linux-kernel - Re: [PATCH] vfs: fsmount: add missing mntget()

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Message-ID: <20190613090305.GA40885@lakrids.cambridge.arm.com>
Date:   Thu, 13 Jun 2019 10:03:06 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: fsmount: add missing mntget()

On Wed, Jun 12, 2019 at 11:43:13AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> sys_fsmount() needs to take a reference to the new mount when adding it
> to the anonymous mount namespace.  Otherwise the filesystem can be
> unmounted while it's still in use, as found by syzkaller.
> 
> Reported-by: Mark Rutland <mark.rutland@....com>
> Reported-by: syzbot+99de05d099a170867f22@...kaller.appspotmail.com
> Reported-by: syzbot+7008b8b8ba7df475fdc8@...kaller.appspotmail.com
> Fixes: 93766fbd2696 ("vfs: syscall: Add fsmount() to create a mount for a superblock")
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

With this patch applied, my reproducer from [1] no longer triggers the
issue. I polled /proc/meminfo, and don't see any leak. FWIW:

Tested-by: Mark Rutland <mark.rutland@....com>

Thanks for fixing this!

Mark.

[1] https://lore.kernel.org/lkml/20190605135401.GB30925@lakrids.cambridge.arm.com/

> ---
>  fs/namespace.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index b26778bdc236e..5dc137a22d406 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3445,6 +3445,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
>  	ns->root = mnt;
>  	ns->mounts = 1;
>  	list_add(&mnt->mnt_list, &ns->list);
> +	mntget(newmount.mnt);
>  
>  	/* Attach to an apparent O_PATH fd with a note that we need to unmount
>  	 * it, not just simply put it.
> -- 
> 2.22.0.rc2.383.gf4fbbf30c2-goog
>