FAILED: patch "[PATCH] s390/smp,vdso: fix ASCE handling" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a2308c11ecbc3471ebb7435ee8075815b1502ef0 Mon Sep 17 00:00:00 2001
From: Heiko Carstens <[email protected]>
Date: Mon, 18 Nov 2019 13:09:52 +0100
Subject: [PATCH] s390/smp,vdso: fix ASCE handling

When a secondary CPU is brought up it must initialize its control registers. CPU A which triggers that a secondary CPU B is brought up stores its control register contents into the lowcore of new CPU B, which then loads these values on startup.

This is problematic in various ways: the control register which contains the home space ASCE will correctly contain the kernel ASCE; however control registers for primary and secondary ASCEs are initialized with whatever values were present in CPU A.

Typically:
- the primary ASCE will contain the user process ASCE of the process that triggered onlining of CPU B.
- the secondary ASCE will contain the percpu VDSO ASCE of CPU A.

Due to lazy ASCE handling we may also end up with other combinations.

When then CPU B switches to a different process (!= idle) it will fixup the primary ASCE. However the problem is that the (wrong) ASCE from CPU A was loaded into control register 1: as soon as an ASCE is attached (aka loaded) a CPU is free to generate TLB entries using that address space.
Even though it is very unlikey that CPU B will actually generate such entries, this could result in TLB entries of the address space of the process that ran on CPU A. These entries shouldn't exist at all and could cause problems later on.

Furthermore the secondary ASCE of CPU B will not be updated correctly. This means that processes may see wrong results or even crash if they access VDSO data on CPU B. The correct VDSO ASCE will eventually be loaded on return to user space as soon as the kernel executed a call to strnlen_user or an atomic futex operation on CPU B.

Fix both issues by intializing the to be loaded control register contents with the correct ASCEs and also enforce (re-)loading of the ASCEs upon first context switch and return to user space.

index 6acdcf1d4074..06dddd7c4290 100644
@@ -262,10 +262,13 @@ static void pcpu_prepare_secondary(struct pcpu *pcpu, int cpu)
 	lc->spinlock_index = 0;
 	lc->percpu_offset = __per_cpu_offset[cpu];
 	lc->kernel_asce = S390_lowcore.kernel_asce;
+	lc->user_asce = S390_lowcore.kernel_asce;
 	lc->machine_flags = S390_lowcore.machine_flags;
 	lc->user_timer = lc->system_timer =
 		lc->steal_timer = lc->avg_steal_timer = 0;
 	__ctl_store(lc->cregs_save_area, 0, 15);
+	lc->cregs_save_area[1] = lc->kernel_asce;
+	lc->cregs_save_area[7] = lc->vdso_asce;
 	save_access_regs((unsigned int *) lc->access_regs_save_area);
 	memcpy(lc->stfle_fac_list, S390_lowcore.stfle_fac_list,
 	       sizeof(lc->stfle_fac_list));

@@ -844,6 +847,8 @@ static void smp_init_secondary(void)
 
 	S390_lowcore.last_update_clock = get_tod_clock();
 	restore_access_regs(S390_lowcore.access_regs_save_area);
+	set_cpu_flag(CIF_ASCE_PRIMARY);
+	set_cpu_flag(CIF_ASCE_SECONDARY);
 	cpu_init();
 	preempt_disable();
 	init_cpu_timer();

Adjusted context for missing ce3dc447493f ("s390: add support for virtually mapped kernel stacks") in 4.19 and queued up.

Thanks,
Sasha