Re: [PATCH 5.15] ovl: fix null pointer dereference in ovl_get_acl_rcu()

[Amir Goldstein]

On Mon, Jul 10, 2023 at 6:29 AM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
>
> [ Upstream commit f4e19e595cc2e76a8a58413eb19d3d9c51328b53 ]
>
> Following process:
>          P1                     P2
>  path_openat
>   link_path_walk
>    may_lookup
>     inode_permission(rcu)
>      ovl_permission
>       acl_permission_check
>        check_acl
>         get_cached_acl_rcu
>          ovl_get_inode_acl
>           realinode = ovl_inode_real(ovl_inode)
>                               drop_cache
>                                __dentry_kill(ovl_dentry)
>                                 iput(ovl_inode)
>                                  ovl_destroy_inode(ovl_inode)
>                                   dput(oi->__upperdentry)
>                                    dentry_kill(upperdentry)
>                                     dentry_unlink_inode
>                                      upperdentry->d_inode = NULL
>             ovl_inode_upper
>              upperdentry = ovl_i_dentry_upper(ovl_inode)
>              d_inode(upperdentry) // returns NULL
>           IS_POSIXACL(realinode) // NULL pointer dereference
> , will trigger an null pointer dereference at realinode:
>   [  205.472797] BUG: kernel NULL pointer dereference, address:
>                  0000000000000028
>   [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
>                  6.3.0-12064-g2edfa098e750-dirty #1216
>   [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
>   [  205.489584] Call Trace:
>   [  205.489812]  <TASK>
>   [  205.490014]  ovl_get_inode_acl+0x26/0x30
>   [  205.490466]  get_cached_acl_rcu+0x61/0xa0
>   [  205.490908]  generic_permission+0x1bf/0x4e0
>   [  205.491447]  ovl_permission+0x79/0x1b0
>   [  205.491917]  inode_permission+0x15e/0x2c0
>   [  205.492425]  link_path_walk+0x115/0x550
>   [  205.493311]  path_lookupat.isra.0+0xb2/0x200
>   [  205.493803]  filename_lookup+0xda/0x240
>   [  205.495747]  vfs_fstatat+0x7b/0xb0
>
> Fetch a reproducer in [Link].
>
> Use the helper ovl_i_path_realinode() to get realinode and then do
> non-nullptr checking.
>
> There are some changes from upstream commit:
> 1. Corrusponds to do_ovl_get_acl() in 5.15 is ovl_get_acl()
> 2. ovl_i_path_real is not imported in 5.15, we can get realinode by
>    ovl_inode_real
> 3. CONFIG_FS_POSIX_ACL checking is dropped in commit
>    ded536561a3674327dfa4bb389085705cae22b8a ("ovl: improve ovl_get_acl()
>    if POSIX ACL support is off"), we still keep it in 5.15.

Zhihao,

Can you please provide also the backport for 6.1.

Basically, the same as this one without the CONFIG_FS_POSIX_ACL check.

Thanks,
Amir.

>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
> Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.15
> Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
> Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
> Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
>  fs/overlayfs/inode.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index d41f0c8e0e2a..65e5e6eb761a 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -453,7 +453,15 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type, bool rcu)
>         const struct cred *old_cred;
>         struct posix_acl *acl;
>
> -       if (!IS_ENABLED(CONFIG_FS_POSIX_ACL) || !IS_POSIXACL(realinode))
> +       if (!IS_ENABLED(CONFIG_FS_POSIX_ACL))
> +               return NULL;
> +
> +       if (!realinode) {
> +               WARN_ON(!rcu);
> +               return ERR_PTR(-ECHILD);
> +       }
> +
> +       if (!IS_POSIXACL(realinode))
>                 return NULL;
>
>         if (rcu)
> --
> 2.39.2
>