smart_pkt: verify packet length exceeds PKT_LEN_SIZE
Each packet line in the Git protocol is prefixed by a four-byte
length of how much data will follow, which we parse in
`git_pkt_parse_line`. The transmitted length can either be equal
to zero in case of a flush packet or has to be at least of length
four, as it also includes the encoded length itself. Not
checking this may result in a buffer overflow as we directly pass
the length to functions which accept a `size_t` length as
parameter.
Fix the issue by verifying that non-flush packets have at least a
length of `PKT_LEN_SIZE`.
diff --git a/src/transports/smart_pkt.c b/src/transports/smart_pkt.c
index 2ea57bb..b1eee3e 100644
--- a/src/transports/smart_pkt.c
+++ b/src/transports/smart_pkt.c
@@ -427,6 +427,14 @@
if (bufflen > 0 && bufflen < (size_t)len)
return GIT_EBUFS;
+ /*
+ * The length has to be exactly 0 in case of a flush
+ * packet or greater than PKT_LEN_SIZE, as the decoded
+ * length includes its own encoded length of four bytes.
+ */
+ if (len != 0 && len < PKT_LEN_SIZE)
+ return GIT_ERROR;
+
line += PKT_LEN_SIZE;
/*
* TODO: How do we deal with empty lines? Try again? with the next
