autofs4: fix indirect mount pending expire race

The selection of a dentry for expiration and the setting of the
AUTOFS_INF_EXPIRING flag isn't done atomically which can lead to lookups
walking into an expiring mount.

What happens is that an expire is initiated by the daemon and a dentry is
selected for expire but, since there is no lock held between the selection
and setting of the expiring flag, a process may find the flag clear and
continue walking into the mount tree at the same time the daemon attempts
the expire it.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 27 insertions, 5 deletions

diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 61d1dca16884..1c2579de1f2e 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -133,7 +133,10 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
         /* Block on any pending expiry here; invalidate the dentry
            when expiration is done to trigger mount request with a new
            dentry */
-        if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
+        spin_lock(&sbi->fs_lock);
+        if (ino->flags & AUTOFS_INF_EXPIRING) {
+                spin_unlock(&sbi->fs_lock);
+
                 DPRINTK("waiting for expire %p name=%.*s",
                          dentry, dentry->d_name.len, dentry->d_name.name);
 
@@ -149,8 +152,11 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
                 status = d_invalidate(dentry);
                 if (status != -EBUSY)
                         return -EAGAIN;
-        }
 
+                goto cont;
+        }
+        spin_unlock(&sbi->fs_lock);
+cont:
         DPRINTK("dentry=%p %.*s ino=%p",
                  dentry, dentry->d_name.len, dentry->d_name.name, dentry->d_inode);
 
@@ -229,15 +235,21 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
                 goto done;
 
         /* If an expire request is pending wait for it. */
-        if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
+        spin_lock(&sbi->fs_lock);
+        if (ino->flags & AUTOFS_INF_EXPIRING) {
+                spin_unlock(&sbi->fs_lock);
+
                 DPRINTK("waiting for active request %p name=%.*s",
                         dentry, dentry->d_name.len, dentry->d_name.name);
 
                 status = autofs4_wait(sbi, dentry, NFY_NONE);
 
                 DPRINTK("request done status=%d", status);
-        }
 
+                goto cont;
+        }
+        spin_unlock(&sbi->fs_lock);
+cont:
         /*
          * If the dentry contains directories then it is an
          * autofs multi-mount with no root mount offset. So
@@ -292,8 +304,11 @@ static int autofs4_revalidate(struct dentry *dentry, struct nameidata *nd)
         int status = 1;
 
         /* Pending dentry */
+        spin_lock(&sbi->fs_lock);
         if (autofs4_ispending(dentry)) {
                 /* The daemon never causes a mount to trigger */
+                spin_unlock(&sbi->fs_lock);
+
                 if (oz_mode)
                         return 1;
 
@@ -316,6 +331,7 @@ static int autofs4_revalidate(struct dentry *dentry, struct nameidata *nd)
 
                 return status;
         }
+        spin_unlock(&sbi->fs_lock);
 
         /* Negative dentry.. invalidate if "old" */
         if (dentry->d_inode == NULL)
@@ -329,6 +345,7 @@ static int autofs4_revalidate(struct dentry *dentry, struct nameidata *nd)
                 DPRINTK("dentry=%p %.*s, emptydir",
                          dentry, dentry->d_name.len, dentry->d_name.name);
                 spin_unlock(&dcache_lock);
+
                 /* The daemon never causes a mount to trigger */
                 if (oz_mode)
                         return 1;
@@ -521,13 +538,18 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
                  * so it must have been successful, so just wait for it.
                  */
                 ino = autofs4_dentry_ino(expiring);
-                while (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
+                spin_lock(&sbi->fs_lock);
+                if (ino->flags & AUTOFS_INF_EXPIRING) {
+                        spin_unlock(&sbi->fs_lock);
                         DPRINTK("wait for incomplete expire %p name=%.*s",
                                 expiring, expiring->d_name.len,
                                 expiring->d_name.name);
                         autofs4_wait(sbi, expiring, NFY_NONE);
                         DPRINTK("request completed");
+                        goto cont;
                 }
+                spin_unlock(&sbi->fs_lock);
+cont:
                 spin_lock(&sbi->lookup_lock);
                 if (!list_empty(&ino->expiring))
                         list_del_init(&ino->expiring);