fs/namespace.c: WARN if mnt_count has become negative (edf7ddbf) · Commits · 戴 / test

Missing calls to mntget() (or equivalently, too many calls to mntput()) are hard to detect because mntput() delays freeing mounts using task_work_add(), then again using call_rcu(). As a result, mnt_count can often be decremented to -1 without getting a KASAN use-after-free report. Such cases are still bugs though, and they point to real use-after-frees being possible. For an example of this, see the bug fixed by commit ("vfs: fsmount: add missing mntget()"), discussed at https://lkml.kernel.org/linux-fsdevel/20190605135401.GB30925@xxxxxxxxxxxxxxxxxxxxxxxxx/T/#u . This bug *should* have been trivial to find. But actually, it wasn't found until syzkaller happened to use fchdir() to manipulate the reference count just right for the bug to be noticeable. Address this by making mntput_no_expire() issue a WARN if mnt_count has become negative. Suggested-by:

Miklos Szeredi <miklos@szeredi.hu> Signed-off-by:

Eric Biggers <ebiggers@google.com> Signed-off-by:

Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

+6 −3

Original line number	Diff line number	Diff line
		@@ -156,10 +156,10 @@ static inline void mnt_add_count(struct mount *mnt, int n)
		/*
		* vfsmount lock must be held for write
		*/
		unsigned int mnt_get_count(struct mount *mnt)
		int mnt_get_count(struct mount *mnt)
		{
		#ifdef CONFIG_SMP
		unsigned int count = 0;
		int count = 0;
		int cpu;

		for_each_possible_cpu(cpu) {
		@@ -1139,6 +1139,7 @@ static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);
		static void mntput_no_expire(struct mount *mnt)
		{
		LIST_HEAD(list);
		int count;

		rcu_read_lock();
		if (likely(READ_ONCE(mnt->mnt_ns))) {
		@@ -1162,7 +1163,9 @@ static void mntput_no_expire(struct mount *mnt)
		*/
		smp_mb();
		mnt_add_count(mnt, -1);
		if (mnt_get_count(mnt)) {
		count = mnt_get_count(mnt);
		if (count != 0) {
		WARN_ON(count < 0);
		rcu_read_unlock();
		unlock_mount_hash();
		return;

fs/pnode.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -44,7 +44,7 @@ int propagate_mount_busy(struct mount *, int);
		void propagate_mount_unlock(struct mount *);
		void mnt_release_group_id(struct mount *);
		int get_dominating_id(struct mount mnt, const struct path root);
		unsigned int mnt_get_count(struct mount *mnt);
		int mnt_get_count(struct mount *mnt);
		void mnt_set_mountpoint(struct mount , struct mountpoint ,
		struct mount *);
		void mnt_change_mountpoint(struct mount parent, struct mountpoint mp,

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