[PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

Subsystems: kernel virtual machine for powerpc (kvm/powerpc), linux for powerpc (32-bit and 64-bit), the rest

STALE2087d LANDED

Landed in mainline as 75b49620267c on 2020-11-16.

6 messages, 4 authors, 2020-11-17 · open the first message on its own page

[PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

From: Cédric Le Goater <clg@kaod.org>
Date: 2020-11-05 13:52:35 

When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

    WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
    CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
    NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
    REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
    MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
    CFAR: c00000000044b160 IRQMASK: 0
    GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
    GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
    GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
    GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
    GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
    NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
    LR [c00000000044b164] __do_fault+0x64/0x220
    Call Trace:
    [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
    [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
    [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
    [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
    [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
    [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
    [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
    [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
    Instruction dump:
    40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
    7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
    ---[ end trace 66c6ff034c53f64f ]---
    xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

Fix that by checking the validity of the KVM XIVE interrupt structure.

Reported-by: Greg Kurz <redacted>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 arch/powerpc/kvm/book3s_xive_native.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_xive_native.c b/arch/powerpc/kvm/book3s_xive_native.c
index d0c2db0e07fa..a59a94f02733 100644
--- a/arch/powerpc/kvm/book3s_xive_native.c
+++ b/arch/powerpc/kvm/book3s_xive_native.c
@@ -251,6 +251,13 @@ static vm_fault_t xive_native_esb_fault(struct vm_fault *vmf)
 	}
 
 	state = &sb->irq_state[src];
+
+	/* Some sanity checking */
+	if (!state->valid) {
+		pr_devel("%s: source %lx invalid !\n", __func__, irq);
+		return VM_FAULT_SIGBUS;
+	}
+
 	kvmppc_xive_select_irq(state, &hw_num, &xd);
 
 	arch_spin_lock(&sb->lock);
-- 
2.26.2

Re: [PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

From: Greg Kurz <hidden>
Date: 2020-11-05 21:59:32 

On Thu, 5 Nov 2020 14:47:13 +0100
Cédric Le Goater [off-list ref] wrote:

When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

    WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
    CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
    NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
    REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
    MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
    CFAR: c00000000044b160 IRQMASK: 0
    GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
    GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
    GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
    GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
    GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
    NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
    LR [c00000000044b164] __do_fault+0x64/0x220
    Call Trace:
    [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
    [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
    [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
    [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
    [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
    [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
    [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
    [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
    Instruction dump:
    40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
    7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
    ---[ end trace 66c6ff034c53f64f ]---
    xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

Fix that by checking the validity of the KVM XIVE interrupt structure.

Reported-by: Greg Kurz <redacted>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
Looks sane to me. QEMU still crashes on SIGBUS but no more oops at least.

Tested-by: Greg Kurz <redacted>
quoted hunk
 arch/powerpc/kvm/book3s_xive_native.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_xive_native.c b/arch/powerpc/kvm/book3s_xive_native.c
index d0c2db0e07fa..a59a94f02733 100644
--- a/arch/powerpc/kvm/book3s_xive_native.c
+++ b/arch/powerpc/kvm/book3s_xive_native.c
@@ -251,6 +251,13 @@ static vm_fault_t xive_native_esb_fault(struct vm_fault *vmf)
 	}
 
 	state = &sb->irq_state[src];
+
+	/* Some sanity checking */
+	if (!state->valid) {
+		pr_devel("%s: source %lx invalid !\n", __func__, irq);
+		return VM_FAULT_SIGBUS;
+	}
+
 	kvmppc_xive_select_irq(state, &hw_num, &xd);
 
 	arch_spin_lock(&sb->lock);





  

    
Re: [PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

    

      From: Michael Ellerman <mpe@ellerman.id.au>

      Date: 2020-11-06 03:20:55 
    

  

  
Cédric Le Goater [off-list ref] writes:
When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

    WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
    CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
    NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
    REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
    MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
    CFAR: c00000000044b160 IRQMASK: 0
    GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
    GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
    GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
    GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
    GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
    NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
    LR [c00000000044b164] __do_fault+0x64/0x220
    Call Trace:
    [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
    [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
    [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
    [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
    [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
    [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
    [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
    [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
    Instruction dump:
    40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
    7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
    ---[ end trace 66c6ff034c53f64f ]---
    xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

Fix that by checking the validity of the KVM XIVE interrupt structure.

Reported-by: Greg Kurz <redacted>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Fixes ?

cheers





  

    
Re: [PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

    

      From: Cédric Le Goater <clg@kaod.org>

      Date: 2020-11-10 22:06:37 
    

  

  
On 11/6/20 4:19 AM, Michael Ellerman wrote:
Cédric Le Goater [off-list ref] writes:
quoted
When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

    WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
    CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
    NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
    REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
    MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
    CFAR: c00000000044b160 IRQMASK: 0
    GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
    GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
    GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
    GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
    GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
    NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
    LR [c00000000044b164] __do_fault+0x64/0x220
    Call Trace:
    [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
    [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
    [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
    [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
    [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
    [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
    [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
    [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
    Instruction dump:
    40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
    7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
    ---[ end trace 66c6ff034c53f64f ]---
    xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

Fix that by checking the validity of the KVM XIVE interrupt structure.

Reported-by: Greg Kurz <redacted>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Fixes ?
Ah yes :/  

Cc: stable@vger.kernel.org # v5.2+
Fixes: 6520ca64cde7 ("KVM: PPC: Book3S HV: XIVE: Add a mapping for the source ESB pages")

Since my provider changed its imap servers, my email filters are really screwed 
up and I miss emails. 

Sorry about that,

C.





  

    
Re: [PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

    

      From: Michael Ellerman <mpe@ellerman.id.au>

      Date: 2020-11-16 12:35:11 
    

  

  
Cédric Le Goater [off-list ref] writes:
On 11/6/20 4:19 AM, Michael Ellerman wrote:
quoted
Cédric Le Goater [off-list ref] writes:
quoted
When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

    WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
    CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
    NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
    REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
    MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
    CFAR: c00000000044b160 IRQMASK: 0
    GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
    GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
    GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
    GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
    GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
    NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
    LR [c00000000044b164] __do_fault+0x64/0x220
    Call Trace:
    [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
    [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
    [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
    [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
    [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
    [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
    [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
    [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
    Instruction dump:
    40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
    7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
    ---[ end trace 66c6ff034c53f64f ]---
    xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

Fix that by checking the validity of the KVM XIVE interrupt structure.

Reported-by: Greg Kurz <redacted>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Fixes ?
Ah yes :/  

Cc: stable@vger.kernel.org # v5.2+
Fixes: 6520ca64cde7 ("KVM: PPC: Book3S HV: XIVE: Add a mapping for the source ESB pages")

Since my provider changed its imap servers, my email filters are really screwed 
up and I miss emails. 

Sorry about that,
No worries.

It doesn't look like Paul has grabbed this, so I'll take it.

cheers





  

    
Re: [PATCH] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page

    

      From: Michael Ellerman <hidden>

      Date: 2020-11-17 10:54:41 
    

  

  
On Thu, 5 Nov 2020 14:47:13 +0100, C��dric Le Goater wrote:
When accessing the ESB page of a source interrupt, the fault handler
will retrieve the page address from the XIVE interrupt 'xive_irq_data'
structure. If the associated KVM XIVE interrupt is not valid, that is
not allocated at the HW level for some reason, the fault handler will
dereference a NULL pointer leading to the oops below :

    WARNING: CPU: 40 PID: 59101 at arch/powerpc/kvm/book3s_xive_native.c:259 xive_native_esb_fault+0xe4/0x240 [kvm]
    CPU: 40 PID: 59101 Comm: qemu-system-ppc Kdump: loaded Tainted: G        W        --------- -  - 4.18.0-240.el8.ppc64le #1
    NIP:  c00800000e949fac LR: c00000000044b164 CTR: c00800000e949ec8
    REGS: c000001f69617840 TRAP: 0700   Tainted: G        W        --------- -  -  (4.18.0-240.el8.ppc64le)
    MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 44044282  XER: 00000000
    CFAR: c00000000044b160 IRQMASK: 0
    GPR00: c00000000044b164 c000001f69617ac0 c00800000e96e000 c000001f69617c10
    GPR04: 05faa2b21e000080 0000000000000000 0000000000000005 ffffffffffffffff
    GPR08: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    GPR12: c00800000e949ec8 c000001ffffd3400 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 c000001f5c065160 c000000001c76f90
    GPR24: c000001f06f20000 c000001f5c065100 0000000000000008 c000001f0eb98c78
    GPR28: c000001dcab40000 c000001dcab403d8 c000001f69617c10 0000000000000011
    NIP [c00800000e949fac] xive_native_esb_fault+0xe4/0x240 [kvm]
    LR [c00000000044b164] __do_fault+0x64/0x220
    Call Trace:
    [c000001f69617ac0] [0000000137a5dc20] 0x137a5dc20 (unreliable)
    [c000001f69617b50] [c00000000044b164] __do_fault+0x64/0x220
    [c000001f69617b90] [c000000000453838] do_fault+0x218/0x930
    [c000001f69617bf0] [c000000000456f50] __handle_mm_fault+0x350/0xdf0
    [c000001f69617cd0] [c000000000457b1c] handle_mm_fault+0x12c/0x310
    [c000001f69617d10] [c00000000007ef44] __do_page_fault+0x264/0xbb0
    [c000001f69617df0] [c00000000007f8c8] do_page_fault+0x38/0xd0
    [c000001f69617e30] [c00000000000a714] handle_page_fault+0x18/0x38
    Instruction dump:
    40c2fff0 7c2004ac 2fa90000 409e0118 73e90001 41820080 e8bd0008 7c2004ac
    7ca90074 39400000 915c0000 7929d182 <0b090000> 2fa50000 419e0080 e89e0018
    ---[ end trace 66c6ff034c53f64f ]---
    xive-kvm: xive_native_esb_fault: accessing invalid ESB page for source 8 !

[...]
Applied to powerpc/fixes.

[1/1] KVM: PPC: Book3S HV: XIVE: Fix possible oops when accessing ESB page
      https://git.kernel.org/powerpc/c/75b49620267c700f0a07fec7f27f69852db70e46

cheers




    
  

  
    

      

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