[PATCH] smb: client: fix use-before-check of ReparseDataLength in reparse_buf_ptr()

[Frank Sorenson <sorenson@redhat.com>]

reparse_buf_ptr() reads buf->ReparseDataLength before checking that
count covers the full fixed header:

    buf = (struct reparse_data_buffer *)((u8 *)io + off);
    len = sizeof(*buf);                          /* 8 bytes */
    rdlen = le16_to_cpu(buf->ReparseDataLength); /* offset 4, 2 bytes */

    if (count < len || count < rdlen + len)      /* check comes after */

struct reparse_data_buffer has ReparseDataLength at offset 4.  If a
server returns OutputCount < 6, the read at offset 4-5 reaches past
the end of the received data.  The off+count bounds against iov_len
were already validated, but that does not protect against count being
smaller than sizeof(*buf).

Split the check: verify count >= sizeof(*buf) before reading
ReparseDataLength, then verify count covers the data region.

Fixes: a158bb66b137 ("smb: client: optimise reparse point querying")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
---
 fs/smb/client/smb2inode.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/smb2inode.c b/fs/smb/client/smb2inode.c
index 213bc298cdf2..2946391bb992 100644
--- a/fs/smb/client/smb2inode.c
+++ b/fs/smb/client/smb2inode.c
@@ -40,9 +40,11 @@ static struct reparse_data_buffer *reparse_buf_ptr(struct kvec *iov)
 
 	buf = (struct reparse_data_buffer *)((u8 *)io + off);
 	len = sizeof(*buf);
-	rdlen = le16_to_cpu(buf->ReparseDataLength);
+	if (count < len)
+		return ERR_PTR(smb_EIO2(smb_eio_trace_reparse_rdlen, count, 0));
 
-	if (count < len || count < rdlen + len)
+	rdlen = le16_to_cpu(buf->ReparseDataLength);
+	if (count < rdlen + len)
 		return ERR_PTR(smb_EIO2(smb_eio_trace_reparse_rdlen, count, rdlen));
 	return buf;
 }
-- 
2.55.0