On Tue, Nov 20, 2018 at 10:34:17PM +0800, Gao Xiang wrote:
> When the managed cache is enabled, the last reference count
> of a workgroup must be used for its workstation.
>
> Otherwise, it could lead to incorrect (un)freezes in
> the reclaim path, and it would be harmful.
>
> A typical race as follows:
>
> Thread 1 (In the reclaim path) Thread 2
> workgroup_freeze(grp, 1) refcnt = 1
> ...
> workgroup_unfreeze(grp, 1) refcnt = 1
> workgroup_get(grp) refcnt = 2 (x)
> workgroup_put(grp) refcnt = 1 (x)
> ...unexpected behaviors
>
> * grp is detached but still used, which violates cache-managed
> freeze constraint.
>
> Reviewed-by: Chao Yu <yuchao0@xxxxxxxxxx>
> Signed-off-by: Gao Xiang <gaoxiang25@xxxxxxxxxx>
> ---
> drivers/staging/erofs/internal.h | 1 +
> drivers/staging/erofs/utils.c | 131 +++++++++++++++++++++++++++------------
> 2 files changed, 93 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/staging/erofs/internal.h b/drivers/staging/erofs/internal.h
> index 57575c7f5635..89dbd0888e53 100644
> --- a/drivers/staging/erofs/internal.h
> +++ b/drivers/staging/erofs/internal.h
> @@ -250,6 +250,7 @@ static inline bool erofs_workgroup_get(struct erofs_workgroup *grp, int *ocnt)
> }
>
> #define __erofs_workgroup_get(grp) atomic_inc(&(grp)->refcount)
> +#define __erofs_workgroup_put(grp) atomic_dec(&(grp)->refcount)
>
> extern int erofs_workgroup_put(struct erofs_workgroup *grp);
>
> diff --git a/drivers/staging/erofs/utils.c b/drivers/staging/erofs/utils.c
> index ea8a962e5c95..90de8d9195b7 100644
> --- a/drivers/staging/erofs/utils.c
> +++ b/drivers/staging/erofs/utils.c
> @@ -83,12 +83,21 @@ int erofs_register_workgroup(struct super_block *sb,
>
> grp = xa_tag_pointer(grp, tag);
>
> - err = radix_tree_insert(&sbi->workstn_tree,
> - grp->index, grp);
> + /*
> + * Bump up reference count before making this workgroup
> + * visible to other users in order to avoid potential UAF
> + * without serialized by erofs_workstn_lock.
> + */
> + __erofs_workgroup_get(grp);
>
> - if (!err) {
> - __erofs_workgroup_get(grp);
> - }
> + err = radix_tree_insert(&sbi->workstn_tree,
> + grp->index, grp);
> + if (unlikely(err))
> + /*
> + * it's safe to decrease since the workgroup isn't visible
> + * and refcount >= 2 (cannot be freezed).
> + */
> + __erofs_workgroup_put(grp);
>
> erofs_workstn_unlock(sbi);
> radix_tree_preload_end();
> @@ -97,19 +106,91 @@ int erofs_register_workgroup(struct super_block *sb,
>
> extern void erofs_workgroup_free_rcu(struct erofs_workgroup *grp);
>
> +static void __erofs_workgroup_free(struct erofs_workgroup *grp)
> +{
> + atomic_long_dec(&erofs_global_shrink_cnt);
> + erofs_workgroup_free_rcu(grp);
> +}
> +
> int erofs_workgroup_put(struct erofs_workgroup *grp)
> {
> int count = atomic_dec_return(&grp->refcount);
>
> if (count == 1)
> atomic_long_inc(&erofs_global_shrink_cnt);
> - else if (!count) {
> - atomic_long_dec(&erofs_global_shrink_cnt);
> - erofs_workgroup_free_rcu(grp);
> - }
> + else if (!count)
> + __erofs_workgroup_free(grp);
> return count;
> }
>
> +#ifdef EROFS_FS_HAS_MANAGED_CACHE
> +/* for cache-managed case, customized reclaim paths exist */
> +static void erofs_workgroup_unfreeze_final(struct erofs_workgroup *grp)
> +{
> + erofs_workgroup_unfreeze(grp, 0);
> + __erofs_workgroup_free(grp);
> +}
> +
> +bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
> + struct erofs_workgroup *grp,
> + bool cleanup)
> +{
> + /*
> + * for managed cache enabled, the refcount of workgroups
> + * themselves could be < 0 (freezed). So there is no guarantee
> + * that all refcount > 0 if managed cache is enabled.
> + */
> + if (!erofs_workgroup_try_to_freeze(grp, 1))
> + return false;
> +
> + /*
> + * note that all cached pages should be unlinked
> + * before delete it from the radix tree.
> + * Otherwise some cached pages of an orphan old workgroup
> + * could be still linked after the new one is available.
> + */
> + if (erofs_try_to_free_all_cached_pages(sbi, grp)) {
> + erofs_workgroup_unfreeze(grp, 1);
> + return false;
> + }
> +
> + /* it is impossible to fail after we freeze the workgroup */
> + BUG_ON(xa_untag_pointer(radix_tree_delete(&sbi->workstn_tree,
> + grp->index)) != grp);
> +
It is not a good idea to crash the system. Please try to recover from
this, a BUG_ON() implies that the developer has no idea how to handle
this type of error so either it can never happen (which means that the
BUG_ON can be removed), or you need to fix the logic to handle this type
of issue when it does happen.
I'm not going to take this as-is, sorry.
greg k-h
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