CVE-2025-21727

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

padata: fix UAF in padata_reorder

A bug was found when run ltp test:

BUG: KASAN: slab-use-after-free in padata_find_next+0x29/0x1a0 Read of size 4 at addr ffff88bbfe003524 by task kworker/u113:2/3039206

CPU: 0 PID: 3039206 Comm: kworker/u113:2 Kdump: loaded Not tainted 6.6.0+ Workqueue: pdecrypt_parallel padata_parallel_worker Call Trace: dump_stack_lvl+0x32/0x50 print_address_description.constprop.0+0x6b/0x3d0 print_report+0xdd/0x2c0 kasan_report+0xa5/0xd0 padata_find_next+0x29/0x1a0 padata_reorder+0x131/0x220 padata_parallel_worker+0x3d/0xc0 process_one_work+0x2ec/0x5a0

If mdelay(10) is added before calling padata_find_next in the padata_reorder function, this issue could be reproduced easily with ltp test (pcrypt_aead01).

This can be explained as bellow:

pcrypt_aead_encrypt … padata_do_parallel refcount_inc(&pd->refcnt); // add refcnt … padata_do_serial padata_reorder // pd while (1) { padata_find_next(pd, true); // using pd queue_work_on … padata_serial_worker crypto_del_alg padata_put_pd_cnt // sub refcnt padata_free_shell padata_put_pd(ps->pd); // pd is freed // loop again, but pd is freed // call padata_find_next, UAF }

In the padata_reorder function, when it loops in while, if the alg is deleted, the refcnt may be decreased to 0 before entering padata_find_next, which leads to UAF.

As mentioned in [1], do_serial is supposed to be called with BHs disabled and always happen under RCU protection, to address this issue, add synchronize_rcu() in padata_free_shell wait for all _do_serial calls to finish.

[1] https://lore.kernel.org/all/20221028160401.cccypv4euxikusiq@parnassus.localdomain/ [2] https://lore.kernel.org/linux-kernel/jfjz5d7zwbytztackem7ibzalm5lnxldi2eofeiczqmqs2m7o6@fq426cwnjtkm/

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 Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.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/0ae2f332cfd2d74cf3ce344ec9938cf3e29c3ccd
• https://git.kernel.org/stable/c/573ac9c70bf7885dc85d82fa44550581bfc3b738
• https://git.kernel.org/stable/c/80231f069240d52e98b6a317456c67b2eafd0781
• https://git.kernel.org/stable/c/bbccae982e9fa1d7abcb23a5ec81cb0ec883f7de
• https://git.kernel.org/stable/c/e01780ea4661172734118d2a5f41bc9720765668
• https://git.kernel.org/stable/c/f3e0b9f790f8e8065d59e67b565a83154d9f3079
• https://git.kernel.org/stable/c/f78170bee51469734b1a306a74fc5f777bb22ba6
• https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html
• https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html