[PATCH v2 2/2] smb: client: reject a tree connect response whose byte count is too small

[Bryam Vargas via B4 Relay]

From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

CIFSTCon() bounds its strnlen() over the byte area with the server's
ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int
and converts to a huge size_t.  The later subtraction wraps the __u16
bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of
up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of the
slab object, and the bytes reach userspace through tcon->nativeFileSystem
in /proc/fs/cifs/DebugData.

Reject a byte area too small for what the parser consumes.  Two bytes is
the least the parse can consume, and no conformant response carries fewer
-- the Service field is at least "A:" and its NUL.

Fixes: cc20c031bb06 ("cifs: convert CIFSTCon to use new unicode helper functions")
Cc: stable@xxxxxxxxxx
Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
---
 fs/smb/client/cifssmb.c | 6 ++++++
 fs/smb/client/trace.h   | 1 +
 2 files changed, 7 insertions(+)

diff --git a/fs/smb/client/cifssmb.c b/fs/smb/client/cifssmb.c
index 1f77512252e7..f5aad5f61dce 100644
--- a/fs/smb/client/cifssmb.c
+++ b/fs/smb/client/cifssmb.c
@@ -615,6 +615,11 @@ CIFSTCon(const unsigned int xid, struct cifs_ses *ses,
 		tcon->tid = smb_buffer_response->Tid;
 		bcc_ptr = pByteArea(smb_buffer_response);
 		bytes_left = get_bcc(smb_buffer_response);
+		if (bytes_left < 2) {
+			rc = smb_EIO2(smb_eio_trace_tcon_bcc_too_small,
+				      bytes_left, 2);
+			goto out;
+		}
 		length = strnlen(bcc_ptr, bytes_left - 2);
 		if (smb_buffer->Flags2 & SMBFLG2_UNICODE)
 			is_unicode = true;
@@ -670,6 +675,7 @@ CIFSTCon(const unsigned int xid, struct cifs_ses *ses,
 			reset_cifs_unix_caps(xid, tcon, NULL, NULL);
 		}
 	}
+out:
 	cifs_buf_release(smb_buffer);
 	return rc;
 }
diff --git a/fs/smb/client/trace.h b/fs/smb/client/trace.h
index 0a91d3aaa079..12241abb8e2e 100644
--- a/fs/smb/client/trace.h
+++ b/fs/smb/client/trace.h
@@ -133,6 +133,7 @@
 	EM(smb_eio_trace_sym_slash,			"sym_slash") \
 	EM(smb_eio_trace_sym_target_len,		"sym_target_len") \
 	EM(smb_eio_trace_symlink_file_size,		"symlink_file_size") \
+	EM(smb_eio_trace_tcon_bcc_too_small,		"tcon_bcc_too_small") \
 	EM(smb_eio_trace_tdis_in_reconnect,		"tdis_in_reconnect") \
 	EM(smb_eio_trace_tx_chained_async,		"tx_chained_async") \
 	EM(smb_eio_trace_tx_compress_failed,		"tx_compress_failed") \

-- 
2.55.0