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/XXXs inode instantiation.

The bug is that pde->proc_ops dont 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: 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: dont do 2 atomic ops on the common path]

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/4b0b8445b6fd41e6f62ac90547a0ea9d348de3fa
• https://git.kernel.org/stable/c/63b53198aff2e4e6c5866a4ff73c7891f958ffa4
• https://git.kernel.org/stable/c/64dc7c68e040251d9ec6e989acb69f8f6ae4a10b
• https://git.kernel.org/stable/c/654b33ada4ab5e926cd9c570196fefa7bec7c1df
• https://git.kernel.org/stable/c/966f331403dc3ed04ff64eaf3930cf1267965e53
• https://git.kernel.org/stable/c/eda279586e571b05dff44d48e05f8977ad05855d
• https://git.kernel.org/stable/c/ede3e8ac90ae106f0b29cd759aadebc1568f1308