proc/sysctl: don't return ENOMEM on lookup when a table is unregistering

proc_sys_lookup can fail with ENOMEM instead of ENOENT when the
corresponding sysctl table is being unregistered. In our case we see
this upon opening /proc/sys/net/*/conf files while network interfaces
are being deleted, which confuses our configuration daemon.

The problem was successfully reproduced and this fix tested on v4.9.122
and v4.20-rc6.

v2: return ERR_PTRs in all cases when proc_sys_make_inode fails instead
of mixing them with NULL. Thanks Al Viro for the feedback.

Fixes: ace0c791e6c3 ("proc/sysctl: Don't grab i_lock under sysctl_lock.")
Cc: stable@vger.kernel.org
Signed-off-by: Ivan Delalande <colona@arista.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 6 insertions, 7 deletions

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 89921a0d2ebb..4d598a399bbf 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -464,7 +464,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 
         inode = new_inode(sb);
         if (!inode)
-                goto out;
+                return ERR_PTR(-ENOMEM);
 
         inode->i_ino = get_next_ino();
 
@@ -474,8 +474,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
         if (unlikely(head->unregistering)) {
                 spin_unlock(&sysctl_lock);
                 iput(inode);
-                inode = NULL;
-                goto out;
+                return ERR_PTR(-ENOENT);
         }
         ei->sysctl = head;
         ei->sysctl_entry = table;
@@ -500,7 +499,6 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
         if (root->set_ownership)
                 root->set_ownership(head, table, &inode->i_uid, &inode->i_gid);
 
-out:
         return inode;
 }
 
@@ -549,10 +547,11 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
                         goto out;
         }
 
-        err = ERR_PTR(-ENOMEM);
         inode = proc_sys_make_inode(dir->i_sb, h ? h : head, p);
-        if (!inode)
+        if (IS_ERR(inode)) {
+                err = ERR_CAST(inode);
                 goto out;
+        }
 
         d_set_d_op(dentry, &proc_sys_dentry_operations);
         err = d_splice_alias(inode, dentry);
@@ -685,7 +684,7 @@ static bool proc_sys_fill_cache(struct file *file,
                 if (d_in_lookup(child)) {
                         struct dentry *res;
                         inode = proc_sys_make_inode(dir->d_sb, head, table);
-                        if (!inode) {
+                        if (IS_ERR(inode)) {
                                 d_lookup_done(child);
                                 dput(child);
                                 return false;