powerpc/spufs: reference context while dropping state mutex in scheduler

Based on an original patch from Christoph Hellwig <hch@lst.de>.

Currently, there is a possible reference-after-free in the spusched
code - contexts may be freed after we have released their state_mutex
in spusched_tick and find_victim.

This change takes a reference to the context before releasing the
mutex, so that the context doesn't get destroyed.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
index 2deeeba7eccf..4b6b0c3a8c95 100644
--- a/arch/powerpc/platforms/cell/spufs/sched.c
+++ b/arch/powerpc/platforms/cell/spufs/sched.c
@@ -641,8 +641,10 @@ static struct spu *find_victim(struct spu_context *ctx)
 
                         if (tmp && tmp->prio > ctx->prio &&
                             !(tmp->flags & SPU_CREATE_NOSCHED) &&
-                            (!victim || tmp->prio > victim->prio))
+                            (!victim || tmp->prio > victim->prio)) {
                                 victim = spu->ctx;
+                                get_spu_context(victim);
+                        }
                 }
                 mutex_unlock(&cbe_spu_info[node].list_mutex);
 
@@ -658,6 +660,7 @@ static struct spu *find_victim(struct spu_context *ctx)
                          * look at another context or give up after X retries.
                          */
                         if (!mutex_trylock(&victim->state_mutex)) {
+                                put_spu_context(victim);
                                 victim = NULL;
                                 goto restart;
                         }
@@ -670,6 +673,7 @@ static struct spu *find_victim(struct spu_context *ctx)
                                  * restart the search.
                                  */
                                 mutex_unlock(&victim->state_mutex);
+                                put_spu_context(victim);
                                 victim = NULL;
                                 goto restart;
                         }
@@ -687,6 +691,7 @@ static struct spu *find_victim(struct spu_context *ctx)
                                 spu_add_to_rq(victim);
 
                         mutex_unlock(&victim->state_mutex);
+                        put_spu_context(victim);
 
                         return spu;
                 }
@@ -985,9 +990,11 @@ static int spusched_thread(void *unused)
                                 struct spu_context *ctx = spu->ctx;
 
                                 if (ctx) {
+                                        get_spu_context(ctx);
                                         mutex_unlock(mtx);
                                         spusched_tick(ctx);
                                         mutex_lock(mtx);
+                                        put_spu_context(ctx);
                                 }
                         }
                         mutex_unlock(mtx);