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

Mike Rapoport
2021-05-02T06:33:01Z

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@redhat.com>

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
> 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.