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>

3 files changed, 65 insertions, 23 deletions

diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index 058e1800caec..5d90ed3b4b43 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -138,18 +138,14 @@ static inline int autofs4_oz_mode(struct autofs_sb_info *sbi) {
 static inline int autofs4_ispending(struct dentry *dentry)
 {
         struct autofs_info *inf = autofs4_dentry_ino(dentry);
-        int pending = 0;
 
         if (dentry->d_flags & DCACHE_AUTOFS_PENDING)
                 return 1;
 
-        if (inf) {
-                spin_lock(&inf->sbi->fs_lock);
-                pending = inf->flags & AUTOFS_INF_EXPIRING;
-                spin_unlock(&inf->sbi->fs_lock);
-        }
+        if (inf->flags & AUTOFS_INF_EXPIRING)
+                return 1;
 
-        return pending;
+        return 0;
 }
 
 static inline void autofs4_copy_atime(struct file *src, struct file *dst)
diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index 894fee54d4d8..19f5bea2704f 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -292,6 +292,8 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
         struct list_head *next;
         int do_now = how & AUTOFS_EXP_IMMEDIATE;
         int exp_leaves = how & AUTOFS_EXP_LEAVES;
+        struct autofs_info *ino;
+        unsigned int ino_count;
 
         if (!root)
                 return NULL;
@@ -316,6 +318,9 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
                 dentry = dget(dentry);
                 spin_unlock(&dcache_lock);
 
+                spin_lock(&sbi->fs_lock);
+                ino = autofs4_dentry_ino(dentry);
+
                 /*
                  * Case 1: (i) indirect mount or top level pseudo direct mount
                  *         (autofs-4.1).
@@ -326,6 +331,11 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
                         DPRINTK("checking mountpoint %p %.*s",
                                 dentry, (int)dentry->d_name.len, dentry->d_name.name);
 
+                        /* Path walk currently on this dentry? */
+                        ino_count = atomic_read(&ino->count) + 2;
+                        if (atomic_read(&dentry->d_count) > ino_count)
+                                goto next;
+
                         /* Can we umount this guy */
                         if (autofs4_mount_busy(mnt, dentry))
                                 goto next;
@@ -343,23 +353,25 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
 
                 /* Case 2: tree mount, expire iff entire tree is not busy */
                 if (!exp_leaves) {
-                        /* Lock the tree as we must expire as a whole */
-                        spin_lock(&sbi->fs_lock);
-                        if (!autofs4_tree_busy(mnt, dentry, timeout, do_now)) {
-                                struct autofs_info *inf = autofs4_dentry_ino(dentry);
+                        /* Path walk currently on this dentry? */
+                        ino_count = atomic_read(&ino->count) + 1;
+                        if (atomic_read(&dentry->d_count) > ino_count)
+                                goto next;
 
-                                /* Set this flag early to catch sys_chdir and the like */
-                                inf->flags |= AUTOFS_INF_EXPIRING;
-                                spin_unlock(&sbi->fs_lock);
+                        if (!autofs4_tree_busy(mnt, dentry, timeout, do_now)) {
                                 expired = dentry;
                                 goto found;
                         }
-                        spin_unlock(&sbi->fs_lock);
                 /*
                  * Case 3: pseudo direct mount, expire individual leaves
                  *         (autofs-4.1).
                  */
                 } else {
+                        /* Path walk currently on this dentry? */
+                        ino_count = atomic_read(&ino->count) + 1;
+                        if (atomic_read(&dentry->d_count) > ino_count)
+                                goto next;
+
                         expired = autofs4_check_leaves(mnt, dentry, timeout, do_now);
                         if (expired) {
                                 dput(dentry);
@@ -367,6 +379,7 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
                         }
                 }
 next:
+                spin_unlock(&sbi->fs_lock);
                 dput(dentry);
                 spin_lock(&dcache_lock);
                 next = next->next;
@@ -377,6 +390,9 @@ next:
 found:
         DPRINTK("returning %p %.*s",
                 expired, (int)expired->d_name.len, expired->d_name.name);
+        ino = autofs4_dentry_ino(expired);
+        ino->flags |= AUTOFS_INF_EXPIRING;
+        spin_unlock(&sbi->fs_lock);
         spin_lock(&dcache_lock);
         list_move(&expired->d_parent->d_subdirs, &expired->d_u.d_child);
         spin_unlock(&dcache_lock);
@@ -390,7 +406,9 @@ int autofs4_expire_run(struct super_block *sb,
                       struct autofs_packet_expire __user *pkt_p)
 {
         struct autofs_packet_expire pkt;
+        struct autofs_info *ino;
         struct dentry *dentry;
+        int ret = 0;
 
         memset(&pkt,0,sizeof pkt);
 
@@ -406,9 +424,14 @@ int autofs4_expire_run(struct super_block *sb,
         dput(dentry);
 
         if ( copy_to_user(pkt_p, &pkt, sizeof(struct autofs_packet_expire)) )
-                return -EFAULT;
+                ret = -EFAULT;
 
-        return 0;
+        spin_lock(&sbi->fs_lock);
+        ino = autofs4_dentry_ino(dentry);
+        ino->flags &= ~AUTOFS_INF_EXPIRING;
+        spin_unlock(&sbi->fs_lock);
+
+        return ret;
 }
 
 /* Call repeatedly until it returns -EAGAIN, meaning there's nothing
@@ -433,9 +456,10 @@ int autofs4_expire_multi(struct super_block *sb, struct vfsmount *mnt,
 
                 /* This is synchronous because it makes the daemon a
                    little easier */
-                ino->flags |= AUTOFS_INF_EXPIRING;
                 ret = autofs4_wait(sbi, dentry, NFY_EXPIRE);
+                spin_lock(&sbi->fs_lock);
                 ino->flags &= ~AUTOFS_INF_EXPIRING;
+                spin_unlock(&sbi->fs_lock);
                 dput(dentry);
         }
 
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);