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Thu, 08 Oct 2026 12:56:22 -0700 (PDT) From: Magnus Lindholm To: richard.henderson@linaro.org, mattst88@gmail.com, linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org Cc: linmag7@gmail.com, stable@vger.kernel.org Subject: [PATCH v2 1/3] alpha: load the MMU context when switch_mm() switches the current task Date: Thu, 8 Oct 2026 21:55:16 +0200 Message-ID: <20261008195608.965266-2-linmag7@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261008195608.965266-1-linmag7@gmail.com> References: <20261008195608.965266-1-linmag7@gmail.com> Precedence: bulk X-Mailing-List: linux-alpha@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit ev5_switch_mm() only prepares the incoming PCB. The context is installed by PAL_swpctx, which alpha_switch_to() issues against that PCB on the way out of the scheduler. Two callers reach switch_mm_irqs_off() without going through alpha_switch_to(): kthread_use_mm(), which borrows an mm for the current kernel thread, and sched_force_init_mm() on the CPU-hotplug teardown path. Neither explicitly loads the context, so the task can carry on running under whatever was loaded before while current->mm says otherwise. The stale page-table root need not be swapper_pg_dir; it may belong to a user process that ran on the CPU earlier, and the kthread's user accesses can then read and write that process's memory wherever the stale mappings allow. Translations taken that way can also end up tagged with the borrowed mm's ASN: ev5_switch_mm() writes that ASN into the PCB, which the next PAL_swpctx installs against the stale ptbr. An address the stale tables do not map faults instead, and since do_page_fault() resolves faults against current->mm without reloading the context, the same access can fault again on return. sched_force_init_mm() needs CONFIG_HOTPLUG_CPU, which alpha does not support, so kthread_use_mm() is the only one of the two reachable in practice; the fix below tests the caller's identity rather than special-casing either one. The scheduler passes the incoming task, which is not current until alpha_switch_to() runs; both direct callers pass current. Test for that and load the context the way activate_mm() does. Both hold interrupts disabled across switch_mm_irqs_off(), so this completes before any shootdown can be taken and needs no asn_lock handshake. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: Signed-off-by: Magnus Lindholm Tested-by: Matt Turner --- arch/alpha/include/asm/mmu_context.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/arch/alpha/include/asm/mmu_context.h b/arch/alpha/include/asm/mmu_context.h index 825d3b9605c9..6ecc8a0676cb 100644 --- a/arch/alpha/include/asm/mmu_context.h +++ b/arch/alpha/include/asm/mmu_context.h @@ -130,6 +130,8 @@ __get_new_mm_context(struct mm_struct *mm, long cpu) return next; } +extern void __load_new_mm_context(struct mm_struct *); + __EXTERN_INLINE void ev5_switch_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm, struct task_struct *next) @@ -139,6 +141,15 @@ ev5_switch_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm, unsigned long mmc; long cpu = smp_processor_id(); + /* + * kthread_use_mm() and sched_force_init_mm() switch current's mm + * without alpha_switch_to(), which is what loads the context. + */ + if (next == current) { + __load_new_mm_context(next_mm); + return; + } + #ifdef CONFIG_SMP cpu_data[cpu].asn_lock = 1; barrier(); @@ -160,7 +171,6 @@ ev5_switch_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm, task_thread_info(next)->pcb.asn = mmc & HARDWARE_ASN_MASK; } -extern void __load_new_mm_context(struct mm_struct *); asmlinkage void do_page_fault(unsigned long address, unsigned long mmcsr, long cause, struct pt_regs *regs); -- 2.43.0