Bug 2351605 (CVE-2025-21857)

Summary: CVE-2025-21857 kernel: net/sched: cls_api: fix error handling causing NULL dereference
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: dfreiber, drow, jburrell, vkumar
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Hardware: All   
OS: Linux   
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Description OSIDB Bzimport 2025-03-12 12:23:14 UTC 
In the Linux kernel, the following vulnerability has been resolved:

net/sched: cls_api: fix error handling causing NULL dereference

tcf_exts_miss_cookie_base_alloc() calls xa_alloc_cyclic() which can
return 1 if the allocation succeeded after wrapping. This was treated as
an error, with value 1 returned to caller tcf_exts_init_ex() which sets
exts->actions to NULL and returns 1 to caller fl_change().

fl_change() treats err == 1 as success, calling tcf_exts_validate_ex()
which calls tcf_action_init() with exts->actions as argument, where it
is dereferenced.

Example trace:

BUG: kernel NULL pointer dereference, address: 0000000000000000
CPU: 114 PID: 16151 Comm: handler114 Kdump: loaded Not tainted 5.14.0-503.16.1.el9_5.x86_64 #1
RIP: 0010:tcf_action_init+0x1f8/0x2c0
Call Trace:
 tcf_action_init+0x1f8/0x2c0
 tcf_exts_validate_ex+0x175/0x190
 fl_change+0x537/0x1120 [cls_flower]
Comment 1 Avinash Hanwate 2025-03-13 07:45:13 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025031215-CVE-2025-21857-ab00@gregkh/T