| Message ID | 20190508144422.13171-4-kirill.shutemov@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Intel MKTME enabling | expand |
On 5/8/19 7:43 AM, Kirill A. Shutemov wrote: > KeyID indicates what key to use to encrypt and decrypt page's content. > Depending on the implementation a cipher text may be tied to physical > address of the page. It means that pages with an identical plain text > would appear different if KSM would look at a cipher text. It effectively > disables KSM for encrypted pages. > > In addition, some implementations may not allow to read cipher text at all. > > KSM compares plain text instead (transparently to KSM code). > > But we still need to make sure that pages with identical plain text will > not be merged together if they are encrypted with different keys. > > To make it work kernel only allows merging pages with the same KeyID. > The approach guarantees that the merged page can be read by all users. I can't really parse this description. Can I suggest replacement text? Problem: KSM compares plain text. It might try to merge two pages that have the same plain text but different ciphertext and possibly different encryption keys. When the kernel encrypted the page, it promised that it would keep it encrypted with _that_ key. That makes it impossible to merge two pages encrypted with different keys. Solution: Never merge encrypted pages with different KeyIDs.
On Fri, May 10, 2019 at 06:07:11PM +0000, Dave Hansen wrote: > On 5/8/19 7:43 AM, Kirill A. Shutemov wrote: > > KeyID indicates what key to use to encrypt and decrypt page's content. > > Depending on the implementation a cipher text may be tied to physical > > address of the page. It means that pages with an identical plain text > > would appear different if KSM would look at a cipher text. It effectively > > disables KSM for encrypted pages. > > > > In addition, some implementations may not allow to read cipher text at all. > > > > KSM compares plain text instead (transparently to KSM code). > > > > But we still need to make sure that pages with identical plain text will > > not be merged together if they are encrypted with different keys. > > > > To make it work kernel only allows merging pages with the same KeyID. > > The approach guarantees that the merged page can be read by all users. > > I can't really parse this description. Can I suggest replacement text? Sure.
diff --git a/include/linux/mm.h b/include/linux/mm.h index 13c40c43ce00..07c36f4673f6 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1606,6 +1606,13 @@ static inline int vma_keyid(struct vm_area_struct *vma) } #endif +#ifndef page_keyid +static inline int page_keyid(struct page *page) +{ + return 0; +} +#endif + #ifdef CONFIG_SHMEM /* * The vma_is_shmem is not inline because it is used only by slow diff --git a/mm/ksm.c b/mm/ksm.c index fc64874dc6f4..91bce4799c45 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -1227,6 +1227,23 @@ static int try_to_merge_one_page(struct vm_area_struct *vma, if (!PageAnon(page)) goto out; + /* + * KeyID indicates what key to use to encrypt and decrypt page's + * content. + * + * KSM compares plain text instead (transparently to KSM code). + * + * But we still need to make sure that pages with identical plain + * text will not be merged together if they are encrypted with + * different keys. + * + * To make it work kernel only allows merging pages with the same KeyID. + * The approach guarantees that the merged page can be read by all + * users. + */ + if (kpage && page_keyid(page) != page_keyid(kpage)) + goto out; + /* * We need the page lock to read a stable PageSwapCache in * write_protect_page(). We use trylock_page() instead of
KeyID indicates what key to use to encrypt and decrypt page's content. Depending on the implementation a cipher text may be tied to physical address of the page. It means that pages with an identical plain text would appear different if KSM would look at a cipher text. It effectively disables KSM for encrypted pages. In addition, some implementations may not allow to read cipher text at all. KSM compares plain text instead (transparently to KSM code). But we still need to make sure that pages with identical plain text will not be merged together if they are encrypted with different keys. To make it work kernel only allows merging pages with the same KeyID. The approach guarantees that the merged page can be read by all users. Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> --- include/linux/mm.h | 7 +++++++ mm/ksm.c | 17 +++++++++++++++++ 2 files changed, 24 insertions(+)