[PATCH] smb: client: validate the data area in SMB2_read()

[Abdifatah Suruur <suruurism@gmail.com>]

SMB2_read() copies le32_to_cpu(rsp->DataLength) bytes from
(char *)rsp + rsp->DataOffset without checking that the range lies
within the response actually received. A READ response with
DataOffset == 0 makes smb2_get_data_area_len() zero the data length, so
smb2_check_message() accepts a frame as short as the fixed response,
while SMB2_read() still uses the server-supplied DataLength, capped only
by CIFS_MAX_MSGSIZE and the caller's length.

A malicious server can answer the synchronous read used by the
mfsymlinks (or SFU) symlink parser with an 81-byte READ response
carrying DataOffset 0 and DataLength 1067, making the client copy 1067
bytes from a 448-byte receive object (KASAN slab-out-of-bounds; up to
16 KiB in the general case).

Clamp a zero (or header-overlapping) DataOffset to read_rsp_size, as
handle_read_data() does for the asynchronous path, and reject the
response unless DataOffset + DataLength fits within rsp_iov.iov_len.
The SMB1 helper was fixed the same way in 5be5bdda5863.

Cc: stable@vger.kernel.org
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
---
 fs/smb/client/smb2pdu.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index f276acf7b..8291eadd7 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/fs/smb/client/smb2pdu.c
@@ -4947,6 +4947,7 @@ SMB2_read(const unsigned int xid, struct cifs_io_parms *io_parms,
 	struct kvec iov[1];
 	struct kvec rsp_iov;
 	unsigned int total_len;
+	u16 data_offset;
 	int flags = CIFS_LOG_ERROR;
 	struct cifs_ses *ses = io_parms->tcon->ses;
 
@@ -5006,8 +5007,29 @@ SMB2_read(const unsigned int xid, struct cifs_io_parms *io_parms,
 		*nbytes = 0;
 	}
 
+	data_offset = rsp->DataOffset;
+	if (data_offset < io_parms->server->vals->read_rsp_size) {
+		/*
+		 * win2k8 sometimes sends an offset of 0 when the read is
+		 * beyond the EOF. Treat it as if the data starts just after
+		 * the header, as handle_read_data() does.
+		 */
+		cifs_dbg(FYI, "data offset %u inside read response header\n",
+			 data_offset);
+		data_offset = io_parms->server->vals->read_rsp_size;
+	}
+
+	/* The data area must lie within the response actually received. */
+	if (*nbytes && (size_t)data_offset + *nbytes > rsp_iov.iov_len) {
+		cifs_dbg(FYI, "bad data offset %u length %u for response of %zu\n",
+			 data_offset, *nbytes, rsp_iov.iov_len);
+		rc = smb_EIO2(smb_eio_trace_read_bad_offset,
+			      data_offset, *nbytes);
+		*nbytes = 0;
+	}
+
 	if (*buf) {
-		memcpy(*buf, (char *)rsp + rsp->DataOffset, *nbytes);
+		memcpy(*buf, (char *)rsp + data_offset, *nbytes);
 		free_rsp_buf(resp_buftype, rsp_iov.iov_base);
 	} else if (resp_buftype != CIFS_NO_BUFFER) {
 		*buf = rsp_iov.iov_base;
-- 
2.53.0