[PATCH vfs/for-next] vfs: fix vfs_get_single_reconf_super error handling

[PATCH vfs/for-next] vfs: fix vfs_get_single_reconf_super error handling

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

syzbot reported an invalid free in debugfs_release_dentry().  The
reproducer tries to mount debugfs with the 'dirsync' option, which is
not allowed.  The bug is that if reconfigure_super() fails in
vfs_get_super(), deactivate_locked_super() is called, but also
fs_context::root is left non-NULL which causes deactivate_super() to be
called again later.

Fix it by releasing fs_context::root in the error path.

Reported-by: syzbot+5aca688dac0796c56129@syzkaller.appspotmail.com
Fixes: e478b48498a7 ("vfs: Add a single-or-reconfig keying to vfs_get_super()")
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/super.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index 0f913376fc4c..99195e15be05 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1194,8 +1194,11 @@ int vfs_get_super(struct fs_context *fc,
 		fc->root = dget(sb->s_root);
 		if (keying == vfs_get_single_reconf_super) {
 			err = reconfigure_super(fc);
-			if (err < 0)
+			if (err < 0) {
+				dput(fc->root);
+				fc->root = NULL;
 				goto error;
+			}
 		}
 	}
 
-- 
2.23.0

Re: WARNING in kfree

[syzbot]

syzbot has bisected this bug to:

commit 3deadeebafcec6a0a7c9397bd32ea5ac6d5191c1
Author: David Howells <dhowells@redhat.com>
Date:   Mon Jan 21 14:04:22 2019 +0000

     vfs: Convert debugfs to use the new mount API

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=115d9e56600000
start commit:   ed2393ca Add linux-next specific files for 20190827
git tree:       linux-next
final crash:    https://syzkaller.appspot.com/x/report.txt?x=135d9e56600000
console output: https://syzkaller.appspot.com/x/log.txt?x=155d9e56600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2ef5940a07ed45f4
dashboard link: https://syzkaller.appspot.com/bug?extid=5aca688dac0796c56129
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1595ee12600000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16df7fd2600000

Reported-by: syzbot+5aca688dac0796c56129@syzkaller.appspotmail.com
Fixes: 3deadeebafce ("vfs: Convert debugfs to use the new mount API")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Re: [PATCH vfs/for-next] vfs: fix vfs_get_single_reconf_super error handling

[Eric Biggers]

On Fri, Aug 30, 2019 at 10:10:24PM -0500, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> syzbot reported an invalid free in debugfs_release_dentry().  The
> reproducer tries to mount debugfs with the 'dirsync' option, which is
> not allowed.  The bug is that if reconfigure_super() fails in
> vfs_get_super(), deactivate_locked_super() is called, but also
> fs_context::root is left non-NULL which causes deactivate_super() to be
> called again later.
> 
> Fix it by releasing fs_context::root in the error path.
> 
> Reported-by: syzbot+5aca688dac0796c56129@syzkaller.appspotmail.com
> Fixes: e478b48498a7 ("vfs: Add a single-or-reconfig keying to vfs_get_super()")
> Cc: David Howells <dhowells@redhat.com>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/super.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 0f913376fc4c..99195e15be05 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1194,8 +1194,11 @@ int vfs_get_super(struct fs_context *fc,
>  		fc->root = dget(sb->s_root);
>  		if (keying == vfs_get_single_reconf_super) {
>  			err = reconfigure_super(fc);
> -			if (err < 0)
> +			if (err < 0) {
> +				dput(fc->root);
> +				fc->root = NULL;
>  				goto error;
> +			}
>  		}
>  	}
>  

Ping.  This is still broken in linux-next.

- Eric