[PATCH v2 1/2] mm/memory: recheck PG_uptodate after ->page_mkwrite()

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

do_shared_fault() unlocks the folio around ->page_mkwrite(). Writeback
can lock it in that window, fail, and clear PG_uptodate with the folio
left in the page cache. ecryptfs_writepages() does this when encryption
fails. filemap_page_mkwrite() relocks the folio and checks only
->mapping, so the fault maps a folio that is not uptodate:

    CPU0                            CPU1
    do_shared_fault()
      folio_unlock()
                                    ecryptfs_writepages()
                                      folio_clear_uptodate()
      filemap_page_mkwrite()
        folio_lock()
      finish_fault()

The next read(2) or fault on the folio reads it in again from the file,
over anything stored through the mapping in the meantime. Those stores
are lost.

Recheck PG_uptodate once ->page_mkwrite() returns with the folio locked,
and retry the fault if it is clear, the same way filemap_fault() retries
on a cached folio that lost PG_uptodate. Skip folios outside the page
cache: driver pages from ->fault() never get PG_uptodate and would
retry forever.

The check stays out of do_page_mkwrite(): wp_page_shared() calls it on
a folio that is still mapped, and a retry there would fault on the same
PTE forever.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/sashiko-outbox-202966@kernel.org/
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/memory.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index fe72e66c9c2e..cc1d0b7d15c7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5613,6 +5613,18 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	return VM_FAULT_OOM;
 }
 
+/*
+ * A page cache folio that is not uptodate holds data that was never read in
+ * or cleared, and mapping it leaks stale memory to userspace.
+ */
+static bool file_folio_not_uptodate(struct folio *folio)
+{
+	if (likely(folio_test_uptodate(folio)))
+		return false;
+	/* Driver pages from ->fault() never get PG_uptodate */
+	return !folio_test_anon(folio) && folio->mapping;
+}
+
 /*
  * Either the VMA lock or the mmap_lock must have been held on entry
  * (see FAULT_FLAG_VMA_LOCK) and may have been released depending on
@@ -6163,6 +6175,16 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
 			folio_put(folio);
 			return tmp;
 		}
+
+		/*
+		 * The folio was unlocked for ->page_mkwrite(). If it lost
+		 * PG_uptodate meanwhile, retry the fault to read it in again.
+		 */
+		if (unlikely(file_folio_not_uptodate(folio))) {
+			folio_unlock(folio);
+			folio_put(folio);
+			return VM_FAULT_NOPAGE;
+		}
 	}
 
 	ret |= finish_fault(vmf);
-- 
2.54.0