Re: [PATCH v1 4/7] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages

[Mike Rapoport]

On Thu, Apr 29, 2021 at 02:25:16PM +0200, David Hildenbrand wrote:
> Let's avoid reading:
> 
> 1) Offline memory sections: the content of offline memory sections is stale
>    as the memory is effectively unused by the kernel. On s390x with standby
>    memory, offline memory sections (belonging to offline storage
>    increments) are not accessible. With virtio-mem and the hyper-v balloon,
>    we can have unavailable memory chunks that should not be accessed inside
>    offline memory sections. Last but not least, offline memory sections
>    might contain hwpoisoned pages which we can no longer identify
>    because the memmap is stale.
> 
> 2) PG_offline pages: logically offline pages that are documented as
>    "The content of these pages is effectively stale. Such pages should not
>     be touched (read/write/dump/save) except by their owner.".
>    Examples include pages inflated in a balloon or unavailble memory
>    ranges inside hotplugged memory sections with virtio-mem or the hyper-v
>    balloon.
> 
> 3) PG_hwpoison pages: Reading pages marked as hwpoisoned can be fatal.
>    As documented: "Accessing is not safe since it may cause another machine
>    check. Don't touch!"
> 
> Reading /proc/kcore now performs similar checks as when reading
> /proc/vmcore for kdump via makedumpfile: problematic pages are exclude.
> It's also similar to hibernation code, however, we don't skip hwpoisoned
> pages when processing pages in kernel/power/snapshot.c:saveable_page() yet.
> 
> Note 1: we can race against memory offlining code, especially
> memory going offline and getting unplugged: however, we will properly tear
> down the identity mapping and handle faults gracefully when accessing
> this memory from kcore code.
> 
> Note 2: we can race against drivers setting PageOffline() and turning
> memory inaccessible in the hypervisor. We'll handle this in a follow-up
> patch.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

> ---
>  fs/proc/kcore.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index ed6fbb3bd50c..92ff1e4436cb 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -465,6 +465,9 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  
>  	m = NULL;
>  	while (buflen) {
> +		struct page *page;
> +		unsigned long pfn;
> +
>  		/*
>  		 * If this is the first iteration or the address is not within
>  		 * the previous entry, search for a matching entry.
> @@ -503,7 +506,16 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  			}
>  			break;
>  		case KCORE_RAM:
> -			if (!pfn_is_ram(__pa(start) >> PAGE_SHIFT)) {
> +			pfn = __pa(start) >> PAGE_SHIFT;
> +			page = pfn_to_online_page(pfn);
> +
> +			/*
> +			 * Don't read offline sections, logically offline pages
> +			 * (e.g., inflated in a balloon), hwpoisoned pages,
> +			 * and explicitly excluded physical ranges.
> +			 */
> +			if (!page || PageOffline(page) ||
> +			    is_page_hwpoison(page) || !pfn_is_ram(pfn)) {
>  				if (clear_user(buffer, tsz)) {
>  					ret = -EFAULT;
>  					goto out;
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.