CVE-2025-21999

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

proc: fix UAF in proc_get_inode()

Fix race between rmmod and /proc/XXX's inode instantiation.

The bug is that pde->proc_ops don't belong to /proc, it belongs to a
module, therefore dereferencing it after /proc entry has been registered
is a bug unless use_pde/unuse_pde() pair has been used.

use_pde/unuse_pde can be avoided (2 atomic ops!) because pde->proc_ops
never changes so information necessary for inode instantiation can be
saved _before_ proc_register() in PDE itself and used later, avoiding
pde->proc_ops->...  dereference.

      rmmod                         lookup
sys_delete_module
                         proc_lookup_de
			   pde_get(de);
			   proc_get_inode(dir->i_sb, de);
  mod->exit()
    proc_remove
      remove_proc_subtree
       proc_entry_rundown(de);
  free_module(mod);

                               if (S_ISREG(inode->i_mode))
	                         if (de->proc_ops->proc_read_iter)
                           --> As module is already freed, will trigger UAF

BUG: unable to handle page fault for address: fffffbfff80a702b
PGD 817fc4067 P4D 817fc4067 PUD 817fc0067 PMD 102ef4067 PTE 0
Oops: Oops: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 26 UID: 0 PID: 2667 Comm: ls Tainted: G
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
RIP: 0010:proc_get_inode+0x302/0x6e0
RSP: 0018:ffff88811c837998 EFLAGS: 00010a06
RAX: dffffc0000000000 RBX: ffffffffc0538140 RCX: 0000000000000007
RDX: 1ffffffff80a702b RSI: 0000000000000001 RDI: ffffffffc0538158
RBP: ffff8881299a6000 R08: 0000000067bbe1e5 R09: 1ffff11023906f20
R10: ffffffffb560ca07 R11: ffffffffb2b43a58 R12: ffff888105bb78f0
R13: ffff888100518048 R14: ffff8881299a6004 R15: 0000000000000001
FS:  00007f95b9686840(0000) GS:ffff8883af100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: fffffbfff80a702b CR3: 0000000117dd2000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 proc_lookup_de+0x11f/0x2e0
 __lookup_slow+0x188/0x350
 walk_component+0x2ab/0x4f0
 path_lookupat+0x120/0x660
 filename_lookup+0x1ce/0x560
 vfs_statx+0xac/0x150
 __do_sys_newstat+0x96/0x110
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

[adobriyan@gmail.com: don't do 2 atomic ops on the common path]