afs: Fix use-after-free due to get/remove race in volume tree

When an afs_volume struct is put, its refcount is reduced to 0 before
the cell->volume_lock is taken and the volume removed from the
cell->volumes tree.

Unfortunately, this means that the lookup code can race and see a volume
with a zero ref in the tree, resulting in a use-after-free:

    refcount_t: addition on 0; use-after-free.
    WARNING: CPU: 3 PID: 130782 at lib/refcount.c:25 refcount_warn_saturate+0x7a/0xda
    ...
    RIP: 0010:refcount_warn_saturate+0x7a/0xda
    ...
    Call Trace:
     afs_get_volume+0x3d/0x55
     afs_create_volume+0x126/0x1de
     afs_validate_fc+0xfe/0x130
     afs_get_tree+0x20/0x2e5
     vfs_get_tree+0x1d/0xc9
     do_new_mount+0x13b/0x22e
     do_mount+0x5d/0x8a
     __do_sys_mount+0x100/0x12a
     do_syscall_64+0x3a/0x94
     entry_SYSCALL_64_after_hwframe+0x62/0x6a

Fix this by:

 (1) When putting, use a flag to indicate if the volume has been removed
     from the tree and skip the rb_erase if it has.

 (2) When looking up, use a conditional ref increment and if it fails
     because the refcount is 0, replace the node in the tree and set the
     removal flag.

Fixes: 20325960f875 ("afs: Reorganise volume and server trees to be rooted on the cell")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index a812952be1c948d059991d1d67562284e340303a..7385d62c8cf50318c10a257479919fa33bff8634 100644 (file)
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -586,6 +586,7 @@ struct afs_volume {
 #define AFS_VOLUME_OFFLINE     4       /* - T if volume offline notice given */
 #define AFS_VOLUME_BUSY                5       /* - T if volume busy notice given */
 #define AFS_VOLUME_MAYBE_NO_IBULK 6    /* - T if some servers don't have InlineBulkStatus */
+#define AFS_VOLUME_RM_TREE     7       /* - Set if volume removed from cell->volumes */
 #ifdef CONFIG_AFS_FSCACHE
        struct fscache_volume   *cache;         /* Caching cookie */
 #endif
@@ -1513,6 +1514,7 @@ extern struct afs_vlserver_list *afs_extract_vlserver_list(struct afs_cell *,
 extern struct afs_volume *afs_create_volume(struct afs_fs_context *);
 extern int afs_activate_volume(struct afs_volume *);
 extern void afs_deactivate_volume(struct afs_volume *);
+bool afs_try_get_volume(struct afs_volume *volume, enum afs_volume_trace reason);
 extern struct afs_volume *afs_get_volume(struct afs_volume *, enum afs_volume_trace);
 extern void afs_put_volume(struct afs_net *, struct afs_volume *, enum afs_volume_trace);
 extern int afs_check_volume_status(struct afs_volume *, struct afs_operation *);

diff --git a/fs/afs/volume.c b/fs/afs/volume.c
index 29d483c80281309c37ee6b3d1cf28affffbd6e14..115c081a8e2ce54e3986a6cd06b33bfb12bc0d5b 100644 (file)
--- a/fs/afs/volume.c
+++ b/fs/afs/volume.c
@@ -32,8 +32,13 @@ static struct afs_volume *afs_insert_volume_into_cell(struct afs_cell *cell,
                } else if (p->vid > volume->vid) {
                        pp = &(*pp)->rb_right;
                } else {
-                       volume = afs_get_volume(p, afs_volume_trace_get_cell_insert);
-                       goto found;
+                       if (afs_try_get_volume(p, afs_volume_trace_get_cell_insert)) {
+                               volume = p;
+                               goto found;
+                       }
+
+                       set_bit(AFS_VOLUME_RM_TREE, &volume->flags);
+                       rb_replace_node_rcu(&p->cell_node, &volume->cell_node, &cell->volumes);
                }
        }
 
@@ -56,7 +61,8 @@ static void afs_remove_volume_from_cell(struct afs_volume *volume)
                                 afs_volume_trace_remove);
                write_seqlock(&cell->volume_lock);
                hlist_del_rcu(&volume->proc_link);
-               rb_erase(&volume->cell_node, &cell->volumes);
+               if (!test_and_set_bit(AFS_VOLUME_RM_TREE, &volume->flags))
+                       rb_erase(&volume->cell_node, &cell->volumes);
                write_sequnlock(&cell->volume_lock);
        }
 }
@@ -231,6 +237,20 @@ static void afs_destroy_volume(struct afs_net *net, struct afs_volume *volume)
        _leave(" [destroyed]");
 }
 
+/*
+ * Try to get a reference on a volume record.
+ */
+bool afs_try_get_volume(struct afs_volume *volume, enum afs_volume_trace reason)
+{
+       int r;
+
+       if (__refcount_inc_not_zero(&volume->ref, &r)) {
+               trace_afs_volume(volume->vid, r + 1, reason);
+               return true;
+       }
+       return false;
+}
+
 /*
  * Get a reference on a volume record.
  */