Re: [PATCH v3] net/core/skbuff: fix passing wrong size to __alloc_skb

[Pavel Skripkin <paskripkin@gmail.com>]

Hi, thanks for reply!

> From: Pavel Skripkin <paskripkin@gmail.com>
> Date: Sat, 27 Feb 2021 20:51:14 +0300
> 
> Hi,
> 
> > 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.

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