This new config option will sanitize all freed pages. This is a pretty low-level change useful to track some cases of use-after-free, help kernel same-page merging in VM environments, and counter a few info leaks. Signed-off-by: Anisse Astier <anisse@xxxxxxxxx> --- mm/Kconfig | 12 ++++++++++++ mm/page_alloc.c | 12 ++++++++++++ 2 files changed, 24 insertions(+) diff --git a/mm/Kconfig b/mm/Kconfig index 390214d..e9fb3bd 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -635,3 +635,15 @@ config MAX_STACK_SIZE_MB changed to a smaller value in which case that is used. A sane initial value is 80 MB. + +config SANITIZE_FREED_PAGES + bool "Sanitize memory pages after free" + default n + help + This option is used to make sure all pages freed are zeroed. This is + quite low-level and doesn't handle your slab buffers. + It has various applications, from preventing some info leaks to + helping kernel same-page merging in virtualised environments. + Depending on your workload it will greatly reduce performance. + + If unsure, say N. diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 4d5ce6e..c29e3a0 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -795,6 +795,12 @@ static bool free_pages_prepare(struct page *page, unsigned int order) debug_check_no_obj_freed(page_address(page), PAGE_SIZE << order); } + +#ifdef CONFIG_SANITIZE_FREED_PAGES + for (i = 0; i < (1 << order); i++) + clear_highpage(page + i); +#endif + arch_free_page(page, order); kernel_map_pages(page, 1 << order, 0); @@ -960,9 +966,15 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags, kernel_map_pages(page, 1 << order, 1); kasan_alloc_pages(page, order); +#ifndef CONFIG_SANITIZE_FREED_PAGES + /* SANITIZE_FREED_PAGES relies implicitly on the fact that pages are + * cleared before use, so we don't need gfp zero in the default case + * because all pages go through the free_pages_prepare code path when + * switching from bootmem to the default allocator */ if (gfp_flags & __GFP_ZERO) for (i = 0; i < (1 << order); i++) clear_highpage(page + i); +#endif if (order && (gfp_flags & __GFP_COMP)) prep_compound_page(page, order); -- 1.9.3 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>