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


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