[PATCH v2] smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2()

coalesce_t2() computes data pointers directly from server-supplied
DataOffset fields with no validation against buffer bounds:

  data_area_of_tgt = (char *)&pSMBt->hdr.Protocol +
                     get_unaligned_le16(&pSMBt->t2_rsp.DataOffset);
  data_area_of_src = (char *)&pSMBs->hdr.Protocol +
                     get_unaligned_le16(&pSMBs->t2_rsp.DataOffset);
  data_area_of_tgt += total_in_tgt;
  ...
  memcpy(data_area_of_tgt, data_area_of_src, total_in_src);

A small DataOffset can push a pointer below the actual byte area,
overwriting header fields; a large one can push it past the buffer
end, causing out-of-bounds heap reads (source) or writes (target).
The BCC overflow guard does not prevent this: BCC reflects how much
data is present, while DataOffset controls where in the buffer it
starts.

The "validate target area" comment present since the function was
first written in 2005 was a placeholder that was never implemented.

Add lower- and upper-bound checks for both data pointers before the
memcpy, and before any target header fields are modified.

Fixes: e4eb295d38b5 ("[PATCH] cifs: Handle multiple response transact2 part 1 of 2")
Cc: [email protected]
Reported-by: Shen Yongchao <[email protected]>
Signed-off-by: Frank Sorenson <[email protected]>
---

 fs/smb/client/smb1transport.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/smb/client/smb1transport.c b/fs/smb/client/smb1transport.c
index 966f2cf83a51..66daa5a37e4a 100644
--- a/fs/smb/client/smb1transport.c
+++ b/fs/smb/client/smb1transport.c
@@ -375,12 +375,31 @@ coalesce_t2(char *second_buf, struct smb_hdr *target_hdr, unsigned int *pdu_len)
 	data_area_of_tgt = (char *)&pSMBt->hdr.Protocol +
 				get_unaligned_le16(&pSMBt->t2_rsp.DataOffset);
 
-	/* validate target area */
 	data_area_of_src = (char *)&pSMBs->hdr.Protocol +
 				get_unaligned_le16(&pSMBs->t2_rsp.DataOffset);
 
 	data_area_of_tgt += total_in_tgt;
 
+	/*
+	 * DataOffset fields are server-supplied and not validated against
+	 * buffer bounds; check both data pointers before mutating the
+	 * target header.
+	 */
+	if (data_area_of_tgt < (char *)target_hdr +
+				sizeof(struct smb_t2_rsp) + sizeof(__le16) ||
+	    data_area_of_tgt + total_in_src >
+	    (char *)target_hdr + CIFSMaxBufSize + MAX_CIFS_HDR_SIZE) {
+		cifs_dbg(VFS, "%s: target data area out of bounds\n", __func__);
+		return -EPROTO;
+	}
+	if (data_area_of_src < second_buf +
+				sizeof(struct smb_t2_rsp) + sizeof(__le16) ||
+	    data_area_of_src + total_in_src >
+	    second_buf + smbCalcSize((struct smb_hdr *)second_buf)) {
+		cifs_dbg(VFS, "%s: secondary data area out of bounds\n", __func__);
+		return -EPROTO;
+	}
+
 	total_in_tgt += total_in_src;
 	/* is the result too big for the field? */
 	if (total_in_tgt > USHRT_MAX) {
-- 
2.55.0