linux-kernel - Re: [PATCH v2 11/11] arm64: ptdump: Add support for guest stage-2 pagetables dumping

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Message-ID: <ZTI85i0PmI_6doz1@google.com>
Date:   Fri, 20 Oct 2023 09:40:06 +0100
From:   Vincent Donnefort <vdonnefort@...gle.com>
To:     Sebastian Ene <sebastianene@...gle.com>
Cc:     will@...nel.org, catalin.marinas@....com, mark.rutland@....com,
        akpm@...ux-foundation.org, maz@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, qperret@...gle.com, smostafa@...gle.com
Subject: Re: [PATCH v2 11/11] arm64: ptdump: Add support for guest stage-2
 pagetables dumping

On Thu, Oct 19, 2023 at 02:40:33PM +0000, Sebastian Ene wrote:
> Register a debugfs file on guest creation to be able to view their
> second translation tables with ptdump. This assumes that the host is in
> control of the guest stage-2 and has direct access to the pagetables.

What about pKVM? The walker you wrote for the host stage-2 should be
reusable in that case?

> 
> Signed-off-by: Sebastian Ene <sebastianene@...gle.com>
> ---
>  arch/arm64/include/asm/ptdump.h | 21 +++++++--
>  arch/arm64/kvm/mmu.c            |  3 ++
>  arch/arm64/mm/ptdump.c          | 84 +++++++++++++++++++++++++++++++++
>  arch/arm64/mm/ptdump_debugfs.c  |  5 +-
>  4 files changed, 108 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
> index 35b883524462..be86244d532b 100644
> --- a/arch/arm64/include/asm/ptdump.h
> +++ b/arch/arm64/include/asm/ptdump.h
> @@ -5,6 +5,8 @@
>  #ifndef __ASM_PTDUMP_H
>  #define __ASM_PTDUMP_H
>  
> +#include <asm/kvm_pgtable.h>
> +
>  #ifdef CONFIG_PTDUMP_CORE
>  
>  #include <linux/mm_types.h>
> @@ -30,14 +32,27 @@ struct ptdump_info {
>  void ptdump_walk(struct seq_file *s, struct ptdump_info *info);
>  #ifdef CONFIG_PTDUMP_DEBUGFS
>  #define EFI_RUNTIME_MAP_END	DEFAULT_MAP_WINDOW_64
> -void __init ptdump_debugfs_register(struct ptdump_info *info, const char *name);
> +struct dentry *ptdump_debugfs_register(struct ptdump_info *info,
> +				       const char *name);
>  #else
> -static inline void ptdump_debugfs_register(struct ptdump_info *info,
> -					   const char *name) { }
> +static inline struct dentry *ptdump_debugfs_register(struct ptdump_info *info,
> +						     const char *name)
> +{
> +	return NULL;
> +}
>  #endif
>  void ptdump_check_wx(void);
>  #endif /* CONFIG_PTDUMP_CORE */
>  
> +#ifdef CONFIG_NVHE_EL2_PTDUMP_DEBUGFS
> +void ptdump_register_guest_stage2(struct kvm_pgtable *pgt, void *lock);
> +void ptdump_unregister_guest_stage2(struct kvm_pgtable *pgt);
> +#else
> +static inline void ptdump_register_guest_stage2(struct kvm_pgtable *pgt,
> +						void *lock) { }
> +static inline void ptdump_unregister_guest_stage2(struct kvm_pgtable *pgt) { }
> +#endif /* CONFIG_NVHE_EL2_PTDUMP_DEBUGFS */

I believe this should be compatible with VHE as well, that option should be
renamed.

> +
>  #ifdef CONFIG_DEBUG_WX
>  #define debug_checkwx()	ptdump_check_wx()
>  #else
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 482280fe22d7..e47988dba34d 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -11,6 +11,7 @@
>  #include <linux/sched/signal.h>
>  #include <trace/events/kvm.h>
>  #include <asm/pgalloc.h>
> +#include <asm/ptdump.h>
>  #include <asm/cacheflush.h>
>  #include <asm/kvm_arm.h>
>  #include <asm/kvm_mmu.h>
> @@ -908,6 +909,7 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
>  	if (err)
>  		goto out_free_pgtable;
>  
> +	ptdump_register_guest_stage2(pgt, &kvm->mmu_lock);
>  	mmu->last_vcpu_ran = alloc_percpu(typeof(*mmu->last_vcpu_ran));
>  	if (!mmu->last_vcpu_ran) {
>  		err = -ENOMEM;
> @@ -1021,6 +1023,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>  	write_unlock(&kvm->mmu_lock);
>  
>  	if (pgt) {
> +		ptdump_unregister_guest_stage2(pgt);
>  		kvm_pgtable_stage2_destroy(pgt);
>  		kfree(pgt);
>  	}
> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
> index 4687840dcb69..facfb15468f5 100644
> --- a/arch/arm64/mm/ptdump.c
> +++ b/arch/arm64/mm/ptdump.c
> @@ -26,6 +26,7 @@
>  #include <asm/ptdump.h>
>  #include <asm/kvm_pkvm.h>
>  #include <asm/kvm_pgtable.h>
> +#include <asm/kvm_host.h>
>  
>  
>  enum address_markers_idx {
> @@ -543,6 +544,22 @@ void ptdump_check_wx(void)
>  #ifdef CONFIG_NVHE_EL2_PTDUMP_DEBUGFS
>  static struct ptdump_info stage2_kernel_ptdump_info;
>  
> +#define GUEST_NAME_LEN	(32U)
> +
> +struct ptdump_registered_guest {
> +	struct list_head		reg_list;
> +	struct ptdump_info		info;
> +	struct mm_struct		mem;
> +	struct kvm_pgtable_snapshot	snapshot;
> +	struct dentry			*dentry;
> +	rwlock_t			*lock;
> +	char				reg_name[GUEST_NAME_LEN];
> +};
> +
> +static LIST_HEAD(ptdump_guest_list);
> +static DEFINE_MUTEX(ptdump_list_lock);
> +static u16 guest_no;

This is not robust enough: If 1 VM starts then 65535 others which are killed.
guest_no overflows. The next number is 0 which is already taken.

Linux has and ID allocation to solve this problem, but I don't think this is
necessary anyway. This should simply reuse the struct kvm->debugfs_dentry.

Also probably most of the informations contained in ptdump_registered_guest can
be found in struct kvm. The debugfs should then probably simply take struct kvm
for the private argument.

> +
>  static phys_addr_t ptdump_host_pa(void *addr)
>  {
>  	return __pa(addr);
> @@ -740,6 +757,73 @@ static void stage2_ptdump_walk(struct seq_file *s, struct ptdump_info *info)
>  
>  	kvm_pgtable_walk(pgtable, start_ipa, end_ipa, &walker);
>  }

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