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Date: Sun, 26 Jan 2025 14:50:21 +0100
From: Florian Westphal <fw@strlen.de>
To: Sunny73Cr <Sunny73Cr@protonmail.com>
Cc: "netfilter@vger.kernel.org" <netfilter@vger.kernel.org>
Subject: Re: Raw Payload Expressions - out of bounds write?
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References: <Bp_PH3aO9-d7-Av1SaQGtAgrvHaPwr0l5r_aR31_U8l5IvbSFaLJ1lXI3vFBV46Xb4IxK-tP-HkcSFS-2KCLyYfRibcIkioxjdem46U0IB8=@protonmail.com>
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Sunny73Cr <Sunny73Cr@protonmail.com> wrote:
>   @ih,58,6 set 0 \
>   @ih,86,6 set 0 \
>   @ih,170,22 set 0 \

> 3. Output ruleset (/usr/sbin/nft list rulset):
> 
> chain output {
>  type filter hook output priority filter; policy accept;
>  @ih,48,16 set @ih,48,16 & 0x3f \
>  @ih,80,16 set @ih,80,16 & 0x3f0 \
>  @ih,160,32 set @ih,160,32 @0x3fffff \
>  accept;
> }
> 
> The mask appears correct for the first re-write, but subsequent re-writes appear to be too large.

What do you mean?
Please elaborate as what is wrong here in listing 3.

The nft interpreter cannot load individual bits,
the length/size has to be byte aligned.

6 bits from offset 86 straddles two bytes (bits 86-7,88-91).

22 from offset 170 touch 4 bytes: bits 168-169, 170-177, 178-185,
186-195.