[stable:PATCH v4.9.334 0/2] arm64: errata: Spectre-BHB fixes

[stable:PATCH v4.9.334 0/2] arm64: errata: Spectre-BHB fixes

[James Morse]

Hello!

The first patch fixes an issue reported by Sami, where linux panic()s
when bringing secondary CPUs online. The problem was the Spectre
workarounds trying to allocate a new slot for mitigating KVM when
those pages are no longer writeable.

While debugging that issue, I spotted the Spectre-BHB KVM mitigation was
over-riding the Spectre-v2 KVM Mitigation. It's supposed to happen the
other way round.

The backports aren't the same as mainline because the spectre mitigation code
was totally rewritten for v5.10, and prior to that the KVM infrastructure
is very different.


Thanks,

James Morse (2):
  arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate
    KVM vectors
  arm64: errata: Fix KVM Spectre-v2 mitigation selection for
    Cortex-A57/A72

 arch/arm64/kernel/cpu_errata.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

-- 
2.30.2


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[stable:PATCH v4.9.334 1/2] arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate KVM vectors

[James Morse]

Sami reports that linux panic()s when resuming from suspend to RAM. This
is because when CPUs are brought back online, they re-enable any
necessary mitigations.

The Spectre-v2 and Spectre-BHB mitigations interact as both need to
done by KVM when exiting a guest. Slots KVM can use as vectors are
allocated, and templates for the mitigation are patched into the vector.

This fails if a new slot needs to be allocated once the kernel has finished
booting as it is no-longer possible to modify KVM's vectors:
| root@adam:/sys/devices/system/cpu/cpu1# echo 1 > online
| Unable to handle kernel write to read-only memory at virtual add>
| Mem abort info:
|   ESR = 0x9600004e
|   Exception class = DABT (current EL), IL = 32 bits
|   SET = 0, FnV = 0
|   EA = 0, S1PTW = 0
| Data abort info:
|   ISV = 0, ISS = 0x0000004e
|   CM = 0, WnR = 1
| swapper pgtable: 4k pages, 48-bit VAs, pgdp = 000000000f07a71c
| [ffff800000b4b800] pgd=00000009ffff8803, pud=00000009ffff7803, p>
| Internal error: Oops: 9600004e [#1] PREEMPT SMP
| Modules linked in:
| Process swapper/1 (pid: 0, stack limit = 0x0000000063153c53)
| CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.252-dirty #14
| Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno De>
| pstate: 000001c5 (nzcv dAIF -PAN -UAO)
| pc : __memcpy+0x48/0x180
| lr : __copy_hyp_vect_bpi+0x64/0x90

| Call trace:
|  __memcpy+0x48/0x180
|  kvm_setup_bhb_slot+0x204/0x2a8
|  spectre_bhb_enable_mitigation+0x1b8/0x1d0
|  __verify_local_cpu_caps+0x54/0xf0
|  check_local_cpu_capabilities+0xc4/0x184
|  secondary_start_kernel+0xb0/0x170
| Code: b8404423 b80044c3 36180064 f8408423 (f80084c3)
| ---[ end trace 859bcacb09555348 ]---
| Kernel panic - not syncing: Attempted to kill the idle task!
| SMP: stopping secondary CPUs
| Kernel Offset: disabled
| CPU features: 0x10,25806086
| Memory Limit: none
| ---[ end Kernel panic - not syncing: Attempted to kill the idle ]

This is only a problem on platforms where there is only one CPU that is
vulnerable to both Spectre-v2 and Spectre-BHB.

The Spectre-v2 mitigation identifies the slot it can re-use by the CPU's
'fn'. It unconditionally writes the slot number and 'template_start'
pointer. The Spectre-BHB mitigation identifies slots it can re-use by
the CPU's template_start pointer, which was previously clobbered by the
Spectre-v2 mitigation.

When there is only one CPU that is vulnerable to both issues, this causes
Spectre-v2 to try to allocate a new slot, which fails.

Change both mitigations to check whether they are changing the slot this
CPU uses before writing the percpu variables again.

This issue only exists in the stable backports for Spectre-BHB which have
to use totally different infrastructure to mainline.

Reported-by: Sami Lee <sami.lee@mediatek.com>
Fixes: 4dd8aae585a5 ("arm64: Mitigate spectre style branch history side channels")
Signed-off-by: James Morse <james.morse@arm.com>
---
 arch/arm64/kernel/cpu_errata.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 17208f1b10a9..f26320103651 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -125,10 +125,12 @@ static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
 		__copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end);
 	}
 
-	__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
-	__this_cpu_write(bp_hardening_data.fn, fn);
-	__this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start);
-	__hardenbp_enab = true;
+	if (fn != __this_cpu_read(bp_hardening_data.fn)) {
+		__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
+		__this_cpu_write(bp_hardening_data.fn, fn);
+		__this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start);
+		__hardenbp_enab = true;
+	}
 	spin_unlock(&bp_lock);
 }
 #else
@@ -828,8 +830,11 @@ static void kvm_setup_bhb_slot(const char *hyp_vecs_start)
 		__copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end);
 	}
 
-	__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
-	__this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start);
+	if (hyp_vecs_start != __this_cpu_read(bp_hardening_data.template_start)) {
+		__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
+		__this_cpu_write(bp_hardening_data.template_start,
+				 hyp_vecs_start);
+	}
 	spin_unlock(&bp_lock);
 }
 #else
-- 
2.30.2


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Patch "arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate KVM vectors" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate KVM vectors

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm64-fix-panic-when-spectre-v2-causes-spectre-bhb-to-re-allocate-kvm-vectors.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From foo@baz Sat Dec  3 02:32:26 PM CET 2022
From: James Morse <james.morse@arm.com>
Date: Wed, 30 Nov 2022 18:29:55 +0000
Subject: arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate KVM vectors
To: stable@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc: Catalin Marinas <catalin.marinas@arm.com>, James Morse <james.morse@arm.com>, Sami Lee <sami.lee@mediatek.com>
Message-ID: <20221130182956.739350-2-james.morse@arm.com>

From: James Morse <james.morse@arm.com>

Sami reports that linux panic()s when resuming from suspend to RAM. This
is because when CPUs are brought back online, they re-enable any
necessary mitigations.

The Spectre-v2 and Spectre-BHB mitigations interact as both need to
done by KVM when exiting a guest. Slots KVM can use as vectors are
allocated, and templates for the mitigation are patched into the vector.

This fails if a new slot needs to be allocated once the kernel has finished
booting as it is no-longer possible to modify KVM's vectors:
| root@adam:/sys/devices/system/cpu/cpu1# echo 1 > online
| Unable to handle kernel write to read-only memory at virtual add>
| Mem abort info:
|   ESR = 0x9600004e
|   Exception class = DABT (current EL), IL = 32 bits
|   SET = 0, FnV = 0
|   EA = 0, S1PTW = 0
| Data abort info:
|   ISV = 0, ISS = 0x0000004e
|   CM = 0, WnR = 1
| swapper pgtable: 4k pages, 48-bit VAs, pgdp = 000000000f07a71c
| [ffff800000b4b800] pgd=00000009ffff8803, pud=00000009ffff7803, p>
| Internal error: Oops: 9600004e [#1] PREEMPT SMP
| Modules linked in:
| Process swapper/1 (pid: 0, stack limit = 0x0000000063153c53)
| CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.252-dirty #14
| Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno De>
| pstate: 000001c5 (nzcv dAIF -PAN -UAO)
| pc : __memcpy+0x48/0x180
| lr : __copy_hyp_vect_bpi+0x64/0x90

| Call trace:
|  __memcpy+0x48/0x180
|  kvm_setup_bhb_slot+0x204/0x2a8
|  spectre_bhb_enable_mitigation+0x1b8/0x1d0
|  __verify_local_cpu_caps+0x54/0xf0
|  check_local_cpu_capabilities+0xc4/0x184
|  secondary_start_kernel+0xb0/0x170
| Code: b8404423 b80044c3 36180064 f8408423 (f80084c3)
| ---[ end trace 859bcacb09555348 ]---
| Kernel panic - not syncing: Attempted to kill the idle task!
| SMP: stopping secondary CPUs
| Kernel Offset: disabled
| CPU features: 0x10,25806086
| Memory Limit: none
| ---[ end Kernel panic - not syncing: Attempted to kill the idle ]

This is only a problem on platforms where there is only one CPU that is
vulnerable to both Spectre-v2 and Spectre-BHB.

The Spectre-v2 mitigation identifies the slot it can re-use by the CPU's
'fn'. It unconditionally writes the slot number and 'template_start'
pointer. The Spectre-BHB mitigation identifies slots it can re-use by
the CPU's template_start pointer, which was previously clobbered by the
Spectre-v2 mitigation.

When there is only one CPU that is vulnerable to both issues, this causes
Spectre-v2 to try to allocate a new slot, which fails.

Change both mitigations to check whether they are changing the slot this
CPU uses before writing the percpu variables again.

This issue only exists in the stable backports for Spectre-BHB which have
to use totally different infrastructure to mainline.

Reported-by: Sami Lee <sami.lee@mediatek.com>
Fixes: 4dd8aae585a5 ("arm64: Mitigate spectre style branch history side channels")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/arm64/kernel/cpu_errata.c |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -125,10 +125,12 @@ static void __install_bp_hardening_cb(bp
 		__copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end);
 	}
 
-	__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
-	__this_cpu_write(bp_hardening_data.fn, fn);
-	__this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start);
-	__hardenbp_enab = true;
+	if (fn != __this_cpu_read(bp_hardening_data.fn)) {
+		__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
+		__this_cpu_write(bp_hardening_data.fn, fn);
+		__this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start);
+		__hardenbp_enab = true;
+	}
 	spin_unlock(&bp_lock);
 }
 #else
@@ -828,8 +830,11 @@ static void kvm_setup_bhb_slot(const cha
 		__copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end);
 	}
 
-	__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
-	__this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start);
+	if (hyp_vecs_start != __this_cpu_read(bp_hardening_data.template_start)) {
+		__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
+		__this_cpu_write(bp_hardening_data.template_start,
+				 hyp_vecs_start);
+	}
 	spin_unlock(&bp_lock);
 }
 #else


Patches currently in stable-queue which might be from james.morse@arm.com are

queue-4.9/arm64-errata-fix-kvm-spectre-v2-mitigation-selection-for-cortex-a57-a72.patch
queue-4.9/arm64-fix-panic-when-spectre-v2-causes-spectre-bhb-to-re-allocate-kvm-vectors.patch

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[stable:PATCH v4.9.334 2/2] arm64: errata: Fix KVM Spectre-v2 mitigation selection for Cortex-A57/A72

[James Morse]

Both the Spectre-v2 and Spectre-BHB mitigations involve running a sequence
immediately after exiting a guest, before any branches. In the stable
kernels these sequences are built by copying templates into an empty vector
slot.

For Spectre-BHB, Cortex-A57 and A72 require the branchy loop with k=8.
If Spectre-v2 needs mitigating at the same time, a firmware call to EL3 is
needed. The work EL3 does at this point is also enough to mitigate
Spectre-BHB.

When enabling the Spectre-BHB mitigation, spectre_bhb_enable_mitigation()
should check if a slot has already been allocated for Spectre-v2, meaning
no work is needed for Spectre-BHB.

This check was missed in the earlier backport, add it.

Fixes: 4dd8aae585a5 ("arm64: Mitigate spectre style branch history side channels")
Signed-off-by: James Morse <james.morse@arm.com>
---
 arch/arm64/kernel/cpu_errata.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index f26320103651..ae11af9c8cbd 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -876,7 +876,13 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 	} else if (spectre_bhb_loop_affected(SCOPE_LOCAL_CPU)) {
 		switch (spectre_bhb_loop_affected(SCOPE_SYSTEM)) {
 		case 8:
-			kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start);
+			/*
+			 * A57/A72-r0 will already have selected the
+			 * spectre-indirect vector, which is sufficient
+			 * for BHB too.
+			 */
+			if (!__this_cpu_read(bp_hardening_data.fn))
+				kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start);
 			break;
 		case 24:
 			kvm_setup_bhb_slot(__spectre_bhb_loop_k24_start);
-- 
2.30.2


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Patch "arm64: errata: Fix KVM Spectre-v2 mitigation selection for Cortex-A57/A72" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    arm64: errata: Fix KVM Spectre-v2 mitigation selection for Cortex-A57/A72

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm64-errata-fix-kvm-spectre-v2-mitigation-selection-for-cortex-a57-a72.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From foo@baz Sat Dec  3 02:32:26 PM CET 2022
From: James Morse <james.morse@arm.com>
Date: Wed, 30 Nov 2022 18:29:56 +0000
Subject: arm64: errata: Fix KVM Spectre-v2 mitigation selection for Cortex-A57/A72
To: stable@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc: Catalin Marinas <catalin.marinas@arm.com>, James Morse <james.morse@arm.com>
Message-ID: <20221130182956.739350-3-james.morse@arm.com>

From: James Morse <james.morse@arm.com>

Both the Spectre-v2 and Spectre-BHB mitigations involve running a sequence
immediately after exiting a guest, before any branches. In the stable
kernels these sequences are built by copying templates into an empty vector
slot.

For Spectre-BHB, Cortex-A57 and A72 require the branchy loop with k=8.
If Spectre-v2 needs mitigating at the same time, a firmware call to EL3 is
needed. The work EL3 does at this point is also enough to mitigate
Spectre-BHB.

When enabling the Spectre-BHB mitigation, spectre_bhb_enable_mitigation()
should check if a slot has already been allocated for Spectre-v2, meaning
no work is needed for Spectre-BHB.

This check was missed in the earlier backport, add it.

Fixes: 4dd8aae585a5 ("arm64: Mitigate spectre style branch history side channels")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/arm64/kernel/cpu_errata.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -876,7 +876,13 @@ void spectre_bhb_enable_mitigation(const
 	} else if (spectre_bhb_loop_affected(SCOPE_LOCAL_CPU)) {
 		switch (spectre_bhb_loop_affected(SCOPE_SYSTEM)) {
 		case 8:
-			kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start);
+			/*
+			 * A57/A72-r0 will already have selected the
+			 * spectre-indirect vector, which is sufficient
+			 * for BHB too.
+			 */
+			if (!__this_cpu_read(bp_hardening_data.fn))
+				kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start);
 			break;
 		case 24:
 			kvm_setup_bhb_slot(__spectre_bhb_loop_k24_start);


Patches currently in stable-queue which might be from james.morse@arm.com are

queue-4.9/arm64-errata-fix-kvm-spectre-v2-mitigation-selection-for-cortex-a57-a72.patch
queue-4.9/arm64-fix-panic-when-spectre-v2-causes-spectre-bhb-to-re-allocate-kvm-vectors.patch

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