[RESEND PATCH net] mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs

[RESEND PATCH net] mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs

[Jann Horn]

The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum
number of references that we might need to create in the fastpath later,
the bump-allocation fastpath only has to modify the non-atomic bias value
that tracks the number of extra references we hold instead of the atomic
refcount. The maximum number of allocations we can serve (under the
assumption that no allocation is made with size 0) is nc->size, so that's
the bias used.

However, even when all memory in the allocation has been given away, a
reference to the page is still held; and in the `offset < 0` slowpath, the
page may be reused if everyone else has dropped their references.
This means that the necessary number of references is actually
`nc->size+1`.

Luckily, from a quick grep, it looks like the only path that can call
page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which
requires CAP_NET_ADMIN in the init namespace and is only intended to be
used for kernel testing and fuzzing.

To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the
`offset < 0` path, below the virt_to_page() call, and then repeatedly call
writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI,
with a vector consisting of 15 elements containing 1 byte each.

Signed-off-by: Jann Horn <jannh@google.com>
---
Resending to davem at the request of akpm.

 mm/page_alloc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 35fdde041f5c..46285d28e43b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 		/* Even if we own the page, we do not use atomic_set().
 		 * This would break get_page_unless_zero() users.
 		 */
-		page_ref_add(page, size - 1);
+		page_ref_add(page, size);
 
 		/* reset page count bias and offset to start of new frag */
 		nc->pfmemalloc = page_is_pfmemalloc(page);
-		nc->pagecnt_bias = size;
+		nc->pagecnt_bias = size + 1;
 		nc->offset = size;
 	}
 
@@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 		size = nc->size;
 #endif
 		/* OK, page count is 0, we can safely set it */
-		set_page_count(page, size);
+		set_page_count(page, size + 1);
 
 		/* reset page count bias and offset to start of new frag */
-		nc->pagecnt_bias = size;
+		nc->pagecnt_bias = size + 1;
 		offset = size - fragsz;
 	}
 
-- 
2.20.1.791.gb4d0f1c61a-goog

Re: [RESEND PATCH net] mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs

[David Miller]

From: Jann Horn <jannh@google.com>
Date: Wed, 13 Feb 2019 22:45:59 +0100

> The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum
> number of references that we might need to create in the fastpath later,
> the bump-allocation fastpath only has to modify the non-atomic bias value
> that tracks the number of extra references we hold instead of the atomic
> refcount. The maximum number of allocations we can serve (under the
> assumption that no allocation is made with size 0) is nc->size, so that's
> the bias used.
> 
> However, even when all memory in the allocation has been given away, a
> reference to the page is still held; and in the `offset < 0` slowpath, the
> page may be reused if everyone else has dropped their references.
> This means that the necessary number of references is actually
> `nc->size+1`.
> 
> Luckily, from a quick grep, it looks like the only path that can call
> page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which
> requires CAP_NET_ADMIN in the init namespace and is only intended to be
> used for kernel testing and fuzzing.
> 
> To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the
> `offset < 0` path, below the virt_to_page() call, and then repeatedly call
> writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI,
> with a vector consisting of 15 elements containing 1 byte each.
> 
> Signed-off-by: Jann Horn <jannh@google.com>

Applied and queued up for -stable.