aio: v4 ensure access to ctx->ring_pages is correctly serialised for migration

As reported by Tang Chen, Gu Zheng and Yasuaki Isimatsu, the following issues
exist in the aio ring page migration support.

As a result, for example, we have the following problem:

            thread 1                      |              thread 2
                                          |
aio_migratepage()                         |
 |-> take ctx->completion_lock            |
 |-> migrate_page_copy(new, old)          |
 |   *NOW*, ctx->ring_pages[idx] == old   |
                                          |
                                          |    *NOW*, ctx->ring_pages[idx] == old
                                          |    aio_read_events_ring()
                                          |     |-> ring = kmap_atomic(ctx->ring_pages[0])
                                          |     |-> ring->head = head;          *HERE, write to the old ring page*
                                          |     |-> kunmap_atomic(ring);
                                          |
 |-> ctx->ring_pages[idx] = new           |
 |   *BUT NOW*, the content of            |
 |    ring_pages[idx] is old.             |
 |-> release ctx->completion_lock         |

As above, the new ring page will not be updated.

Fix this issue, as well as prevent races in aio_ring_setup() by holding
the ring_lock mutex during kioctx setup and page migration.  This avoids
the overhead of taking another spinlock in aio_read_events_ring() as Tang's
and Gu's original fix did, pushing the overhead into the migration code.

Note that to handle the nesting of ring_lock inside of mmap_sem, the
migratepage operation uses mutex_trylock().  Page migration is not a 100%
critical operation in this case, so the ocassional failure can be
tolerated.  This issue was reported by Sasha Levin.

Based on feedback from Linus, avoid the extra taking of ctx->completion_lock.
Instead, make page migration fully serialised by mapping->private_lock, and
have aio_free_ring() simply disconnect the kioctx from the mapping by calling
put_aio_ring_file() before touching ctx->ring_pages[].  This simplifies the
error handling logic in aio_migratepage(), and should improve robustness.

v4: always do mutex_unlock() in cases when kioctx setup fails.

Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Gu Zheng <guz.fnst@cn.fujitsu.com>
Cc: stable@vger.kernel.org

1 files changed, 67 insertions, 53 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 062a5f6a1448..12a3de0ee6da 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -52,7 +52,8 @@
 struct aio_ring {
         unsigned        id;     /* kernel internal index number */
         unsigned        nr;     /* number of io_events */
-        unsigned        head;
+        unsigned        head;   /* Written to by userland or under ring_lock
+                                 * mutex by aio_read_events_ring(). */
         unsigned        tail;
 
         unsigned        magic;
@@ -243,6 +244,11 @@ static void aio_free_ring(struct kioctx *ctx)
 {
         int i;
 
+        /* Disconnect the kiotx from the ring file.  This prevents future
+         * accesses to the kioctx from page migration.
+         */
+        put_aio_ring_file(ctx);
+
         for (i = 0; i < ctx->nr_pages; i++) {
                 struct page *page;
                 pr_debug("pid(%d) [%d] page->count=%d\n", current->pid, i,
@@ -254,8 +260,6 @@ static void aio_free_ring(struct kioctx *ctx)
                 put_page(page);
         }
 
-        put_aio_ring_file(ctx);
-
         if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages) {
                 kfree(ctx->ring_pages);
                 ctx->ring_pages = NULL;
@@ -283,29 +287,38 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
 {
         struct kioctx *ctx;
         unsigned long flags;
+        pgoff_t idx;
         int rc;
 
         rc = 0;
 
-        /* Make sure the old page hasn't already been changed */
+        /* mapping->private_lock here protects against the kioctx teardown.  */
         spin_lock(&mapping->private_lock);
         ctx = mapping->private_data;
-        if (ctx) {
-                pgoff_t idx;
-                spin_lock_irqsave(&ctx->completion_lock, flags);
-                idx = old->index;
-                if (idx < (pgoff_t)ctx->nr_pages) {
-                        if (ctx->ring_pages[idx] != old)
-                                rc = -EAGAIN;
-                } else
-                        rc = -EINVAL;
-                spin_unlock_irqrestore(&ctx->completion_lock, flags);
+        if (!ctx) {
+                rc = -EINVAL;
+                goto out;
+        }
+
+        /* The ring_lock mutex.  The prevents aio_read_events() from writing
+         * to the ring's head, and prevents page migration from mucking in
+         * a partially initialized kiotx.
+         */
+        if (!mutex_trylock(&ctx->ring_lock)) {
+                rc = -EAGAIN;
+                goto out;
+        }
+
+        idx = old->index;
+        if (idx < (pgoff_t)ctx->nr_pages) {
+                /* Make sure the old page hasn't already been changed */
+                if (ctx->ring_pages[idx] != old)
+                        rc = -EAGAIN;
         } else
                 rc = -EINVAL;
-        spin_unlock(&mapping->private_lock);
 
         if (rc != 0)
-                return rc;
+                goto out_unlock;
 
         /* Writeback must be complete */
         BUG_ON(PageWriteback(old));
@@ -314,38 +327,26 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
         rc = migrate_page_move_mapping(mapping, new, old, NULL, mode, 1);
         if (rc != MIGRATEPAGE_SUCCESS) {
                 put_page(new);
-                return rc;
+                goto out_unlock;
         }
 
-        /* We can potentially race against kioctx teardown here.  Use the
-         * address_space's private data lock to protect the mapping's
-         * private_data.
+        /* Take completion_lock to prevent other writes to the ring buffer
+         * while the old page is copied to the new.  This prevents new
+         * events from being lost.
          */
-        spin_lock(&mapping->private_lock);
-        ctx = mapping->private_data;
-        if (ctx) {
-                pgoff_t idx;
-                spin_lock_irqsave(&ctx->completion_lock, flags);
-                migrate_page_copy(new, old);
-                idx = old->index;
-                if (idx < (pgoff_t)ctx->nr_pages) {
-                        /* And only do the move if things haven't changed */
-                        if (ctx->ring_pages[idx] == old)
-                                ctx->ring_pages[idx] = new;
-                        else
-                                rc = -EAGAIN;
-                } else
-                        rc = -EINVAL;
-                spin_unlock_irqrestore(&ctx->completion_lock, flags);
-        } else
-                rc = -EBUSY;
-        spin_unlock(&mapping->private_lock);
+        spin_lock_irqsave(&ctx->completion_lock, flags);
+        migrate_page_copy(new, old);
+        BUG_ON(ctx->ring_pages[idx] != old);
+        ctx->ring_pages[idx] = new;
+        spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
-        if (rc == MIGRATEPAGE_SUCCESS)
-                put_page(old);
-        else
-                put_page(new);
+        /* The old page is no longer accessible. */
+        put_page(old);
 
+out_unlock:
+        mutex_unlock(&ctx->ring_lock);
+out:
+        spin_unlock(&mapping->private_lock);
         return rc;
 }
 #endif
@@ -380,7 +381,7 @@ static int aio_setup_ring(struct kioctx *ctx)
         file = aio_private_file(ctx, nr_pages);
         if (IS_ERR(file)) {
                 ctx->aio_ring_file = NULL;
-                return -EAGAIN;
+                return -ENOMEM;
         }
 
         ctx->aio_ring_file = file;
@@ -415,7 +416,7 @@ static int aio_setup_ring(struct kioctx *ctx)
 
         if (unlikely(i != nr_pages)) {
                 aio_free_ring(ctx);
-                return -EAGAIN;
+                return -ENOMEM;
         }
 
         ctx->mmap_size = nr_pages * PAGE_SIZE;
@@ -429,7 +430,7 @@ static int aio_setup_ring(struct kioctx *ctx)
         if (IS_ERR((void *)ctx->mmap_base)) {
                 ctx->mmap_size = 0;
                 aio_free_ring(ctx);
-                return -EAGAIN;
+                return -ENOMEM;
         }
 
         pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
@@ -556,6 +557,10 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
                                         rcu_read_unlock();
                                         spin_unlock(&mm->ioctx_lock);
 
+                                        /* While kioctx setup is in progress,
+                                         * we are protected from page migration
+                                         * changes ring_pages by ->ring_lock.