On Tue, May 16, 2023 at 5:19 PM Zhihao Cheng <[email protected]> wrote:
>
> Following process:
> P1 P2
> path_lookupat
> link_path_walk
> inode_permission
> ovl_permission
> ovl_i_path_real(inode, &realpath)
> path->dentry = ovl_i_dentry_upper(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
> realinode = d_inode(realpath.dentry) // return NULL
> inode_permission(realinode)
> inode->i_sb // NULL pointer dereference
> , will trigger an null pointer dereference at realinode:
> [ 335.664979] BUG: kernel NULL pointer dereference,
> address: 0000000000000002
> [ 335.668032] CPU: 0 PID: 2592 Comm: ls Not tainted 6.3.0
> [ 335.669956] RIP: 0010:inode_permission+0x33/0x2c0
> [ 335.678939] Call Trace:
> [ 335.679165] <TASK>
> [ 335.679371] ovl_permission+0xde/0x320
> [ 335.679723] inode_permission+0x15e/0x2c0
> [ 335.680090] link_path_walk+0x115/0x550
> [ 335.680771] path_lookupat.isra.0+0xb2/0x200
> [ 335.681170] filename_lookup+0xda/0x240
> [ 335.681922] vfs_statx+0xa6/0x1f0
> [ 335.682233] 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.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217405
> Fixes: 4b7791b2e958 ("ovl: handle idmappings in ovl_permission()")
> Signed-off-by: Zhihao Cheng <[email protected]>
> Suggested-by: Christian Brauner <[email protected]>
> Suggested-by: Amir Goldstein <[email protected]>
Reviewed-by: Amir Goldstein <[email protected]>
> ---
> fs/overlayfs/inode.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 541cf3717fc2..ca56b1328a2c 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -288,8 +288,8 @@ int ovl_permission(struct mnt_idmap *idmap,
> int err;
>
> /* Careful in RCU walk mode */
> - ovl_i_path_real(inode, &realpath);
> - if (!realpath.dentry) {
> + realinode = ovl_i_path_real(inode, &realpath);
> + if (!realinode) {
> WARN_ON(!(mask & MAY_NOT_BLOCK));
> return -ECHILD;
> }
> @@ -302,7 +302,6 @@ int ovl_permission(struct mnt_idmap *idmap,
> if (err)
> return err;
>
> - realinode = d_inode(realpath.dentry);
> old_cred = ovl_override_creds(inode->i_sb);
> if (!upperinode &&
> !special_file(realinode->i_mode) && mask & MAY_WRITE) {
> --
> 2.39.2
>