[PATCH v2 0/2] KVM: arm64: Prevent kmemleak from accessing HYP data

[PATCH v2 0/2] KVM: arm64: Prevent kmemleak from accessing HYP data

[Marc Zyngier]

This is a rework of the patch previously posted at [1].

The gist of the problem is that kmemleak can legitimately access data
that has been removed from the kernel view, for two reasons:

(1) .hyp.rodata is lumped together with the BSS
(2) there is no separation of the HYP BSS from the kernel BSS

(1) can easily be addressed by moving the .hyp.rodata section into the
    kernel's RO zone, which avoids any kmemleak into that section.
(2) must be addressed by telling kmemleak about the address range.

Tested on a SC2A11 system, in protected and non-protected modes with
kmemleak active. Both patches are stable candidates.

[1] https://lore.kernel.org/r/20210729135016.3037277-1-maz@kernel.org

Marc Zyngier (2):
  arm64: Move .hyp.rodata outside of the _sdata.._edata range
  KVM: arm64: Unregister HYP sections from kmemleak in protected mode

 arch/arm64/kernel/vmlinux.lds.S | 4 ++--
 arch/arm64/kvm/arm.c            | 7 +++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

-- 
2.30.2

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[PATCH v2 1/2] arm64: Move .hyp.rodata outside of the _sdata.._edata range

[Marc Zyngier]

The HYP rodata section is currently lumped together with the BSS,
which isn't exactly what is expected (it gets registered with
kmemleak, for example).

Move it away so that it is actually marked RO. As an added
benefit, it isn't registered with kmemleak anymore.

Fixes: 380e18ade4a5 ("KVM: arm64: Introduce a BSS section for use at Hyp")
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org #5.13
---
 arch/arm64/kernel/vmlinux.lds.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 709d2c433c5e..f6b1a88245db 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -181,6 +181,8 @@ SECTIONS
 	/* everything from this point to __init_begin will be marked RO NX */
 	RO_DATA(PAGE_SIZE)
 
+	HYPERVISOR_DATA_SECTIONS
+
 	idmap_pg_dir = .;
 	. += IDMAP_DIR_SIZE;
 	idmap_pg_end = .;
@@ -260,8 +262,6 @@ SECTIONS
 	_sdata = .;
 	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
 
-	HYPERVISOR_DATA_SECTIONS
-
 	/*
 	 * Data written with the MMU off but read with the MMU on requires
 	 * cache lines to be invalidated, discarding up to a Cache Writeback
-- 
2.30.2

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Re: [PATCH v2 1/2] arm64: Move .hyp.rodata outside of the _sdata.._edata range

[Quentin Perret]

Hi Marc,

On Monday 02 Aug 2021 at 13:38:29 (+0100), Marc Zyngier wrote:
> The HYP rodata section is currently lumped together with the BSS,
> which isn't exactly what is expected (it gets registered with
> kmemleak, for example).
> 
> Move it away so that it is actually marked RO. As an added
> benefit, it isn't registered with kmemleak anymore.

2d7bf218ca73 ("KVM: arm64: Add .hyp.data..ro_after_init ELF section")
states explicitly that the hyp ro_after_init section should remain RW in
the host as it is expected to modify it before initializing EL2. But I
can't seem to trigger anything with this patch applied, so I'll look
into this a bit more.

> Fixes: 380e18ade4a5 ("KVM: arm64: Introduce a BSS section for use at Hyp")

Not sure this is the patch to blame?

> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org #5.13
> ---
>  arch/arm64/kernel/vmlinux.lds.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 709d2c433c5e..f6b1a88245db 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -181,6 +181,8 @@ SECTIONS
>  	/* everything from this point to __init_begin will be marked RO NX */
>  	RO_DATA(PAGE_SIZE)
>  
> +	HYPERVISOR_DATA_SECTIONS
> +
>  	idmap_pg_dir = .;
>  	. += IDMAP_DIR_SIZE;
>  	idmap_pg_end = .;
> @@ -260,8 +262,6 @@ SECTIONS
>  	_sdata = .;
>  	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
>  
> -	HYPERVISOR_DATA_SECTIONS
> -
>  	/*
>  	 * Data written with the MMU off but read with the MMU on requires
>  	 * cache lines to be invalidated, discarding up to a Cache Writeback
> -- 
> 2.30.2

Thanks,
Quentin
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Re: [PATCH v2 1/2] arm64: Move .hyp.rodata outside of the _sdata.._edata range

[Marc Zyngier]

Hi Quentin,

On Mon, 02 Aug 2021 14:11:07 +0100,
Quentin Perret <qperret@google.com> wrote:
> 
> Hi Marc,
> 
> On Monday 02 Aug 2021 at 13:38:29 (+0100), Marc Zyngier wrote:
> > The HYP rodata section is currently lumped together with the BSS,
> > which isn't exactly what is expected (it gets registered with
> > kmemleak, for example).
> > 
> > Move it away so that it is actually marked RO. As an added
> > benefit, it isn't registered with kmemleak anymore.
> 
> 2d7bf218ca73 ("KVM: arm64: Add .hyp.data..ro_after_init ELF section")
> states explicitly that the hyp ro_after_init section should remain RW in
> the host as it is expected to modify it before initializing EL2. But I
> can't seem to trigger anything with this patch applied, so I'll look
> into this a bit more.

The switch to RO happens quite late. And if the host was to actually
try and change things there, it would be screwed anyway (we will have
already removed the pages from its S2).

I wouldn't be surprised if this was a consequence of the way we now
build the HYP object, and the comment in the original commit may not
be valid anymore.

> 
> > Fixes: 380e18ade4a5 ("KVM: arm64: Introduce a BSS section for use at Hyp")
> 
> Not sure this is the patch to blame?

My bad, this is plain wrong. I'm not sure it can be applied earlier
though if my rambling above is correct.

Thanks,

	M.

-- 
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Re: [PATCH v2 1/2] arm64: Move .hyp.rodata outside of the _sdata.._edata range

[Quentin Perret]

On Monday 02 Aug 2021 at 14:20:42 (+0100), Marc Zyngier wrote:
> Hi Quentin,
> 
> On Mon, 02 Aug 2021 14:11:07 +0100,
> Quentin Perret <qperret@google.com> wrote:
> > 
> > Hi Marc,
> > 
> > On Monday 02 Aug 2021 at 13:38:29 (+0100), Marc Zyngier wrote:
> > > The HYP rodata section is currently lumped together with the BSS,
> > > which isn't exactly what is expected (it gets registered with
> > > kmemleak, for example).
> > > 
> > > Move it away so that it is actually marked RO. As an added
> > > benefit, it isn't registered with kmemleak anymore.
> > 
> > 2d7bf218ca73 ("KVM: arm64: Add .hyp.data..ro_after_init ELF section")
> > states explicitly that the hyp ro_after_init section should remain RW in
> > the host as it is expected to modify it before initializing EL2. But I
> > can't seem to trigger anything with this patch applied, so I'll look
> > into this a bit more.
> 
> The switch to RO happens quite late. And if the host was to actually
> try and change things there, it would be screwed anyway (we will have
> already removed the pages from its S2).

Yes, clearly mapping rodata RO in host happens much later than I
thought, so this should indeed be fine.

> I wouldn't be surprised if this was a consequence of the way we now
> build the HYP object, and the comment in the original commit may not
> be valid anymore.

Just had a quick look and that still seems valid, at least for some
things (e.g. see how we set hyp_cpu_logical_map[] early from EL1 while
it is clearly annotated as __ro_after_init in the EL2 code).

> > 
> > > Fixes: 380e18ade4a5 ("KVM: arm64: Introduce a BSS section for use at Hyp")
> > 
> > Not sure this is the patch to blame?
> 
> My bad, this is plain wrong. I'm not sure it can be applied earlier
> though if my rambling above is correct.

By the look of it going all the way back to 2d7bf218ca73 (in David's
PSCI proxy series) should actually be correct. But not sure if that's
really going to make a difference before the patch you've mentioned
above as the kmemleak issue will only be visible once we have a host
stage-2, so no big deal.

Thanks,
Quentin
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Re: [PATCH v2 1/2] arm64: Move .hyp.rodata outside of the _sdata.._edata range

[Catalin Marinas]

On Mon, Aug 02, 2021 at 01:38:29PM +0100, Marc Zyngier wrote:
> The HYP rodata section is currently lumped together with the BSS,
> which isn't exactly what is expected (it gets registered with
> kmemleak, for example).
> 
> Move it away so that it is actually marked RO. As an added
> benefit, it isn't registered with kmemleak anymore.
> 
> Fixes: 380e18ade4a5 ("KVM: arm64: Introduce a BSS section for use at Hyp")
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org #5.13

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
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[PATCH v2 2/2] KVM: arm64: Unregister HYP sections from kmemleak in protected mode

[Marc Zyngier]

Booting a KVM host in protected mode with kmemleak quickly results
in a pretty bad crash, as kmemleak doesn't know that the HYP sections
have been taken away. This is specially true for the BSS section,
which is part of the kernel BSS section and registered at boot time
by kmemleak itself.

Unregister the HYP part of the BSS before making that section
HYP-private. The rest of the HYP-specific data is obtained via
the page allocator or lives in other sections, none of which is
subjected to kmemleak.

Fixes: 90134ac9cabb ("KVM: arm64: Protect the .hyp sections from the host")
Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: stable@vger.kernel.org # 5.13
---
 arch/arm64/kvm/arm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index e9a2b8f27792..52242f32c4be 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -15,6 +15,7 @@
 #include <linux/fs.h>
 #include <linux/mman.h>
 #include <linux/sched.h>
+#include <linux/kmemleak.h>
 #include <linux/kvm.h>
 #include <linux/kvm_irqfd.h>
 #include <linux/irqbypass.h>
@@ -1982,6 +1983,12 @@ static int finalize_hyp_mode(void)
 	if (ret)
 		return ret;
 
+	/*
+	 * Exclude HYP BSS from kmemleak so that it doesn't get peeked
+	 * at, which would end badly once the section is inaccessible.
+	 * None of other sections should ever be introspected.
+	 */
+	kmemleak_free_part(__hyp_bss_start, __hyp_bss_end - __hyp_bss_start);
 	ret = pkvm_mark_hyp_section(__hyp_bss);
 	if (ret)
 		return ret;
-- 
2.30.2

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Re: [PATCH v2 2/2] KVM: arm64: Unregister HYP sections from kmemleak in protected mode

[Catalin Marinas]

On Mon, Aug 02, 2021 at 01:38:30PM +0100, Marc Zyngier wrote:
> Booting a KVM host in protected mode with kmemleak quickly results
> in a pretty bad crash, as kmemleak doesn't know that the HYP sections
> have been taken away. This is specially true for the BSS section,
> which is part of the kernel BSS section and registered at boot time
> by kmemleak itself.
> 
> Unregister the HYP part of the BSS before making that section
> HYP-private. The rest of the HYP-specific data is obtained via
> the page allocator or lives in other sections, none of which is
> subjected to kmemleak.
> 
> Fixes: 90134ac9cabb ("KVM: arm64: Protect the .hyp sections from the host")
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: stable@vger.kernel.org # 5.13

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
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Re: [PATCH v2 0/2] KVM: arm64: Prevent kmemleak from accessing HYP data

[Marc Zyngier]

On Mon, 2 Aug 2021 13:38:28 +0100, Marc Zyngier wrote:
> This is a rework of the patch previously posted at [1].
> 
> The gist of the problem is that kmemleak can legitimately access data
> that has been removed from the kernel view, for two reasons:
> 
> (1) .hyp.rodata is lumped together with the BSS
> (2) there is no separation of the HYP BSS from the kernel BSS
> 
> [...]

Applied to next, thanks!

[1/2] arm64: Move .hyp.rodata outside of the _sdata.._edata range
      commit: eb48d154cd0dade56a0e244f0cfa198ea2925ed3
[2/2] KVM: arm64: Unregister HYP sections from kmemleak in protected mode
      commit: 47e6223c841e029bfc23c3ce594dac5525cebaf8

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.


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