[PATCH v5 1/8] alpha: load the MMU context when switch_mm() switches the current task

[Magnus Lindholm]

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.

Direct callers reach switch_mm() without going through alpha_switch_to():
kthread_use_mm(), which borrows an mm for the current kernel thread,
sched_force_init_mm() on the CPU-hotplug teardown path, and
do_shoot_lazy_tlb(), which switches to init_mm from an IPI. None 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 and do_shoot_lazy_tlb()
needs CONFIG_MMU_LAZY_TLB_SHOOTDOWN. Alpha selects neither, so only
kthread_use_mm() is reachable in practice. Test the caller's identity
rather than special-casing individual callers.

The scheduler passes the incoming task, which is not current until
alpha_switch_to() runs; all three direct callers pass current. Test for
that and load the context the way activate_mm() does. All three run with
interrupts disabled across the switch, so this completes before any
shootdown can be taken and needs no asn_lock handshake.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Tested-by: Matt Turner <mattst88@xxxxxxxxx>
Reviewed-by: Matt Turner <mattst88@xxxxxxxxx>
---
 arch/alpha/include/asm/mmu_context.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/include/asm/mmu_context.h b/arch/alpha/include/asm/mmu_context.h
index eee8fe836a59..d366f89fee58 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 +141,16 @@ ev5_switch_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm,
 	unsigned long mmc;
 	long cpu = smp_processor_id();
 
+	/*
+	 * kthread_use_mm(), sched_force_init_mm() and do_shoot_lazy_tlb()
+	 * switch current's mm without alpha_switch_to(), which loads the
+	 * context. The latter two require options Alpha does not select.
+	 */
+	if (next == current) {
+		__load_new_mm_context(next_mm);
+		return;
+	}
+
 #ifdef CONFIG_SMP
 	cpu_data[cpu].asn_lock = 1;
 	barrier();
@@ -160,7 +172,6 @@ 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