On 12/13/2023 5:25 AM, David Howells wrote:
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@xxxxxxxxxx> cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx> cc: linux-afs@xxxxxxxxxxxxxxxxxxx --- 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. */
Reviewed-by: Jeffrey Altman <jaltman@xxxxxxxxxxxx>
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