[PATCH v2 3/5] util, lineread, tap: Overflow checks on long signed sums and subtractions

[Stefano Brivio <sbrivio@redhat.com>]

Potential sum and subtraction overflows were reported by Coverity in a
few places where we use size_t and ssize_t types.

Strictly speaking, those are not false positives even if I don't see a
way to trigger those: for instance, if we receive up to n bytes from a
socket up, the return value from recv() is already constrained and
can't overflow for the usage we make in tap_handler_passt().

In any case, take care of those by adding two functions that
explicitly check for overflows in sums and subtractions of long signed
values, and using them.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 lineread.c |  5 +++--
 tap.c      | 21 +++++++++++++++------
 util.c     |  5 +++++
 util.h     | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 78 insertions(+), 8 deletions(-)

diff --git a/lineread.c b/lineread.c
index 0387f4a..f72a14e 100644
--- a/lineread.c
+++ b/lineread.c
@@ -17,6 +17,7 @@
 #include <string.h>
 #include <stdbool.h>
 #include <unistd.h>
+#include <errno.h>
 
 #include "lineread.h"
 #include "util.h"
@@ -102,8 +103,8 @@ ssize_t lineread_get(struct lineread *lr, char **line)
 
 		if (rc == 0)
 			eof = true;
-		else
-			lr->count += rc;
+		else if (sadd_overflow(lr->count, rc, &lr->count))
+			return -ERANGE;
 	}
 
 	*line = lr->buf + lr->next_line;
diff --git a/tap.c b/tap.c
index ec994a2..e5c1693 100644
--- a/tap.c
+++ b/tap.c
@@ -1031,7 +1031,11 @@ redo:
 		 */
 		if (l2len > n) {
 			rem = recv(c->fd_tap, p + n, l2len - n, 0);
-			if ((n += rem) != l2len)
+
+			if (sadd_overflow(n, rem, &n))
+				return;
+
+			if (n != l2len)
 				return;
 		}
 
@@ -1046,7 +1050,9 @@ redo:
 
 next:
 		p += l2len;
-		n -= l2len;
+
+		if (ssub_overflow(n, l2len, &n))
+			return;
 	}
 
 	tap_handler(c, now);
@@ -1077,17 +1083,20 @@ redo:
 	tap_flush_pools();
 restart:
 	while ((len = read(c->fd_tap, pkt_buf + n, TAP_BUF_BYTES - n)) > 0) {
-
 		if (len < (ssize_t)sizeof(struct ethhdr) ||
 		    len > (ssize_t)ETH_MAX_MTU) {
-			n += len;
+			if (sadd_overflow(n, len, &n))
+				return;
+
 			continue;
 		}
 
-
 		tap_add_packet(c, len, pkt_buf + n);
 
-		if ((n += len) == TAP_BUF_BYTES)
+		if (sadd_overflow(n, len, &n))
+			return;
+
+		if (n == TAP_BUF_BYTES)
 			break;
 	}
 
diff --git a/util.c b/util.c
index dd2e57f..4de872b 100644
--- a/util.c
+++ b/util.c
@@ -585,6 +585,11 @@ int write_remainder(int fd, const struct iovec *iov, int iovcnt, size_t skip)
 		if (rc < 0)
 			return -1;
 
+		if (skip > (size_t)SIZE_MAX - rc) {
+			errno = -ERANGE;
+			return -1;
+		}
+
 		skip += rc;
 	}
 
diff --git a/util.h b/util.h
index eebb027..1da2616 100644
--- a/util.h
+++ b/util.h
@@ -223,6 +223,61 @@ static inline bool mod_between(unsigned x, unsigned i, unsigned j, unsigned m)
 	return mod_sub(x, i, m) < mod_sub(j, i, m);
 }
 
+#if defined __has_builtin
+# if __has_builtin(__builtin_add_overflow)
+#  define sadd_overflow(a, b, res) __builtin_add_overflow(a, b, res)
+# endif
+# if __has_builtin(__builtin_sub_overflow)
+#  define ssub_overflow(a, b, res) __builtin_sub_overflow(a, b, res)
+# endif
+#endif
+
+#ifndef SSIZE_MIN
+# if SSIZE_MAX == LONG_MAX
+#  define SSIZE_MIN	LONG_MIN
+# else
+#  define SSIZE_MIN	INT_MIN
+# endif
+#endif
+
+#ifndef sadd_overflow
+/**
+ * sadd_overflow() - Sum with overflow check for ssize_t values
+ * @a:		First value
+ * @b:		Second value
+ * @sum:	Pointer to result of sum, if it doesn't overflow
+ *
+ * Return: true if the sum would overflow, false otherwise
+ */
+static inline bool sadd_overflow(ssize_t a, ssize_t b, ssize_t *sum)
+{
+	if ((a > 0 && a > SSIZE_MAX - b) || (b < 0 && a < SSIZE_MIN - b))
+		return true;
+
+	*sum = a + b;
+	return false;
+}
+#endif
+
+#ifndef ssub_overflow
+/**
+ * ssub_overflow() - Subtraction with overflow check for ssize_t values
+ * @a:		Minuend
+ * @b:		Subtrahend
+ * @sum:	Pointer to result of subtraction, if it doesn't overflow
+ *
+ * Return: true if the subtraction would overflow, false otherwise
+ */
+static inline bool ssub_overflow(ssize_t a, ssize_t b, ssize_t *diff)
+{
+	if ((b > 0 && a < SSIZE_MIN + b) || (b < 0 && a > SSIZE_MAX + b))
+		return true;
+
+	*diff = a - b;
+	return false;
+}
+#endif
+
 /*
  * Workarounds for https://github.com/llvm/llvm-project/issues/58992
  *
-- 
@@ -223,6 +223,61 @@ static inline bool mod_between(unsigned x, unsigned i, unsigned j, unsigned m)
 	return mod_sub(x, i, m) < mod_sub(j, i, m);
 }
 
+#if defined __has_builtin
+# if __has_builtin(__builtin_add_overflow)
+#  define sadd_overflow(a, b, res) __builtin_add_overflow(a, b, res)
+# endif
+# if __has_builtin(__builtin_sub_overflow)
+#  define ssub_overflow(a, b, res) __builtin_sub_overflow(a, b, res)
+# endif
+#endif
+
+#ifndef SSIZE_MIN
+# if SSIZE_MAX == LONG_MAX
+#  define SSIZE_MIN	LONG_MIN
+# else
+#  define SSIZE_MIN	INT_MIN
+# endif
+#endif
+
+#ifndef sadd_overflow
+/**
+ * sadd_overflow() - Sum with overflow check for ssize_t values
+ * @a:		First value
+ * @b:		Second value
+ * @sum:	Pointer to result of sum, if it doesn't overflow
+ *
+ * Return: true if the sum would overflow, false otherwise
+ */
+static inline bool sadd_overflow(ssize_t a, ssize_t b, ssize_t *sum)
+{
+	if ((a > 0 && a > SSIZE_MAX - b) || (b < 0 && a < SSIZE_MIN - b))
+		return true;
+
+	*sum = a + b;
+	return false;
+}
+#endif
+
+#ifndef ssub_overflow
+/**
+ * ssub_overflow() - Subtraction with overflow check for ssize_t values
+ * @a:		Minuend
+ * @b:		Subtrahend
+ * @sum:	Pointer to result of subtraction, if it doesn't overflow
+ *
+ * Return: true if the subtraction would overflow, false otherwise
+ */
+static inline bool ssub_overflow(ssize_t a, ssize_t b, ssize_t *diff)
+{
+	if ((b > 0 && a < SSIZE_MIN + b) || (b < 0 && a > SSIZE_MAX + b))
+		return true;
+
+	*diff = a - b;
+	return false;
+}
+#endif
+
 /*
  * Workarounds for https://github.com/llvm/llvm-project/issues/58992
  *
-- 
2.43.0