[PATCH v2 2/2] mm/memory: refuse to map page cache folios that are not uptodate

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

A page cache folio that is not uptodate holds data that was never read
in or cleared. shmem fallocate(2), for one, adds such folios and clears
them only on first use. Mapping one hands userspace whatever the memory
held before.

Each path that maps page cache checks for this itself: fault-around in
next_uptodate_folio(), a fault in filemap_fault() or shmem_get_folio().
Nothing in the mapping code does, so a caller that misses the check
leaks stale memory silently. MADV_COLLAPSE did exactly that with
fallocated shmem THPs until recently.

Warn and refuse the mapping in __do_fault() and do_set_pmd().

__do_fault() checks the folio ->fault() returned while it is still
locked. do_shared_fault() unlocks it for ->page_mkwrite(), and a
writeback error can clear PG_uptodate after that; that case retries the
fault instead. For CoW, __do_fault() sees the page cache folio that
do_cow_fault() copies from. do_set_pmd() covers MADV_COLLAPSE, which
maps without a fault.

The fault gets SIGBUS, as for a folio that fails to read in.
MADV_COLLAPSE fails and leaves the range to be faulted in.

Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/memory.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index cc1d0b7d15c7..2324c04d632f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5684,6 +5684,13 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
 	else
 		VM_BUG_ON_PAGE(!folio_test_locked(folio), vmf->page);
 
+	if (WARN_ON_ONCE(file_folio_not_uptodate(folio))) {
+		folio_unlock(folio);
+		folio_put(folio);
+		vmf->page = NULL;
+		return VM_FAULT_SIGBUS;
+	}
+
 	return ret;
 }
 
@@ -5727,6 +5734,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
 		return ret;
 	page = &folio->page;
 
+	if (WARN_ON_ONCE(file_folio_not_uptodate(folio)))
+		return VM_FAULT_SIGBUS;
+
 	/*
 	 * Just backoff if any subpage of a THP is corrupted otherwise
 	 * the corrupted page may mapped by PMD silently to escape the
-- 
2.54.0