ipc/sem: avoid ipc_rcu_alloc()

Instead of using ipc_rcu_alloc() which only performs the refcount bump,
open code it to perform better sem-specific checks.  This also allows
for sem_array structure layout to be randomized in the future.

[manfred@colorfullife.com: Rediff, because the memset was temporarily inside ipc_rcu_alloc()]
Link: http://lkml.kernel.org/r/20170525185107.12869-10-manfred@colorfullife.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 5 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index a04c4d6d120c..445a5b5eb88f 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -451,6 +451,25 @@ static inline void sem_rmid(struct ipc_namespace *ns, struct sem_array *s)
         ipc_rmid(&sem_ids(ns), &s->sem_perm);
 }
 
+static struct sem_array *sem_alloc(size_t nsems)
+{
+        struct sem_array *sma;
+        size_t size;
+
+        if (nsems > (INT_MAX - sizeof(*sma)) / sizeof(sma->sems[0]))
+                return NULL;
+
+        size = sizeof(*sma) + nsems * sizeof(sma->sems[0]);
+        sma = kvmalloc(size, GFP_KERNEL);
+        if (unlikely(!sma))
+                return NULL;
+
+        memset(sma, 0, size);
+        atomic_set(&sma->sem_perm.refcount, 1);
+
+        return sma;
+}
+
 /**
  * newary - Create a new semaphore set
  * @ns: namespace
@@ -463,7 +482,6 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
         int id;
         int retval;
         struct sem_array *sma;
-        int size;
         key_t key = params->key;
         int nsems = params->u.nsems;
         int semflg = params->flg;
@@ -474,10 +492,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
         if (ns->used_sems + nsems > ns->sc_semmns)
                 return -ENOSPC;
 
-        BUILD_BUG_ON(offsetof(struct sem_array, sem_perm) != 0);
-
-        size = sizeof(*sma) + nsems * sizeof(sma->sems[0]);
-        sma = container_of(ipc_rcu_alloc(size), struct sem_array, sem_perm);
+        sma = sem_alloc(nsems);
         if (!sma)
                 return -ENOMEM;