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Bug#1127597: linux-image-6.12.48+deb13-amd64: GRE6 decap fails

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Hello Salvatore,

I would actually give reverting that one a shot first. Let me see if I
can quickly build a kernel without that specific patch.

With best regards,
Tobias

On Tue, 2026-02-10 at 13:18 +0100, Salvatore Bonaccorso wrote:
> Control: tags -1 + moreinfo
> 
> Hi BAstien, hi Tobias,
> 
> On Tue, Feb 10, 2026 at 09:57:15AM +0100, Bastien Durel wrote:
> > Source: linux
> > Version: 6.12.48+deb13-amd64
> > Severity: important
> > Tags: ipv6
> > X-Debbugs-Cc: debian-amd64@lists.debian.org
> > User: debian-amd64@lists.debian.org
> > Usertags: amd64
> > 
> > Dear Maintainer,
> > 
> > When I boot on the 6.12.48+deb13-amd64 version of the kernel, my
> > GRE6 tunnels stop to work, it seems that decapsulation fail
> > 
> > Rolling back to 6.12.63+deb13-amd64 make them work again, so I
> > _think_ this is not a configuration issue.
> > 
> > I have two GRE6 tunnels on which I run BGP sessions.
> > 
> > Here's a capture of the external interface, filtered on ip6[40] =
> > 47
> > 
> > 21:56:44.411270 IP6 (flowlabel 0x7759c, hlim 64, next-header
> > unknown (60) payload length: 92) 2001:41d0:302:2000::2144 >
> > 2a0c:b641:6::: DSTOPT (opt_type 0x04: len=1)(padn) GREv0, Flags
> > [none], length 84
> > 	IP6 (class 0xc0, flowlabel 0x7759c, hlim 255, next-header
> > TCP (6) payload length: 40) 2a0c:b640:fffe::21:2834:2.47098 >
> > 2a0c:b640:fffe::21:2834:1.179: Flags [S], cksum 0xc046 (correct),
> > seq 2822657693, win 65236, options [mss 1388,sackOK,TS val
> > 903776896 ecr 0,nop,wscale 7], length 0
> > 21:56:44.421735 IP6 (flowlabel 0xc3931, hlim 242, next-header
> > unknown (60) payload length: 92) 2a0c:b641:6:: >
> > 2001:41d0:302:2000::2144: DSTOPT (opt_type 0x04: len=1)(padn)
> > GREv0, Flags [none], length 84
> > 	IP6 (class 0xc0, flowlabel 0xc3931, hlim 255, next-header
> > TCP (6) payload length: 40) 2a0c:b640:fffe::21:2834:1.179 >
> > 2a0c:b640:fffe::21:2834:2.47098: Flags [S.], cksum 0x03b1
> > (correct), seq 1998626730, ack 2822657694, win 64672, options [mss
> > 1388,sackOK,TS val 911965588 ecr 903773822,nop,wscale 7], length 0
> > 
> > We can see the SYN sent by bird, and the SYN/ACK returned by the
> > peer.
> > Meanwile, when capturing on the gre interfact itself, only the SYNs
> > sent by my side shows up
> > 
> > 21:56:43.387145 IP6 (class 0xc0, flowlabel 0xd8556, hlim 255, next-
> > header TCP (6) payload length: 40) 2a0c:b640:fffe::21:2834:2.47098
> > > 2a0c:b640:fffe::21:2834:1.179: Flags [S], cksum 0x1173 (incorrect
> > -> 0xc446), seq 2822657693, win 65236, options [mss 1388,sackOK,TS
> > val 903775872 ecr 0,nop,wscale 7], length 0
> > 21:56:44.411239 IP6 (class 0xc0, flowlabel 0x7759c, hlim 255, next-
> > header TCP (6) payload length: 40) 2a0c:b640:fffe::21:2834:2.47098
> > > 2a0c:b640:fffe::21:2834:1.179: Flags [S], cksum 0x1173 (incorrect
> > -> 0xc046), seq 2822657693, win 65236, options [mss 1388,sackOK,TS
> > val 903776896 ecr 0,nop,wscale 7], length 0
> > 21:56:45.435116 IP6 (class 0xc0, flowlabel 0x9203f, hlim 255, next-
> > header TCP (6) payload length: 40) 2a0c:b640:fffe::21:2834:2.47098
> > > 2a0c:b640:fffe::21:2834:1.179: Flags [S], cksum 0x1173 (incorrect
> > -> 0xbc46), seq 2822657693, win 65236, options [mss 1388,sackOK,TS
> > val 903777920 ecr 0,nop,wscale 7], length 0
> > 
> > ICMP inside the tunnels behave the same (seen from the outer
> > interface, not on the GRE one)
> 
> Thanks for your both reports. So this look we introduced a regression
> from the preivous 6.12.63-1 to 6.12.69-1 with gre6. 
> 
> I will have a look at this shortly.
> 
> If any of you both has a testing environment where you can bisect the
> upstream changes between 6.12.63 and 6.12.69 to clearly isolate the
> breaking change that would be great, let me know if you need
> instructions on how to do it. It might be the ip6_gre change
> db5b4e39c4e6 ("ip6_gre: make ip6gre_header() robust") in 6.12.64.
> 
> Regards,
> Salvatore

-- 
Dr.-Ing. Tobias Fiebig
T +31 616 80 98 99
M tobias@fiebig.nl

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Thread

Bug#1127597: linux-image-6.12.48+deb13-amd64: GRE6 decap fails Bastien Durel <bastien.durel@geekwu.org> - 2026-02-10 10:20 +0100
  Bug#1127597: additional info Bastien Durel <bastien@durel.org> - 2026-02-10 11:10 +0100
  Bug#1127597: linux-image-6.12.48+deb13-amd64: GRE6 decap fails Tobias Fiebig <tobias@fiebig.nl> - 2026-02-10 13:30 +0100
  Processed: Re: Bug#1127597: linux-image-6.12.48+deb13-amd64: GRE6  decap fails "Debian Bug Tracking System" <owner@bugs.debian.org> - 2026-02-10 13:30 +0100
  Bug#1127597: Info received (Bug#1127597: linux-image-6.12.48+deb13-amd64: GRE6 decap fails) Tobias Fiebig <tobias@fiebig.nl> - 2026-02-10 13:50 +0100
  Bug#1127597: linux-image-6.19.0+debian+tj: v6.12.69 - ip6gre regression Tj <tj.iam.tj@proton.me> - 2026-02-10 23:00 +0100
  Bug#1127597: Regression: v6.12.67 ip6_tunnel: ip6gre decapsulation fails Tj <tj.iam.tj@proton.me> - 2026-02-11 06:10 +0100
    Bug#1127597: Regression: v6.12.67 ip6_tunnel: ip6gre decapsulation fails Greg KH <gregkh@linuxfoundation.org> - 2026-02-11 06:40 +0100
  Bug#1127597: Regression: v6.12.67 ip6_tunnel: ip6gre decapsulation fails F!nTcH <fintch@fintch.org> - 2026-03-04 18:50 +0100
    Bug#1127597: Regression: v6.12.67 ip6_tunnel: ip6gre decapsulation fails Salvatore Bonaccorso <carnil@debian.org> - 2026-03-05 10:30 +0100
  Bug#1127597: marked as done (linux-image-6.12.48+deb13-amd64:  GRE6 decap fails) "Debian Bug Tracking System" <owner@bugs.debian.org> - 2026-03-13 19:40 +0100

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