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SubjectRe: [PATCH net] udp: fix segmentation crash for GRO packet without fraglist
From
Date
On 4/28/24 9:48 AM, Lena Wang (王娜) wrote:
> On Sat, 2024-04-27 at 09:28 -0400, Willem de Bruijn wrote:
>>
>> External email : Please do not click links or open attachments until
>> you have verified the sender or the content.
>>
>> Daniel Borkmann wrote:
>>> On 4/26/24 11:52 AM, Lena Wang (王娜) wrote:
>>> [...]
>>>>>> From 301da5c9d65652bac6091d4cd64b751b3338f8bb Mon Sep 17
>> 00:00:00
>>>>> 2001
>>>>>> From: Shiming Cheng <shiming.cheng@mediatek.com>
>>>>>> Date: Wed, 24 Apr 2024 13:42:35 +0800
>>>>>> Subject: [PATCH net] net: prevent BPF pulling SKB_GSO_FRAGLIST
>> skb
>>>>>>
>>>>>> A SKB_GSO_FRAGLIST skb can't be pulled data
>>>>>> from its fraglist as it may result an invalid
>>>>>> segmentation or kernel exception.
>>>>>>
>>>>>> For such structured skb we limit the BPF pulling
>>>>>> data length smaller than skb_headlen() and return
>>>>>> error if exceeding.
>>>>>>
>>>>>> Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
>>>>>> Signed-off-by: Shiming Cheng <shiming.cheng@mediatek.com>
>>>>>> Signed-off-by: Lena Wang <lena.wang@mediatek.com>
>>>>>> ---
>>>>>> net/core/filter.c | 5 +++++
>>>>>> 1 file changed, 5 insertions(+)
>>>>>>
>>>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>>>> index 8adf95765cdd..8ed4d5d87167 100644
>>>>>> --- a/net/core/filter.c
>>>>>> +++ b/net/core/filter.c
>>>>>> @@ -1662,6 +1662,11 @@ static DEFINE_PER_CPU(struct
>> bpf_scratchpad,
>>>>>> bpf_sp);
>>>>>> static inline int __bpf_try_make_writable(struct sk_buff
>> *skb,
>>>>>> unsigned int write_len)
>>>>>> {
>>>>>> +if (skb_is_gso(skb) &&
>>>>>> + (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) &&
>>>>>> + write_len > skb_headlen(skb)) {
>>>>>> +return -ENOMEM;
>>>>>> +}
>>>>>> return skb_ensure_writable(skb, write_len);
>>>
>>> Dumb question, but should this guard be more generically part of
>> skb_ensure_writable()
>>> internals, presumably that would be inside pskb_may_pull_reason(),
>> or only if we ever
>>> see more code instances similar to this?
>>
>> Good point. Most callers of skb_ensure_writable correctly pull only
>> headers, so wouldn't cause this problem. But it also adds coverage to
>> things like tc pedit.
>
> Updated:
>
> From 3be30b8cf6e629f2615ef4eafe3b2a1c0d68c530 Mon Sep 17 00:00:00 2001
> From: Shiming Cheng <shiming.cheng@mediatek.com>
> Date: Sun, 28 Apr 2024 15:03:12 +0800
> Subject: [PATCH net] net: prevent pulling SKB_GSO_FRAGLIST skb
>
> BPF or TC callers may pull in a length longer than skb_headlen()
> for a SKB_GSO_FRAGLIST skb. The data in fraglist will be pulled
> into the linear space. However it destroys the skb's structure
> and may result in an invalid segmentation or kernel exception.
>
> So we should add protection to stop the operation and return
> error to remind callers.
>
> Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
> Signed-off-by: Shiming Cheng <shiming.cheng@mediatek.com>
> Signed-off-by: Lena Wang <lena.wang@mediatek.com>
> ---
> include/linux/skbuff.h | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 9d24aec064e8..3eef65b3db24 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2740,6 +2740,12 @@ pskb_may_pull_reason(struct sk_buff *skb,
> unsigned int len)
> if (unlikely(len > skb->len))
> return SKB_DROP_REASON_PKT_TOO_SMALL;
>
> + if (skb_is_gso(skb) &&
> + (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) &&
> + write_len > skb_headlen(skb)) {
> + return SKB_DROP_REASON_NOMEM;
> + }

The 'write_len > skb_headlen(skb)' test is redundant, no ?

It is covered by the earlier test :

if (likely(len <= skb_headlen(skb)))
return SKB_NOT_DROPPED_YET;

Also, was this patch even compile tested since there is no write_len var ?

> if (unlikely(!__pskb_pull_tail(skb, len - skb_headlen(skb))))
> return SKB_DROP_REASON_NOMEM;
>
>


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