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Commit 7ea600b5 authored by Al Viro's avatar Al Viro
Browse files

Nest rename_lock inside vfsmount_lock 



... lest we get livelocks between path_is_under() and d_path() and friends.

The thing is, wrt fairness lglocks are more similar to rwsems than to rwlocks;
it is possible to have thread B spin on attempt to take lock shared while thread
A is already holding it shared, if B is on lower-numbered CPU than A and there's
a thread C spinning on attempt to take the same lock exclusive.

As the result, we need consistent ordering between vfsmount_lock (lglock) and
rename_lock (seq_lock), even though everything that takes both is going to take
vfsmount_lock only shared.

Spotted-by: default avatarBrad Spengler <spender@grsecurity.net>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 06ae43f3

fs/dcache.c

+11 −5
Original line number Diff line number Diff line
@@ -2542,7 +2542,6 @@ static int prepend_path(const struct path *path, 
	bool slash = false;

	int error = 0;



	br_read_lock(&vfsmount_lock);

	while (dentry != root->dentry || vfsmnt != root->mnt) {

		struct dentry * parent;
@@ -2572,8 +2571,6 @@ static int prepend_path(const struct path *path, 
	if (!error && !slash)

		error = prepend(buffer, buflen, "/", 1);



out:

	br_read_unlock(&vfsmount_lock);

	return error;



global_root:
@@ -2590,7 +2587,7 @@ global_root: 
		error = prepend(buffer, buflen, "/", 1);

	if (!error)

		error = is_mounted(vfsmnt) ? 1 : 2;

	goto out;

	return error;

}



/**
@@ -2617,9 +2614,11 @@ char *__d_path(const struct path *path, 
	int error;



	prepend(&res, &buflen, "\0", 1);

	br_read_lock(&vfsmount_lock);

	write_seqlock(&rename_lock);

	error = prepend_path(path, root, &res, &buflen);

	write_sequnlock(&rename_lock);

	br_read_unlock(&vfsmount_lock);



	if (error < 0)

		return ERR_PTR(error);
@@ -2636,9 +2635,11 @@ char *d_absolute_path(const struct path *path, 
	int error;



	prepend(&res, &buflen, "\0", 1);

	br_read_lock(&vfsmount_lock);

	write_seqlock(&rename_lock);

	error = prepend_path(path, &root, &res, &buflen);

	write_sequnlock(&rename_lock);

	br_read_unlock(&vfsmount_lock);



	if (error > 1)

		error = -EINVAL;
@@ -2702,11 +2703,13 @@ char *d_path(const struct path *path, char *buf, int buflen) 
		return path->dentry->d_op->d_dname(path->dentry, buf, buflen);



	get_fs_root(current->fs, &root);

	br_read_lock(&vfsmount_lock);

	write_seqlock(&rename_lock);

	error = path_with_deleted(path, &root, &res, &buflen);

	write_sequnlock(&rename_lock);

	br_read_unlock(&vfsmount_lock);

	if (error < 0)

		res = ERR_PTR(error);

	write_sequnlock(&rename_lock);

	path_put(&root);

	return res;

}
@@ -2830,6 +2833,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) 
	get_fs_root_and_pwd(current->fs, &root, &pwd);



	error = -ENOENT;

	br_read_lock(&vfsmount_lock);

	write_seqlock(&rename_lock);

	if (!d_unlinked(pwd.dentry)) {

		unsigned long len;
@@ -2839,6 +2843,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) 
		prepend(&cwd, &buflen, "\0", 1);

		error = prepend_path(&pwd, &root, &cwd, &buflen);

		write_sequnlock(&rename_lock);

		br_read_unlock(&vfsmount_lock);



		if (error < 0)

			goto out;
@@ -2859,6 +2864,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size) 
		}

	} else {

		write_sequnlock(&rename_lock);

		br_read_unlock(&vfsmount_lock);

	}



out: