aio: fix use-after-free in aio_migratepage
Dmitry Vyukov managed to trigger a case where aio_migratepage can cause a
use-after-free during teardown of the aio ring buffer's mapping. This turns
out to be caused by access to the ioctx's ring_pages via the migratepage
operation which was not being protected by any locks during ioctx freeing.
Use the address_space's private_lock to protect use and updates of the mapping's
private_data, and make ioctx teardown unlink the ioctx from the address space.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
diff --git a/fs/aio.c b/fs/aio.c
index 6b868f0..067e3d3 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -167,10 +167,25 @@
}
__initcall(aio_setup);
+static void put_aio_ring_file(struct kioctx *ctx)
+{
+ struct file *aio_ring_file = ctx->aio_ring_file;
+ if (aio_ring_file) {
+ truncate_setsize(aio_ring_file->f_inode, 0);
+
+ /* Prevent further access to the kioctx from migratepages */
+ spin_lock(&aio_ring_file->f_inode->i_mapping->private_lock);
+ aio_ring_file->f_inode->i_mapping->private_data = NULL;
+ ctx->aio_ring_file = NULL;
+ spin_unlock(&aio_ring_file->f_inode->i_mapping->private_lock);
+
+ fput(aio_ring_file);
+ }
+}
+
static void aio_free_ring(struct kioctx *ctx)
{
int i;
- struct file *aio_ring_file = ctx->aio_ring_file;
for (i = 0; i < ctx->nr_pages; i++) {
pr_debug("pid(%d) [%d] page->count=%d\n", current->pid, i,
@@ -178,14 +193,10 @@
put_page(ctx->ring_pages[i]);
}
+ put_aio_ring_file(ctx);
+
if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages)
kfree(ctx->ring_pages);
-
- if (aio_ring_file) {
- truncate_setsize(aio_ring_file->f_inode, 0);
- fput(aio_ring_file);
- ctx->aio_ring_file = NULL;
- }
}
static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)
@@ -207,9 +218,8 @@
static int aio_migratepage(struct address_space *mapping, struct page *new,
struct page *old, enum migrate_mode mode)
{
- struct kioctx *ctx = mapping->private_data;
+ struct kioctx *ctx;
unsigned long flags;
- unsigned idx = old->index;
int rc;
/* Writeback must be complete */
@@ -224,10 +234,23 @@
get_page(new);
- spin_lock_irqsave(&ctx->completion_lock, flags);
- migrate_page_copy(new, old);
- ctx->ring_pages[idx] = new;
- spin_unlock_irqrestore(&ctx->completion_lock, flags);
+ /* We can potentially race against kioctx teardown here. Use the
+ * address_space's private data lock to protect the mapping's
+ * private_data.
+ */
+ 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)
+ ctx->ring_pages[idx] = new;
+ spin_unlock_irqrestore(&ctx->completion_lock, flags);
+ } else
+ rc = -EBUSY;
+ spin_unlock(&mapping->private_lock);
return rc;
}
@@ -617,8 +640,7 @@
out_freeref:
free_percpu(ctx->users.pcpu_count);
out_freectx:
- if (ctx->aio_ring_file)
- fput(ctx->aio_ring_file);
+ put_aio_ring_file(ctx);
kmem_cache_free(kioctx_cachep, ctx);
pr_debug("error allocating ioctx %d\n", err);
return ERR_PTR(err);