Re: [PATCH v2 15/25] x86/sev: Introduce snp leaked pages list

[Vlastimil Babka]

On 1/26/24 05:11, Michael Roth wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> 
> Pages are unsafe to be released back to the page-allocator, if they
> have been transitioned to firmware/guest state and can't be reclaimed
> or transitioned back to hypervisor/shared state. In this case add
> them to an internal leaked pages list to ensure that they are not freed
> or touched/accessed to cause fatal page faults.
> 
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> [mdr: relocate to arch/x86/virt/svm/sev.c]
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Some minor nitpicks:

> ---
>  arch/x86/include/asm/sev.h |  2 ++
>  arch/x86/virt/svm/sev.c    | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index d3ccb7a0c7e9..435ba9bc4510 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -264,6 +264,7 @@ void snp_dump_hva_rmpentry(unsigned long address);
>  int psmash(u64 pfn);
>  int rmp_make_private(u64 pfn, u64 gpa, enum pg_level level, int asid, bool immutable);
>  int rmp_make_shared(u64 pfn, enum pg_level level);
> +void snp_leak_pages(u64 pfn, unsigned int npages);
>  #else
>  static inline bool snp_probe_rmptable_info(void) { return false; }
>  static inline int snp_lookup_rmpentry(u64 pfn, bool *assigned, int *level) { return -ENODEV; }
> @@ -275,6 +276,7 @@ static inline int rmp_make_private(u64 pfn, u64 gpa, enum pg_level level, int as
>  	return -ENODEV;
>  }
>  static inline int rmp_make_shared(u64 pfn, enum pg_level level) { return -ENODEV; }
> +static inline void snp_leak_pages(u64 pfn, unsigned int npages) {}
>  #endif
>  
>  #endif
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index 1a13eff78c9d..649ac1bb6b0e 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -65,6 +65,11 @@ static u64 probed_rmp_base, probed_rmp_size;
>  static struct rmpentry *rmptable __ro_after_init;
>  static u64 rmptable_max_pfn __ro_after_init;
>  
> +static LIST_HEAD(snp_leaked_pages_list);
> +static DEFINE_SPINLOCK(snp_leaked_pages_list_lock);
> +
> +static unsigned long snp_nr_leaked_pages;
> +
>  #undef pr_fmt
>  #define pr_fmt(fmt)	"SEV-SNP: " fmt
>  
> @@ -505,3 +510,32 @@ int rmp_make_shared(u64 pfn, enum pg_level level)
>  	return rmpupdate(pfn, &state);
>  }
>  EXPORT_SYMBOL_GPL(rmp_make_shared);
> +
> +void snp_leak_pages(u64 pfn, unsigned int npages)
> +{
> +	struct page *page = pfn_to_page(pfn);
> +
> +	pr_warn("Leaking PFN range 0x%llx-0x%llx\n", pfn, pfn + npages);
> +
> +	spin_lock(&snp_leaked_pages_list_lock);
> +	while (npages--) {
> +		/*
> +		 * Reuse the page's buddy list for chaining into the leaked
> +		 * pages list. This page should not be on a free list currently
> +		 * and is also unsafe to be added to a free list.
> +		 */
> +		if (likely(!PageCompound(page)) ||
> +		    (PageHead(page) && compound_nr(page) <= npages))
> +			/*
> +			 * Skip inserting tail pages of compound page as
> +			 * page->buddy_list of tail pages is not usable.
> +			 */
> +			list_add_tail(&page->buddy_list, &snp_leaked_pages_list);

Even though it's not necessary for correctness, with the comment I'd put the
whole block into { } to make easier to follow. Or just move the comment
above the if() itself?

> +		dump_rmpentry(pfn);
> +		snp_nr_leaked_pages++;
> +		pfn++;
> +		page++;
> +	}
> +	spin_unlock(&snp_leaked_pages_list_lock);
> +}
> +EXPORT_SYMBOL_GPL(snp_leak_pages);