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: Brad Spengler <spender@grsecurity.net>
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 11 insertions, 5 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index fbfae008ba44..e8bc3420d63e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -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: