CVE-2026-23025: mm/page_alloc: prevent pcp corruption with SMP=n

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

mm/pagealloc: prevent pcp corruption with SMP=n

The kernel test robot has reported:

BUG: spinlock trylock failure on UP on CPU#0, kcompactd0/28 lock: 0xffff888807e35ef0, .magic: dead4ead, .owner: kcompactd0/28, .ownercpu: 0 CPU: 0 UID: 0 PID: 28 Comm: kcompactd0 Not tainted 6.18.0-rc5-00127-ga06157804399 #1 PREEMPT 8cc09ef94dcec767faa911515ce9e609c45db470 Call Trace: <IRQ> dumpstack (lib/dumpstack.c:95) dumpstacklvl (lib/dumpstack.c:123) dumpstack (lib/dumpstack.c:130) spindump (kernel/locking/spinlockdebug.c:71) dorawspintrylock (kernel/locking/spinlockdebug.c:?) rawspintrylock (include/linux/spinlockapismp.h:89 kernel/locking/spinlock.c:138) freefrozenpages (mm/pagealloc.c:2973) freepages (mm/pagealloc.c:5295) freepages (mm/pagealloc.c:5334) tlbremovetablercu (include/linux/mm.h:? include/linux/mm.h:3122 include/asm-generic/tlb.h:220 mm/mmugather.c:227 mm/mmugather.c:290) ? cfitlbremovetablercu (mm/mmugather.c:289) ? rcucore (kernel/rcu/tree.c:?) rcucore (include/linux/rcupdate.h:341 kernel/rcu/tree.c:2607 kernel/rcu/tree.c:2861) rcucoresi (kernel/rcu/tree.c:2879) handlesoftirqs (arch/x86/include/asm/jumplabel.h:36 include/trace/events/irq.h:142 kernel/softirq.c:623) irqexitrcu (arch/x86/include/asm/jumplabel.h:36 kernel/softirq.c:725) irqexitrcu (kernel/softirq.c:741) sysvecapictimerinterrupt (arch/x86/kernel/apic/apic.c:1052) </IRQ> <TASK> RIP: 0010:rawspinunlockirqrestore (arch/x86/include/asm/preempt.h:95 include/linux/spinlockapismp.h:152 kernel/locking/spinlock.c:194) freepcppagesbulk (mm/pagealloc.c:1494) drainpageszone (include/linux/spinlock.h:391 mm/pagealloc.c:2632) drainallpages (mm/pagealloc.c:2731) drainallpages (mm/pagealloc.c:2747) kcompactd (mm/compaction.c:3115) kthread (kernel/kthread.c:465) ? cfikcompactd (mm/compaction.c:3166) ? cfikthread (kernel/kthread.c:412) retfromfork (arch/x86/kernel/process.c:164) ? cfikthread (kernel/kthread.c:412) retfromforkasm (arch/x86/entry/entry64.S:255) </TASK>

Matthew has analyzed the report and identified that in drainpagezone() we are in a section protected by spinlock(&pcp->lock) and then get an interrupt that attempts spintrylock() on the same lock. The code is designed to work this way without disabling IRQs and occasionally fail the trylock with a fallback. However, the SMP=n spinlock implementation assumes spintrylock() will always succeed, and thus it's normally a no-op. Here the enabled lock debugging catches the problem, but otherwise it could cause a corruption of the pcp structure.

The problem has been introduced by commit 574907741599 ("mm/pagealloc: leave IRQs enabled for per-cpu page allocations"). The pcp locking scheme recognizes the need for disabling IRQs to prevent nesting spintrylock() sections on SMP=n, but the need to prevent the nesting in spinlock() has not been recognized. Fix it by introducing local wrappers that change the spinlock() to spinlockiqsave() with SMP=n and use them in all places that do spinlock(&pcp->lock).

[vbabka@suse.cz: add pcp prefix to the spinlockirqsave wrappers, per Steven]