linux-kernel - [PATCH] ipc/sem: Avoid indexing past end of sem

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Message-ID: <20170508222345.GA52073@beast>
Date:   Mon, 8 May 2017 15:23:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Fabian Frederick <fabf@...net.be>, linux-kernel@...r.kernel.org
Subject: [PATCH] ipc/sem: Avoid indexing past end of sem_array

This changes the struct + trailing data pattern to using a void * so that
the end of sem_array is found without possibly indexing past the end which
can upset some static analyzers. Mostly, this ends up avoiding a cast
between different non-void types, which the future randstruct GCC plugin
was warning about.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 ipc/sem.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 947dc2348271..f7cae2b35d62 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -475,6 +475,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 {
 	int id;
 	int retval;
+	void *sem_alloc;
 	struct sem_array *sma;
 	int size;
 	key_t key = params->key;
@@ -488,11 +489,14 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 		return -ENOSPC;
 
 	size = sizeof(*sma) + nsems * sizeof(struct sem);
-	sma = ipc_rcu_alloc(size);
-	if (!sma)
+	sem_alloc = ipc_rcu_alloc(size);
+	if (!sem_alloc)
 		return -ENOMEM;
 
-	memset(sma, 0, size);
+	memset(sem_alloc, 0, size);
+
+	sma = sem_alloc;
+	sma->sem_base = sem_alloc + sizeof(*sma);
 
 	sma->sem_perm.mode = (semflg & S_IRWXUGO);
 	sma->sem_perm.key = key;
@@ -504,8 +508,6 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 		return retval;
 	}
 
-	sma->sem_base = (struct sem *) &sma[1];
-
 	for (i = 0; i < nsems; i++) {
 		INIT_LIST_HEAD(&sma->sem_base[i].pending_alter);
 		INIT_LIST_HEAD(&sma->sem_base[i].pending_const);
-- 
2.7.4


-- 
Kees Cook
Pixel Security