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Subject: Re: [PATCH v1] s390/smp: Fix calling smp_call_ipl_cpu() from ipl CPU
To:     linux-s390@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>,
        Cornelia Huck <cohuck@redhat.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
References: <20190111141822.17262-1-david@redhat.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <3107a059-0117-fe29-05d6-4cba539a113d@redhat.com>
Date:   Fri, 11 Jan 2019 16:25:52 +0100
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On 11.01.19 15:18, David Hildenbrand wrote:
> When calling smp_call_ipl_cpu() from the IPL CPU, we will try to read
> from pcpu_devices->lowcore. However, due to prefixing, that will result
> in reading from absolute address 0 on that CPU. We have to go via the
> actual lowcore instead.
> 
> This means that right now, we will read lc->nodat_stack == 0 and
> therfore work on a very wrong stack.
> 
> This BUG essentially broke rebooting under QEMU TCG (which will report
> a low address protection exception). And checking under KVM, it is
> also broken under KVM. With 1 VCPU it can be easily triggered.
> 
> :/# echo 1 > /proc/sys/kernel/sysrq
> :/# echo b > /proc/sysrq-trigger
> [   28.476745] sysrq: SysRq : Resetting
> [   28.476793] Kernel stack overflow.
> [   28.476817] CPU: 0 PID: 424 Comm: sh Not tainted 5.0.0-rc1+ #13
> [   28.476820] Hardware name: IBM 2964 NE1 716 (KVM/Linux)
> [   28.476826] Krnl PSW : 0400c00180000000 0000000000115c0c (pcpu_delegate+0x12c/0x140)
> [   28.476861]            R:0 T:1 IO:0 EX:0 Key:0 M:0 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
> [   28.476863] Krnl GPRS: ffffffffffffffff 0000000000000000 000000000010dff8 0000000000000000
> [   28.476864]            0000000000000000 0000000000000000 0000000000ab7090 000003e0006efbf0
> [   28.476864]            000000000010dff8 0000000000000000 0000000000000000 0000000000000000
> [   28.476865]            000000007fffc000 0000000000730408 000003e0006efc58 0000000000000000
> [   28.476887] Krnl Code: 0000000000115bfe: 4170f000            la      %r7,0(%r15)
> [   28.476887]            0000000000115c02: 41f0a000            la      %r15,0(%r10)
> [   28.476887]           #0000000000115c06: e370f0980024        stg     %r7,152(%r15)
> [   28.476887]           >0000000000115c0c: c0e5fffff86e        brasl   %r14,114ce8
> [   28.476887]            0000000000115c12: 41f07000            la      %r15,0(%r7)
> [   28.476887]            0000000000115c16: a7f4ffa8            brc     15,115b66
> [   28.476887]            0000000000115c1a: 0707                bcr     0,%r7
> [   28.476887]            0000000000115c1c: 0707                bcr     0,%r7
> [   28.476901] Call Trace:
> [   28.476902] Last Breaking-Event-Address:
> [   28.476920]  [<0000000000a01c4a>] arch_call_rest_init+0x22/0x80
> [   28.476927] Kernel panic - not syncing: Corrupt kernel stack, can't continue.
> [   28.476930] CPU: 0 PID: 424 Comm: sh Not tainted 5.0.0-rc1+ #13
> [   28.476932] Hardware name: IBM 2964 NE1 716 (KVM/Linux)
> [   28.476932] Call Trace:
> 

I guess this has been broken for a long time. Most probably
ce3dc447493f ("s390: add support for virtually mapped kernel stacks")
finally made it trigger the low address protection.

@@ -372,8 +382,7 @@ void smp_call_online_cpu(void (*func)(void *), void
*data)
 void smp_call_ipl_cpu(void (*func)(void *), void *data)
 {
        pcpu_delegate(&pcpu_devices[0], func, data,
-                     pcpu_devices->lowcore->panic_stack -
-                     PANIC_FRAME_OFFSET + PAGE_SIZE);
+                     pcpu_devices->lowcore->nodat_stack);
 }

The old "- PANIC_FRAME_OFFSET + PAGE_SIZE" Made sure that we were not
trying to write at absolute address 0, but instead something >= 512,
whereby low address protection would not trigger.


> Reported-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  arch/s390/kernel/smp.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
> index f82b3d3c36e2..be32dd0b4191 100644
> --- a/arch/s390/kernel/smp.c
> +++ b/arch/s390/kernel/smp.c
> @@ -381,8 +381,13 @@ void smp_call_online_cpu(void (*func)(void *), void *data)
>   */
>  void smp_call_ipl_cpu(void (*func)(void *), void *data)
>  {
> +	struct lowcore *lc = pcpu_devices->lowcore;
> +
> +	if (pcpu_devices[0].address == stap())
> +		lc = &S390_lowcore;
> +
>  	pcpu_delegate(&pcpu_devices[0], func, data,
> -		      pcpu_devices->lowcore->nodat_stack);
> +		      lc->nodat_stack);
>  }
>  
>  int smp_find_processor_id(u16 address)
> 


-- 

Thanks,

David / dhildenb