4.16-stable review patch. If anyone has any objections, please let me know.
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From: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp
commit 8e04944f0ea8b838399049bdcda920ab36ae3b04 upstream.
syzbot is catching so many bugs triggered by commit 9ee332d99e4d5a97 ("sget(): handle failures of register_shrinker()"). That commit expected that calling kill_sb() from deactivate_locked_super() without successful fill_super() is safe, but the reality was different; some callers assign attributes which are needed for kill_sb() after sget() succeeds.
For example, [1] is a report where sb->s_mode (which seems to be either FMODE_READ | FMODE_EXCL | FMODE_WRITE or FMODE_READ | FMODE_EXCL) is not assigned unless sget() succeeds. But it does not worth complicate sget() so that register_shrinker() failure path can safely call kill_block_super() via kill_sb(). Making alloc_super() fail if memory allocation for register_shrinker() failed is much simpler. Let's avoid calling deactivate_locked_super() from sget_userns() by preallocating memory for the shrinker and making register_shrinker() in sget_userns() never fail.
[1] https://syzkaller.appspot.com/bug?id=588996a25a2587be2e3a54e8646728fb9cae44e...
Signed-off-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Reported-by: syzbot syzbot+5a170e19c963a2e0df79@syzkaller.appspotmail.com Cc: Al Viro viro@zeniv.linux.org.uk Cc: Michal Hocko mhocko@suse.com Signed-off-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/super.c | 9 ++++----- include/linux/shrinker.h | 7 +++++-- mm/vmscan.c | 21 ++++++++++++++++++++- 3 files changed, 29 insertions(+), 8 deletions(-)
--- a/fs/super.c +++ b/fs/super.c @@ -166,6 +166,7 @@ static void destroy_unused_super(struct security_sb_free(s); put_user_ns(s->s_user_ns); kfree(s->s_subtype); + free_prealloced_shrinker(&s->s_shrink); /* no delays needed */ destroy_super_work(&s->destroy_work); } @@ -251,6 +252,8 @@ static struct super_block *alloc_super(s s->s_shrink.count_objects = super_cache_count; s->s_shrink.batch = 1024; s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE; + if (prealloc_shrinker(&s->s_shrink)) + goto fail; return s;
fail: @@ -517,11 +520,7 @@ retry: hlist_add_head(&s->s_instances, &type->fs_supers); spin_unlock(&sb_lock); get_filesystem(type); - err = register_shrinker(&s->s_shrink); - if (err) { - deactivate_locked_super(s); - s = ERR_PTR(err); - } + register_shrinker_prepared(&s->s_shrink); return s; }
--- a/include/linux/shrinker.h +++ b/include/linux/shrinker.h @@ -75,6 +75,9 @@ struct shrinker { #define SHRINKER_NUMA_AWARE (1 << 0) #define SHRINKER_MEMCG_AWARE (1 << 1)
-extern int register_shrinker(struct shrinker *); -extern void unregister_shrinker(struct shrinker *); +extern int prealloc_shrinker(struct shrinker *shrinker); +extern void register_shrinker_prepared(struct shrinker *shrinker); +extern int register_shrinker(struct shrinker *shrinker); +extern void unregister_shrinker(struct shrinker *shrinker); +extern void free_prealloced_shrinker(struct shrinker *shrinker); #endif --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -258,7 +258,7 @@ unsigned long lruvec_lru_size(struct lru /* * Add a shrinker callback to be called from the vm. */ -int register_shrinker(struct shrinker *shrinker) +int prealloc_shrinker(struct shrinker *shrinker) { size_t size = sizeof(*shrinker->nr_deferred);
@@ -268,10 +268,29 @@ int register_shrinker(struct shrinker *s shrinker->nr_deferred = kzalloc(size, GFP_KERNEL); if (!shrinker->nr_deferred) return -ENOMEM; + return 0; +} + +void free_prealloced_shrinker(struct shrinker *shrinker) +{ + kfree(shrinker->nr_deferred); + shrinker->nr_deferred = NULL; +}
+void register_shrinker_prepared(struct shrinker *shrinker) +{ down_write(&shrinker_rwsem); list_add_tail(&shrinker->list, &shrinker_list); up_write(&shrinker_rwsem); +} + +int register_shrinker(struct shrinker *shrinker) +{ + int err = prealloc_shrinker(shrinker); + + if (err) + return err; + register_shrinker_prepared(shrinker); return 0; } EXPORT_SYMBOL(register_shrinker);