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

David Howells
2023-12-21T13:57:36Z

Hi Linus,

Could you apply this fix please?

David
---
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:
<TASK>
? __warn+0x8b/0xf5
? report_bug+0xbf/0x11b
? refcount_warn_saturate+0x7a/0xda
? handle_bug+0x3c/0x5b
? exc_invalid_op+0x13/0x59
? asm_exc_invalid_op+0x16/0x20
? refcount_warn_saturate+0x7a/0xda
? refcount_warn_saturate+0x7a/0xda
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
---
fs/afs/internal.h | 2 ++
fs/afs/volume.c | 26 +++++++++++++++++++++++---
2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index a812952be1c9..7385d62c8cf5 100644
--- 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 29d483c80281..115c081a8e2c 100644
--- 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.
*/