[10/32] VFS: Implement a filesystem superblock creation/configuration context [ver #8]

Commit Message

David Howells July 3, 2018, 9:53 p.m. UTC

Eric Biggers <[e-mail]> wrote:

> ->s_umount is released once here and again in destroy_unused_super().

Good catch, thanks.  The interface has changed over the lifetime of the
patches.  How about the attached patch?

David
---
commit b3899e214a6a0e0551f6dc707b28d61b11e718a5
Author: David Howells <[e-mail]>
Date:   Tue Jul 3 22:35:28 2018 +0100

    vfs: Locking fix for sget_fc()
    
    In sget_fc(), don't drop the s_umount lock before calling
    destroy_unused_super() as that will drop the lock.
    
    Fixes: 8a2e54b8af88 ("vfs: Implement a filesystem superblock creation/configuration context")
    Reported-by: Eric Biggers <[e-mail]>
    Signed-off-by: David Howells <[e-mail]>

Comments

Al Viro July 3, 2018, 9:58 p.m. UTC | #1

On Tue, Jul 03, 2018 at 10:53:22PM +0100, David Howells wrote:
> -				if (s) {
> -					up_write(&s->s_umount);
> +				if (s)
>  					destroy_unused_super(s);

static void destroy_unused_super(struct super_block *s)
{
        if (!s)
                return;
	...
}

IOW, all of those should be unconditional.

Patch

diff --git a/fs/super.c b/fs/super.c
index 43400f5fa33a..b014cd48a451 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -516,19 +516,14 @@  struct super_block *sget_fc(struct fs_context *fc,
 				continue;
 			if (fc->user_ns != old->s_user_ns) {
 				spin_unlock(&sb_lock);
-				if (s) {
-					up_write(&s->s_umount);
+				if (s)
 					destroy_unused_super(s);
-				}
 				return ERR_PTR(-EBUSY);
 			}
 			if (!grab_super(old))
 				goto retry;
-			if (s) {
-				up_write(&s->s_umount);
+			if (s)
 				destroy_unused_super(s);
-				s = NULL;
-			}
 			return old;
 		}
 	}
@@ -545,7 +540,6 @@  struct super_block *sget_fc(struct fs_context *fc,
 	if (err) {
 		s->s_fs_info = NULL;
 		spin_unlock(&sb_lock);
-		up_write(&s->s_umount);
 		destroy_unused_super(s);
 		return ERR_PTR(err);
 	}