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.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
mm/memory.c | 55 +++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 43 insertions(+), 12 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index ab3153252cfe..ba23d13b8410 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3499,6 +3499,25 @@ 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
+ * owner. Try freeing the swapcache to get rid of the swapcache
+ * reference in case it's likely that we will succeed.
+ */
+ 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.
@@ -3640,6 +3659,16 @@ 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);
@@ -3658,19 +3687,25 @@ 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;
@@ -3696,10 +3731,6 @@ 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) {
/*
--
2.34.1
