[PATCH RFC 5/5] kdbus: improve tests on incrementing quota

[Sergei Zviagintsev]

 1) Rewrite

        quota->memory + memory > U32_MAX

    as
        U32_MAX - quota->memory < memory

    and provide the comment on why we need that check.

    We have no overflow issue in the original expression when size_t is
    32-bit because the previous one (available - quota->memory < memory)
    guarantees that quota->memory + memory doesn't exceed `available'
    which is <= U32_MAX in that case.

    But lets stay explicit rather than implicit, it would save us from
    describing HOW the code works.

 2) Add WARN_ON when quota->msgs > KDBUS_CONN_MAX_MSGS

    This is somewhat inconsistent, so we need to properly report it.

 3) Replace

        quota->fds + fds < quota->fds ||
        quota->fds + fds > KDBUS_CONN_MAX_FDS_PER_USER

    with

        KDBUS_CONN_MAX_FDS_PER_USER - quota->fds < fds

    and add explicit WARN_ON in the case
    quota->fds > KDBUS_CONN_MAX_FDS_PER_USER.

    Reading the code one can assume that the first expression is
    there to ensure that we won't have an overflow in quota->fds after
    quota->fds += fds, but what it really does is testing for size_t
    overflow in `quota->fds + fds' to be safe in the second expression
    (as fds is size_t, quota->fds is converted to bigger type).

    Rewrite it in more obvious way. KDBUS_CONN_MAX_FDS_PER_USER is
    checked at compile time to fill in quota->fds type (there is
    BUILD_BUG_ON), so no further checks for quota->fds overflow are
    needed.

Signed-off-by: Sergei Zviagintsev <sergei@xxxxxxxx>
---
 ipc/kdbus/connection.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/ipc/kdbus/connection.c b/ipc/kdbus/connection.c
index 12e32de310f5..6556a0f9d44c 100644
--- a/ipc/kdbus/connection.c
+++ b/ipc/kdbus/connection.c
@@ -701,13 +701,21 @@ int kdbus_conn_quota_inc(struct kdbus_conn *c, struct kdbus_user *u,
 	available = (available - accounted + quota->memory) / 3;
 
 	if (available < quota->memory ||
-	    available - quota->memory < memory ||
-	    quota->memory + memory > U32_MAX)
+	    available - quota->memory < memory)
 		return -ENOBUFS;
-	if (quota->msgs >= KDBUS_CONN_MAX_MSGS)
+
+	/*
+	 * available is size_t and thus it could be greater than U32_MAX.
+	 * Ensure that quota->memory won't overflow.
+	 */
+	if (U32_MAX - quota->memory < memory)
+		return -ENOBUFS;
+
+	if (WARN_ON(quota->msgs > KDBUS_CONN_MAX_MSGS) ||
+	    quota->msgs == KDBUS_CONN_MAX_MSGS)
 		return -ENOBUFS;
-	if (quota->fds + fds < quota->fds ||
-	    quota->fds + fds > KDBUS_CONN_MAX_FDS_PER_USER)
+	if (WARN_ON(quota->fds > KDBUS_CONN_MAX_FDS_PER_USER) ||
+	    KDBUS_CONN_MAX_FDS_PER_USER - quota->fds < fds)
 		return -EMFILE;
 
 	quota->memory += memory;
-- 
1.8.3.1

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