[PATCH uci 2/4] file: Check buffer size after strtok()

[Hauke Mehrtens]

This fixes a heap overflow in the parsing of the uci line.

The line which is parsed and put into pctx->buf is null terminated and
stored on the heap. In the uci_parse_line() function we use strtok() to
split this string in multiple parts after divided by a space or tab.
strtok() replaces these characters with a NULL byte. If the next byte is
NULL we assume that this NULL byte was added by strtok() and try to
parse the string after this NULL byte. If this NULL byte was not added
by strtok(), but by fgets() to mark the end of the string we would read
over this end of the string in uninitialized memory and later over the
allocated buffer.

Fix this problem by storing how long the line we read was and check if
we would read over the end of the string here.

This also adds the input which detected this crash to the corpus of the
fuzzer.

Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
---
 file.c         | 19 ++++++++++++++++---
 uci_internal.h |  1 +
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/file.c b/file.c
index 003f6c6..57bb836 100644
--- a/file.c
+++ b/file.c
@@ -68,6 +68,7 @@ __private void uci_getln(struct uci_context *ctx, size_t offset)
 			return;
 
 		ofs += strlen(p);
+		pctx->buf_filled = ofs;
 		if (pctx->buf[ofs - 1] == '\n') {
 			pctx->line++;
 			return;
@@ -125,6 +126,15 @@ static inline void addc(struct uci_context *ctx, size_t *pos_dest, size_t *pos_s
 	*pos_src += 1;
 }
 
+static int uci_increase_pos(struct uci_parse_context *pctx, size_t add)
+{
+	if (pctx->pos + add > pctx->buf_filled)
+		return -EINVAL;
+
+	pctx->pos += add;
+	return 0;
+}
+
 /*
  * parse a double quoted string argument from the command line
  */
@@ -389,7 +399,8 @@ static void uci_parse_package(struct uci_context *ctx, bool single)
 	char *name;
 
 	/* command string null-terminated by strtok */
-	pctx->pos += strlen(pctx_cur_str(pctx)) + 1;
+	if (uci_increase_pos(pctx, strlen(pctx_cur_str(pctx) + 1)))
+		return;
 
 	ofs_name = next_arg(ctx, true, true, true);
 	assert_eol(ctx);
@@ -421,7 +432,8 @@ static void uci_parse_config(struct uci_context *ctx)
 	}
 
 	/* command string null-terminated by strtok */
-	pctx->pos += strlen(pctx_cur_str(pctx)) + 1;
+	if (uci_increase_pos(pctx, strlen(pctx_cur_str(pctx) + 1)))
+		return;
 
 	ofs_type = next_arg(ctx, true, false, false);
 	type = pctx_str(pctx, ofs_type);
@@ -471,7 +483,8 @@ static void uci_parse_option(struct uci_context *ctx, bool list)
 		uci_parse_error(ctx, "option/list command found before the first section");
 
 	/* command string null-terminated by strtok */
-	pctx->pos += strlen(pctx_cur_str(pctx)) + 1;
+	if (uci_increase_pos(pctx, strlen(pctx_cur_str(pctx) + 1)))
+		return;
 
 	ofs_name = next_arg(ctx, true, true, false);
 	ofs_value = next_arg(ctx, false, false, false);
diff --git a/uci_internal.h b/uci_internal.h
index 3a94dbb..34528f0 100644
--- a/uci_internal.h
+++ b/uci_internal.h
@@ -33,6 +33,7 @@ struct uci_parse_context
 	const char *name;
 	char *buf;
 	size_t bufsz;
+	size_t buf_filled;
 	size_t pos;
 };
 #define pctx_pos(pctx)		((pctx)->pos)
-- 
2.20.1