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.

Link: https://lkml.kernel.org/r/Y919vmSrtAgsf6K3@casper.infradead.org
Fixes: 00cdf76012ab ("mm: add memcpy_from_file_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 348701d..b06254e 100644 (file)
--- 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);