Hi, thanks for reply!

From: Pavel Skripkin <redacted>
Date: Sat, 27 Feb 2021 20:51:14 +0300

Hi,

quoted

syzbot found WARNING in __alloc_pages_nodemask()[1] when order >=
MAX_ORDER.
It was caused by __netdev_alloc_skb(), which doesn't check len
value after adding NET_SKB_PAD.
Order will be >= MAX_ORDER and passed to __alloc_pages_nodemask()
if size > KMALLOC_MAX_SIZE.
Same happens in __napi_alloc_skb.

static void *kmalloc_large_node(size_t size, gfp_t flags, int node)
{
	struct page *page;
	void *ptr = NULL;
	unsigned int order = get_order(size);
...
	page = alloc_pages_node(node, flags, order);
...

[1] WARNING in __alloc_pages_nodemask+0x5f8/0x730
mm/page_alloc.c:5014
Call Trace:
 __alloc_pages include/linux/gfp.h:511 [inline]
 __alloc_pages_node include/linux/gfp.h:524 [inline]
 alloc_pages_node include/linux/gfp.h:538 [inline]
 kmalloc_large_node+0x60/0x110 mm/slub.c:3999
 __kmalloc_node_track_caller+0x319/0x3f0 mm/slub.c:4496
 __kmalloc_reserve net/core/skbuff.c:150 [inline]
 __alloc_skb+0x4e4/0x5a0 net/core/skbuff.c:210
 __netdev_alloc_skb+0x70/0x400 net/core/skbuff.c:446
 netdev_alloc_skb include/linux/skbuff.h:2832 [inline]
 qrtr_endpoint_post+0x84/0x11b0 net/qrtr/qrtr.c:442
 qrtr_tun_write_iter+0x11f/0x1a0 net/qrtr/tun.c:98
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x426/0x650 fs/read_write.c:518
 vfs_write+0x791/0xa30 fs/read_write.c:605
 ksys_write+0x12d/0x250 fs/read_write.c:658
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Ah, by the way. Have you tried to seek for the root cause, why
a request for such insanely large (at least 4 Mib) skb happens
in QRTR? I don't believe it's intended to be like this.
Now I feel that silencing this error with early return isn't
really correct approach for this.

Yeah, i figured it out. Syzbot provides reproducer for thig bug:

void loop(void)
{
  intptr_t res = 0;
  memcpy((void*)0x20000000, "/dev/qrtr-tun\000", 14);
  res = syscall(__NR_openat, 0xffffffffffffff9cul, 0x20000000ul, 1ul,
0);
  if (res != -1)
    r[0] = res;
  memcpy((void*)0x20000040, "\x02", 1);
  syscall(__NR_write, r[0], 0x20000040ul, 0x400000ul);
}

So, simply it writes to /dev/qrtr-tun 0x400000 bytes.
In qrtr_tun_write_iter there is a check, that the length is smaller
than KMALLOC_MAX_VSIZE. Then the length is passed to
__netdev_alloc_skb, where it becomes more than KMALLOC_MAX_VSIZE.

quoted

Reported-by: syzbot+80dccaee7c6630fa9dcf@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <redacted>

---
Changes from v3:
* Removed Change-Id and extra tabs in net/core/skbuff.c

Changes from v2:
* Added length check to __napi_alloc_skb
* Added unlikely() in checks

Change from v1:
* Added length check to __netdev_alloc_skb
---
 net/core/skbuff.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 785daff48030..ec7ba8728b61 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -443,6 +443,9 @@ struct sk_buff *__netdev_alloc_skb(struct

net_device *dev, unsigned int len,
 	if (len <= SKB_WITH_OVERHEAD(1024) ||
 	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
 	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
+		if (unlikely(len > KMALLOC_MAX_SIZE))
+			return NULL;
+
 		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX,
NUMA_NO_NODE);
 		if (!skb)
 			goto skb_fail;

@@ -517,6 +520,9 @@ struct sk_buff *__napi_alloc_skb(struct

napi_struct *napi, unsigned int len,
 	if (len <= SKB_WITH_OVERHEAD(1024) ||
 	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
 	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
+		if (unlikely(len > KMALLOC_MAX_SIZE))
+			return NULL;
+
 		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX,
NUMA_NO_NODE);
 		if (!skb)
 			goto skb_fail;
--
2.25.1

Thanks,
Al

With regards,
Pavel Skripkin