[PATCH net] net: tun: limit first seg size to avoid oversized linearization

[Ziyang Xuan]

Recently, we found a syzkaller problem as following:

========================================================
WARNING: CPU: 1 PID: 17965 at mm/page_alloc.c:5295 __alloc_pages+0x1308/0x16c4 mm/page_alloc.c:5295
...
Call trace:
 __alloc_pages+0x1308/0x16c4 mm/page_alloc.c:5295
 __alloc_pages_node include/linux/gfp.h:550 [inline]
 alloc_pages_node include/linux/gfp.h:564 [inline]
 kmalloc_large_node+0x94/0x350 mm/slub.c:4038
 __kmalloc_node_track_caller+0x620/0x8e4 mm/slub.c:4545
 __kmalloc_reserve.constprop.0+0x1e4/0x2b0 net/core/skbuff.c:151
 pskb_expand_head+0x130/0x8b0 net/core/skbuff.c:1654
 __skb_grow include/linux/skbuff.h:2779 [inline]
 tun_napi_alloc_frags+0x144/0x610 drivers/net/tun.c:1477
 tun_get_user+0x31c/0x2010 drivers/net/tun.c:1835
 tun_chr_write_iter+0x98/0x100 drivers/net/tun.c:2036

It is because the first seg size of the iov_iter from user space is
very big, it is 2147479538 which is bigger than the threshold value
for bail out early in __alloc_pages(). And skb->pfmemalloc is true,
__kmalloc_reserve() would use pfmemalloc reserves without __GFP_NOWARN
flag. Thus we got a warning.

I noticed that non-first segs size are required less than PAGE_SIZE in
tun_napi_alloc_frags(). The first seg should not be a special case, and
oversized linearization is also unreasonable. Limit the first seg size to
PAGE_SIZE to avoid oversized linearization.

Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
---
 drivers/net/tun.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 259b2b84b2b3..7db515f94667 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1454,12 +1454,12 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
 					    size_t len,
 					    const struct iov_iter *it)
 {
+	size_t linear = iov_iter_single_seg_count(it);
 	struct sk_buff *skb;
-	size_t linear;
 	int err;
 	int i;
 
-	if (it->nr_segs > MAX_SKB_FRAGS + 1)
+	if (it->nr_segs > MAX_SKB_FRAGS + 1 || linear > PAGE_SIZE)
 		return ERR_PTR(-EMSGSIZE);
 
 	local_bh_disable();
@@ -1468,7 +1468,6 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
 	if (!skb)
 		return ERR_PTR(-ENOMEM);
 
-	linear = iov_iter_single_seg_count(it);
 	err = __skb_grow(skb, linear);
 	if (err)
 		goto free;
-- 
2.25.1