mm: streamline COW logic in do_swap_page()

Currently we have a different COW logic when:
* triggering a read-fault to swapin first and then trigger a write-fault
  -> do_swap_page() + do_wp_page()
* triggering a write-fault to swapin
  -> do_swap_page() + do_wp_page() only if we fail reuse in do_swap_page()

The COW logic in do_swap_page() is different than our reuse logic in
do_wp_page().  The COW logic in do_wp_page() -- page_count() == 1 -- makes
currently sure that we certainly don't have a remaining reference, e.g.,
via GUP, on the target page we want to reuse: if there is any unexpected
reference, we have to copy to avoid information leaks.

As do_swap_page() behaves differently, in environments with swap enabled
we can currently have an unintended information leak from the parent to
the child, similar as known from CVE-2020-29374:

	1. Parent writes to anonymous page
	-> Page is mapped writable and modified
	2. Page is swapped out
	-> Page is unmapped and replaced by swap entry
	3. fork()
	-> Swap entries are copied to child
	4. Child pins page R/O
	-> Page is mapped R/O into child
	5. Child unmaps page
	-> Child still holds GUP reference
	6. Parent writes to page
	-> Page is reused in do_swap_page()
	-> Child can observe changes

Exchanging 2. and 3. should have the same effect.

Let's apply the same COW logic as in do_wp_page(), conditionally trying to
remove the page from the swapcache after freeing the swap entry, however,
before actually mapping our page.  We can change the order now that we use
try_to_free_swap(), which doesn't care about the mapcount, instead of
reuse_swap_page().

To handle references from the LRU pagevecs, conditionally drain the local
LRU pagevecs when required, however, don't consider the page_count() when
deciding whether to drain to keep it simple for now.

Link: https://lkml.kernel.org/r/20220131162940.210846-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Liang Zhang <zhangliang5@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 8e30675..f721735 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3489,6 +3489,25 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
        return 0;
 }
 
+static inline bool should_try_to_free_swap(struct page *page,
+                                          struct vm_area_struct *vma,
+                                          unsigned int fault_flags)
+{
+       if (!PageSwapCache(page))
+               return false;
+       if (mem_cgroup_swap_full(page) || (vma->vm_flags & VM_LOCKED) ||
+           PageMlocked(page))
+               return true;
+       /*
+        * If we want to map a page that's in the swapcache writable, we
+        * have to detect via the refcount if we're really the exclusive
+        * user. Try freeing the swapcache to get rid of the swapcache
+        * reference only in case it's likely that we'll be the exlusive user.
+        */
+       return (fault_flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+               page_count(page) == 2;
+}
+
 /*
  * We enter with non-exclusive mmap_lock (to exclude vma changes,
  * but allow concurrent faults), and pte mapped but not yet locked.
@@ -3630,6 +3649,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
                        page = swapcache;
                        goto out_page;
                }
+
+               /*
+                * If we want to map a page that's in the swapcache writable, we
+                * have to detect via the refcount if we're really the exclusive
+                * owner. Try removing the extra reference from the local LRU
+                * pagevecs if required.
+                */
+               if ((vmf->flags & FAULT_FLAG_WRITE) && page == swapcache &&
+                   !PageKsm(page) && !PageLRU(page))
+                       lru_add_drain();
        }
 
        cgroup_throttle_swaprate(page, GFP_KERNEL);
@@ -3648,19 +3677,25 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
        }
 
        /*
-        * The page isn't present yet, go ahead with the fault.
-        *
-        * Be careful about the sequence of operations here.
-        * To get its accounting right, reuse_swap_page() must be called
-        * while the page is counted on swap but not yet in mapcount i.e.
-        * before page_add_anon_rmap() and swap_free(); try_to_free_swap()
-        * must be called after the swap_free(), or it will never succeed.
+        * Remove the swap entry and conditionally try to free up the swapcache.
+        * We're already holding a reference on the page but haven't mapped it
+        * yet.
         */
+       swap_free(entry);
+       if (should_try_to_free_swap(page, vma, vmf->flags))
+               try_to_free_swap(page);
 
        inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
        dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
        pte = mk_pte(page, vma->vm_page_prot);
-       if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
+
+       /*
+        * Same logic as in do_wp_page(); however, optimize for fresh pages
+        * that are certainly not shared because we just allocated them without
+        * exposing them to the swapcache.
+        */
+       if ((vmf->flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+           (page != swapcache || page_count(page) == 1)) {
                pte = maybe_mkwrite(pte_mkdirty(pte), vma);
                vmf->flags &= ~FAULT_FLAG_WRITE;
                ret |= VM_FAULT_WRITE;
@@ -3686,10 +3721,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
        set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
        arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
 
-       swap_free(entry);
-       if (mem_cgroup_swap_full(page) ||
-           (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
-               try_to_free_swap(page);
        unlock_page(page);
        if (page != swapcache && swapcache) {
                /*