bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs

Overview

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

bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs

commit 4f375ade6aa9 ("bpf: Avoid RCU context warning when unpinning htab with internal structs") moved inode cleanup from ->free_inode() into ->destroy_inode() to avoid sleeping in RCU context when calling bpf_any_put(). However this removed the RCU delay on freeing the inode itself and the cached symlink body (i_link), both of which can be accessed by RCU pathwalk (pick_link, may_lookup etc.).

This causes a use-after-free when a concurrent unlinkat() drops the last inode reference and destroy_inode() frees the inode immediately, while another task is still walking the path in RCU mode and reads inode->i_opflags (offset +2) inside current_time() -> is_mgtime().

KASAN reports: BUG: KASAN: slab-use-after-free in is_mgtime include/linux/fs.h:2313 Read of size 2 at addr ffff8880407e4282 (offset +2 = i_opflags)

The rules (per Al Viro): ->destroy_inode() called immediately, can sleep, use for blocking cleanup e.g. bpf_any_put() ->free_inode() called after RCU grace period, use for freeing inode and anything RCU-accessible e.g. i_link

Fix: split the two concerns properly:

• keep bpf_any_put() in bpf_destroy_inode() since it is blocking and needs to run promptly
• introduce bpf_free_inode() to handle kfree(i_link) and free_inode_nonrcu() with proper RCU delay, preventing the UAF

Affected

• Linux >= e28616ca3d67e745ecb2f10eba4a626e1fc9a203 < ea1c243c39e32b7fc1c2edfe32081ff7e30a877c
• Linux >= 743a620c661994c7f0938e6dd32fb0883fb1e0ea < 5fecb71c10c28aef276ba49c718dc961745fdcf0
• Linux >= b6e9645be9eb93f7aff3ca887f8edb6f1d63358f < c70d0f9114c3cc156f6029a400c4eb7e6f7c82b2
• Linux >= ee04cff9ed4d6bb25802f5cecfcd0750500410f3 < 53649846e0437d1d9b7cb993cfe54c367addf7ae
• Linux >= 4f375ade6aa9f37fd72d7a78682f639772089eed < 61f19729728243c82476dee31315143ed3275e7f
• Linux >= 4f375ade6aa9f37fd72d7a78682f639772089eed < 0497ff765746d9b2d17445c8f7cc737b36c0152a
• Linux >= 4f375ade6aa9f37fd72d7a78682f639772089eed < b93c55b4932dd7e32dca8cf34a3443cc87a02906
• Linux de2d2baecc84cc7fca52eec2b9b55d89c93e3565
• Linux >= 5.15.195 < 5.15.212
• Linux >= 6.1.157 < 6.1.178
• Linux >= 6.6.113 < 6.6.145
• Linux >= 6.12.54 < 6.12.97
• Linux >= 6.17.4 < 6.18
• Linux 6.18

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.