Re: [merged mm-stable] mm-add-memcpy_from_file_folio.patch removed from -mm tree

[Matthew Wilcox <willy@infradead.org>]

On Thu, Feb 02, 2023 at 10:38:49PM -0800, Andrew Morton wrote:
> This patch was dropped because it was merged into the mm-stable branch
> of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

I pick the worst time to find bugs in my own code.

From cf8f62107c965eafc2fa9ad0f839269fcb11991d Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Fri, 3 Feb 2023 16:28:40 -0500
Subject: [PATCH] mm: Fix memcpy_from_file_folio() integer underflow

If we have a HIGHMEM system with a large folio, 'offset' may be larger
than PAGE_SIZE, and so min_t will cap at 'len' instead of the intended
end-of-page.  That can overflow into the next page which is likely to
be unmapped and fault, but could theoretically copy the wrong data.

Fixes: [no commit ID yet] ("mm: add memcpy_from_file_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/highmem.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 348701dae77f..b06254e76d99 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -431,9 +431,10 @@ static inline size_t memcpy_from_file_folio(char *to, struct folio *folio,
 	size_t offset = offset_in_folio(folio, pos);
 	char *from = kmap_local_folio(folio, offset);
 
-	if (folio_test_highmem(folio))
+	if (folio_test_highmem(folio)) {
+		offset = offset_in_page(offset);
 		len = min_t(size_t, len, PAGE_SIZE - offset);
-	else
+	} else
 		len = min(len, folio_size(folio) - offset);
 
 	memcpy(to, from, len);
-- 
2.35.1