ovl: fix null pointer dereference in ovl_permission()

In the Linux kernel, the following vulnerability has been resolved:

ovl: fix null pointer dereference in ovl_permission()

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.