linux-kernel - Re: [PATCH] fs/namespace.c: fix use-after-free of mount in mnt_warn_timestamp

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Date:   Sun, 13 Oct 2019 19:04:10 -0700
From:   Deepa Dinamani <deepa.kernel@...il.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Jeff Layton <jlayton@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] fs/namespace.c: fix use-after-free of mount in mnt_warn_timestamp_expiry()

Thanks for the fix.

Would it be better to move the check and warning to a place where the
access is still safe?

-Deepa

> On Oct 9, 2019, at 12:19 AM, Eric Biggers <ebiggers@...nel.org> wrote:
>
> From: Eric Biggers <ebiggers@...gle.com>


On Wed, Oct 9, 2019 at 12:19 AM Eric Biggers <ebiggers@...nel.org> wrote:
>
> From: Eric Biggers <ebiggers@...gle.com>
>
> After do_add_mount() returns success, the caller doesn't hold a
> reference to the 'struct mount' anymore.  So it's invalid to access it
> in mnt_warn_timestamp_expiry().
>
> Fix it by instead passing the super_block and the mnt_flags.  It's safe
> to access the super_block because it's pinned by fs_context::root.
>
> Reported-by: syzbot+da4f525235510683d855@...kaller.appspotmail.com
> Fixes: f8b92ba67c5d ("mount: Add mount warning for impending timestamp expiry")
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  fs/namespace.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index fe0e9e1410fe..7ef8edaaed69 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2466,12 +2466,11 @@ static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
>         unlock_mount_hash();
>  }
>
> -static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
> +static void mnt_warn_timestamp_expiry(struct path *mountpoint,
> +                                     struct super_block *sb, int mnt_flags)
>  {
> -       struct super_block *sb = mnt->mnt_sb;
> -
> -       if (!__mnt_is_readonly(mnt) &&
> -          (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
> +       if (!(mnt_flags & MNT_READONLY) && !sb_rdonly(sb) &&
> +           (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
>                 char *buf = (char *)__get_free_page(GFP_KERNEL);
>                 char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
>                 struct tm tm;
> @@ -2512,7 +2511,7 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
>                 set_mount_attributes(mnt, mnt_flags);
>         up_write(&sb->s_umount);
>
> -       mnt_warn_timestamp_expiry(path, &mnt->mnt);
> +       mnt_warn_timestamp_expiry(path, sb, mnt_flags);
>
>         return ret;
>  }
> @@ -2555,7 +2554,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
>                 up_write(&sb->s_umount);
>         }
>
> -       mnt_warn_timestamp_expiry(path, &mnt->mnt);
> +       mnt_warn_timestamp_expiry(path, sb, mnt_flags);
>
>         put_fs_context(fc);
>         return err;
> @@ -2770,7 +2769,7 @@ static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
>                 return error;
>         }
>
> -       mnt_warn_timestamp_expiry(mountpoint, mnt);
> +       mnt_warn_timestamp_expiry(mountpoint, sb, mnt_flags);
>
>         return error;
>  }
> --
> 2.23.0
>