Re: [PATCH v5 3/4] KVM: arm64: Register ptdump with debugfs on guest creation

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Feb 07, 2024 at 02:48:32PM +0000, Sebastian Ene wrote:
> While arch/*/mem/ptdump handles the kernel pagetable dumping code,
> introduce KVM/ptdump which shows the guest stage-2 pagetables. The
> separation is necessary because most of the definitions from the
> stage-2 pagetable reside in the KVM path and we will be invoking
> functionality **specific** to KVM.
> 
> When a guest is created, register a new file entry under the guest
> debugfs dir which allows userspace to show the contents of the guest
> stage-2 pagetables when accessed.
> 
> Signed-off-by: Sebastian Ene <sebastianene@google.com>
> ---
>  arch/arm64/kvm/Kconfig      | 13 ++++++
>  arch/arm64/kvm/Makefile     |  1 +
>  arch/arm64/kvm/debug.c      |  7 ++++
>  arch/arm64/kvm/kvm_ptdump.h | 20 ++++++++++
>  arch/arm64/kvm/ptdump.c     | 79 +++++++++++++++++++++++++++++++++++++
>  5 files changed, 120 insertions(+)
>  create mode 100644 arch/arm64/kvm/kvm_ptdump.h
>  create mode 100644 arch/arm64/kvm/ptdump.c
> 
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 6c3c8ca73e7f..28097dd72174 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -68,4 +68,17 @@ config PROTECTED_NVHE_STACKTRACE
>  
>  	  If unsure, or not using protected nVHE (pKVM), say N.
>  
> +config PTDUMP_STAGE2_DEBUGFS
> +       bool "Present the stage-2 pagetables to debugfs"
> +       depends on PTDUMP_DEBUGFS && KVM
> +       default n
> +       help
> +         Say Y here if you want to show the stage-2 kernel pagetables
> +         layout in a debugfs file. This information is only useful for kernel developers
> +         who are working in architecture specific areas of the kernel.
> +         It is probably not a good idea to enable this feature in a production
> +         kernel.
> +
> +         If in doubt, say N.
> +
>  endif # VIRTUALIZATION
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index c0c050e53157..190eac17538c 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -23,6 +23,7 @@ kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
>  	 vgic/vgic-its.o vgic/vgic-debug.o
>  
>  kvm-$(CONFIG_HW_PERF_EVENTS)  += pmu-emul.o pmu.o
> +kvm-$(CONFIG_PTDUMP_STAGE2_DEBUGFS) += ptdump.o
>  
>  always-y := hyp_constants.h hyp-constants.s
>  
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 8725291cb00a..aef52836cd90 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -14,6 +14,8 @@
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_emulate.h>
>  
> +#include <kvm_ptdump.h>
> +
>  #include "trace.h"
>  
>  /* These are the bits of MDSCR_EL1 we may manipulate */
> @@ -342,3 +344,8 @@ void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_SPE);
>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
>  }
> +
> +int kvm_arch_create_vm_debugfs(struct kvm *kvm)
> +{
> +	return kvm_ptdump_guest_register(kvm);
> +}
> diff --git a/arch/arm64/kvm/kvm_ptdump.h b/arch/arm64/kvm/kvm_ptdump.h
> new file mode 100644
> index 000000000000..a7c00a28481b
> --- /dev/null
> +++ b/arch/arm64/kvm/kvm_ptdump.h
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) Google, 2023
> + * Author: Sebastian Ene <sebastianene@google.com>
> + */
> +
> +#ifndef __KVM_PTDUMP_H
> +#define __KVM_PTDUMP_H
> +
> +#include <linux/kvm_host.h>
> +#include <asm/ptdump.h>
> +
> +
> +#ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
> +int kvm_ptdump_guest_register(struct kvm *kvm);
> +#else
> +static inline int kvm_ptdump_guest_register(struct kvm *kvm) { return 0; }
> +#endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
> +
> +#endif /* __KVM_PTDUMP_H */
> diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
> new file mode 100644
> index 000000000000..a4e984da8aa7
> --- /dev/null
> +++ b/arch/arm64/kvm/ptdump.c
> @@ -0,0 +1,79 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +//
> +// Debug helper used to dump the stage-2 pagetables of the system and their
> +// associated permissions.
> +//
> +// Copyright (C) Google, 2023
> +// Author: Sebastian Ene <sebastianene@google.com>

Same comment as last time about ... the comment :)

Should be of the form

/*
 *
 */

> +#include <linux/debugfs.h>
> +#include <linux/kvm_host.h>
> +#include <linux/seq_file.h>
> +
> +#include <asm/kvm_pkvm.h>

is this needed?

> +#include <kvm_ptdump.h>
> +
> +
> +static int kvm_ptdump_guest_open(struct inode *inode, struct file *file);
> +static int kvm_ptdump_guest_show(struct seq_file *m, void *);

can you structure the file in a way to avoid forward declarations?

> +static const struct file_operations kvm_ptdump_guest_fops = {
> +	.open		= kvm_ptdump_guest_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= single_release,
> +};
> +
> +static int kvm_ptdump_guest_open(struct inode *inode, struct file *file)
> +{
> +	return single_open(file, kvm_ptdump_guest_show, inode->i_private);
> +}
> +

Shouldn't we take a reference on the KVM struct at open to avoid UAF?

	struct kvm *kvm = inode->i_private;

	if (!kvm_get_kvm_safe(kvm))
		return -ENOENT;

Then you can do a put on it at close().

> +static int kvm_ptdump_visitor(const struct kvm_pgtable_visit_ctx *ctx,
> +			      enum kvm_pgtable_walk_flags visit)
> +{
> +	struct pg_state *st = ctx->arg;
> +	struct ptdump_state *pt_st = &st->ptdump;
> +
> +	note_page(pt_st, ctx->addr, ctx->level, ctx->old);
> +	return 0;
> +}
> +
> +static int kvm_ptdump_show_common(struct seq_file *m,
> +				  struct kvm_pgtable *pgtable,
> +				  struct pg_state *parser_state)
> +{
> +	struct kvm_pgtable_walker walker = (struct kvm_pgtable_walker) {
> +		.cb     = kvm_ptdump_visitor,
> +		.arg	= parser_state,
> +		.flags	= KVM_PGTABLE_WALK_LEAF,
> +	};
> +
> +	return kvm_pgtable_walk(pgtable, 0, BIT(pgtable->ia_bits), &walker);
> +}
> +
> +static int kvm_ptdump_guest_show(struct seq_file *m, void *)
> +{
> +	struct kvm *guest_kvm = m->private;
> +	struct kvm_s2_mmu *mmu = &guest_kvm->arch.mmu;
> +	struct pg_state parser_state = {0};
> +	int ret;
> +
> +	write_lock(&guest_kvm->mmu_lock);
> +	ret = kvm_ptdump_show_common(m, mmu->pgt, &parser_state);
> +	write_unlock(&guest_kvm->mmu_lock);
> +
> +	return ret;
> +}
> +
> +int kvm_ptdump_guest_register(struct kvm *kvm)
> +{
> +	struct dentry *parent;
> +
> +	parent = debugfs_create_file("stage2_page_tables", 0400,
> +				     kvm->debugfs_dentry, kvm,
> +				     &kvm_ptdump_guest_fops);
> +	if (IS_ERR(parent))
> +		return PTR_ERR(parent);

This makes the otherwise benign debugfs failure into something fatal for
VM creation, no?

From the documentation on debugfs_create_file():

 * NOTE: it's expected that most callers should _ignore_ the errors returned
 * by this function. Other debugfs functions handle the fact that the "dentry"
 * passed to them could be an error and they don't crash in that case.
 * Drivers should generally work fine even if debugfs fails to init anyway.

The fact that kvm_arch_create_vm_debugfs() has a return value is a bit
of an anti-pattern to begin with.

-- 
Thanks,
Oliver

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