[PATCH 3/4] mm/usercopy: Detect large folio overruns

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Move the compound page overrun detection out of
CONFIG_HARDENED_USERCOPY_PAGESPAN and convert it to use folios so it's
enabled for more people.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Kees Cook <keescook@chromium.org>
---
 mm/usercopy.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/usercopy.c b/mm/usercopy.c
index dcf71b7e3098..e1cb98087a05 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -164,7 +164,6 @@ static inline void check_page_span(const void *ptr, unsigned long n,
 {
 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
 	const void *end = ptr + n - 1;
-	struct page *endpage;
 	bool is_reserved, is_cma;
 
 	/*
@@ -195,11 +194,6 @@ static inline void check_page_span(const void *ptr, unsigned long n,
 		   ((unsigned long)end & (unsigned long)PAGE_MASK)))
 		return;
 
-	/* Allow if fully inside the same compound (__GFP_COMP) page. */
-	endpage = virt_to_head_page(end);
-	if (likely(endpage == page))
-		return;
-
 	/*
 	 * Reject if range is entirely either Reserved (i.e. special or
 	 * device memory), or CMA. Otherwise, reject since the object spans
@@ -259,6 +253,10 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 	if (folio_test_slab(folio)) {
 		/* Check slab allocator for flags and size. */
 		__check_heap_object(ptr, n, folio_slab(folio), to_user);
+	} else if (folio_test_large(folio)) {
+		unsigned long offset = ptr - folio_address(folio);
+		if (offset + n > folio_size(folio))
+			usercopy_abort("page alloc", NULL, to_user, offset, n);
 	} else {
 		/* Verify object does not incorrectly span multiple pages. */
 		check_page_span(ptr, n, folio_page(folio, 0), to_user);
-- 
2.33.0