st 1. 6. 2022 v 17:05 odesílatel Alexander Duyck
[off-list ref] napsal:

quoted

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e6f211d..ac60a97 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -5580,6 +5580,7 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
                /* reset page count bias and offset to start of new frag */
                nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
                offset = size - fragsz;
+               BUG_ON(offset < 0);
        }

        nc->pagecnt_bias--;

I think I could be onboard with a patch like this. The test shouldn't
add more than 1 instruction since it is essentially just a jump if
signed test which will be performed after the size - fragsz check.

FYI, I hit this problem a few days ago with the nfp network driver, it uses
page_frag_alloc() with a frag size larger than PAGE_SIZE when MTU is
set to 9000,
this may result in memory corruptions when the system runs out of memory.

The solution I was working on was something like the following, this
makes the allocation
fail if fragsz is greater than the cache size.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4dc0d333279f..c6b40b85c55d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -5544,12 +5544,17 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
                /* if size can vary use size else just use PAGE_SIZE */
                size = nc->size;
 #endif
-               /* OK, page count is 0, we can safely set it */
-               set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
-
                /* reset page count bias and offset to start of new frag */
                nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
                offset = size - fragsz;
+               if (unlikely(offset < 0)) {
+                       free_the_page(page, compound_order(page));
+                       nc->va = NULL;
+                       return NULL;
+               }
+
+               /* OK, page count is 0, we can safely set it */
+               set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
        }

        nc->pagecnt_bias--;

Maurizio