernel heap
into the EA name comparison and influence which WSL xattr the data is
interpreted as.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Cc: stable <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2inode.c

@@ -128,7 +128,7 @@ static int check_wsl_eas(struct kvec *rsp_iov)
 		nlen = ea->ea_name_length;
 		vlen = le16_to_cpu(ea->ea_value_length);
 		if (nlen != SMB2_WSL_XATTR_NAME_LEN ||
-		    (u8 *)ea + nlen + 1 + vlen > end)
+		    (u8 *)ea->ea_data + nlen + 1 + vlen > end)
 			return -EINVAL;
 
 		switch (vlen) {