On Sun, May 17, 2026 at 6:50 PM Gary <saclug@garymcglinn.com> wrote:

Each VM has to maintain it's own state. You can't keep that in common at the hypervisor level. At least, I think, it would be extremely inefficient to try. Each of my VM instances must have it's own kernel instance, since they don't all have the same kernel version. There may be a lot of things that interact with the hardware, like system calls, in the hypervisor, but the basic kernels have to be independent.`

On Sun, May 17, 2026 at 01:48:29PM -0700, Brian E. Lavender wrote:
> This is not KVM. This is Xen.
>
> It is time to imagine multiple kernels share and collaborate. That is
> the whole point of para virtualization. When you boot a guest in
> paravirtualized mode, it says "Oh, I am a guest, I will let the
> hypervisor kernel do the work." Below is some output from a guest kernel
> booting. Notice how it detects it's just a guest, and it's letting the
> host kernel (Dom 0) do the work. It looks like Xen is managing the mmu.
> so I am willing to bet that if the Dom0 is patched, it will mitigate.
> I need to grab the code to exploit and see if this is true. I think the
> question is if this data structure is somehow defined in a table
> maintained by the guest.
>
> [ 0.000000] Linux version 6.1.0-48-amd64 (debian-kernel@lists.debian.org) (gcc-12 (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Deb
> ian) 2.40) #1 SMP PREEMPT_DYNAMIC Debian 6.1.172-1 (2026-05-15)
> [ 0.000000] Command line: root=UUID=718446e1-8c8b-46d8-88ff-ac3fd707ebda ro pti=off console=hvc0 root=/dev/xvda2 ro
> [ 0.000000] ACPI in unprivileged domain disabled
> [ 0.000000] Released 0 page(s)
> [ 0.000000] BIOS-provided physical RAM map:
> [ 0.000000] Xen: [mem 0x0000000000000000-0x000000000009ffff] usable
> [ 0.000000] Xen: [mem 0x00000000000a0000-0x00000000000fffff] reserved
> [ 0.000000] Xen: [mem 0x0000000000100000-0x000000007fffffff] usable
> [ 0.000000] NX (Execute Disable) protection: active
> [ 0.000000] DMI not present or invalid.
> [ 0.000000] Hypervisor detected: Xen PV
> [snip]
> [ 0.615786] Booting kernel on Xen
> [ 0.615788] Xen version: 4.20.3-pre (preserve-AD)
> [snip]
> [ 0.623442] xen: PV spinlocks enabled
> [ 0.623452] PV qspinlock hash table entries: 256 (order: 0, 4096 bytes, linear)
> [snip]
> [ 0.659881] xen:events: Using FIFO-based ABI
> [ 0.659909] rcu: srcu_init: Setting srcu_struct sizes based on contention.
> [ 0.660029] Console: colour dummy device 80x25
> [ 0.660247] printk: console [tty0] enabled
> [ 0.660885] printk: console [hvc0] enabled
> [ 0.660954] clocksource: xen: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
> [ 0.660982] Xen: using vcpuop timer interface
> [ 0.660992] installing Xen timer for CPU 0
> [snip]
> [ 0.750926] VPMU disabled by hypervisor.
> [snip]
> [ 0.752944] installing Xen timer for CPU 1
> [snip]
> [ 0.842354] xen:grant_table: Grant tables using version 1 layout
> [ 0.842398] Grant table initialized
> [ 0.842597] DMA: preallocated 256 KiB GFP_KERNEL pool for atomic allocations
> [ 0.842686] DMA: preallocated 256 KiB GFP_KERNEL|GFP_DMA pool for atomic allocations
> [ 0.857200] DMA: preallocated 256 KiB GFP_KERNEL|GFP_DMA32 pool for atomic allocations
> [snip]
> [ 0.859176] PCI: setting up Xen PCI frontend stub
> [ 0.859205] PCI: pci_cache_line_size set to 64 bytes
> [ 0.862141] kprobes: kprobe jump-optimization is enabled. All kprobes are optimized if possible.
> [ 0.885503] ACPI: Interpreter disabled.
> [ 0.885544] xen:balloon: Initialising balloon driver
> [ 0.897269] iommu: Default domain type: Translated
> [ 0.897269] iommu: DMA domain TLB invalidation policy: lazy mode
> [snip]
>
> Brian
>
> On Sat, May 16, 2026 at 09:06:20AM -0700, Gary wrote:
> > There is also KVM, which may handle system calls differently. Don't know.
> >
> > But, the flaw is in kernel memory management. There is nothing changed "in" the system calls per se. I can't imagine how multiple instances could share the same kernel memory, or module to manipulate it.
> >
> > Specifically crypto/aglif_aead.c has to be patched to change how it uses local memory. This is used by a kernel module that the system calls use.
> >
> >
> >
> > On Fri, May 15, 2026 at 02:51:56PM -0700, Brian E. Lavender wrote:
> > > I think your analysis to patch everywhere may be mistaken.
> > >
> > > https://wiki.xenproject.org/wiki/Xen_FAQ_Design_and_in_Depth#How_does_Xen_process_System_Calls_on_para-virtualized_guest? "Xen maintains two IDT's, one global IDT (its own) and other per domain
> > > IDT. Xen uses global IDT to register the entire trap handler except for
> > > system call handler (int 0x80). When a VM gets scheduled, its system
> > > call handler (from per domain IDT table) is registered with the
> > > processor. Hence when a domain/VM executes a system call, its own
> > > handler is executed. "
> > >
> > > IDT - Interrupt Descriptor Table (IDT)
> > >
> > > https://xint.io/blog/copy-fail-linux-distributions
> > >
> > > So, the scheduler puts in the IDT for the guest. The guest does a socket
> > > open for copy/fail CVE. It seems that the socket system call will be
> > > implemented by the Xen host hypervisor. Therefore, the patched hypervisor
> > > will mark it dirty and the vulnerabiltiy mitigated.
> > >
> > > Anybody want to try and find a contradiction?
> > >
> > > Brian
> > >
> > > On Sat, May 02, 2026 at 10:09:55AM -0700, Gary wrote:
> > > > It's a flaw in the kernel, so you have to patch the kernel in anything that can run a shell where you don't want privilege escalation.
> > > >
> > > > Which I would think is everywhere.
> > > >
> > > > Have you seen any info on patch availability or what versions of Fedora and other distros will have available patches?
> > > >
> > > > I saw some instructions where you can disable a kernel module using modprobe to avoid the vulnerability. There was no information on what functionality was affected by making the change. It sounded like it reverted to doing the copy correctly, which would be a relatively easy and low impact fix.
> > > >
> > > >
> > > >
> > > > On Fri, May 01, 2026 at 05:54:26PM -0700, Brian E. Lavender wrote:
> > > > > I take it most have heard about the copy fail vulnerability if you have
> > > > > local shell?
> > > > >
> > > > > https://arstechnica.com/security/2026/04/as-the-most-severe-linux-threat-in-years-surfaces-the-world-scrambles/
> > > > >
> > > > > I wonder if you patch the hypervisor when using PV guests if you have to
> > > > > update the guests too?
> > > > >
> > > > >
> > > > > --
> > > > > Brian Lavender
> > > > > https://www.brie.com/brian/
> > > > >
> > > > > "There are two ways of constructing a software design. One way is to
> > > > > make it so simple that there are obviously no deficiencies. And the other
> > > > > way is to make it so complicated that there are no obvious deficiencies."
> > > > >
> > > > > Professor C. A. R. Hoare
> > > > > The 1980 Turing award lecture
> > > > > _______________________________________________
> > > > > Lug-nuts mailing list -- lug-nuts@bigbrie.com
> > > > > To unsubscribe send an email to lug-nuts-leave@bigbrie.com
> > > >
> > > > --
> > > > -Gary
> > > >
> > > > It is a simple thing to make things complex,
> > > > a complex thing to make things simple.
> > > >
> > > >
> > > >
> > > > _______________________________________________
> > > > Lug-nuts mailing list -- lug-nuts@bigbrie.com
> > > > To unsubscribe send an email to lug-nuts-leave@bigbrie.com
> > >
> > > --
> > > Brian Lavender
> > > https://www.brie.com/brian/
> > >
> > > "There are two ways of constructing a software design. One way is to
> > > make it so simple that there are obviously no deficiencies. And the other
> > > way is to make it so complicated that there are no obvious deficiencies."
> > >
> > > Professor C. A. R. Hoare
> > > The 1980 Turing award lecture
> > > _______________________________________________
> > > Lug-nuts mailing list -- lug-nuts@bigbrie.com
> > > To unsubscribe send an email to lug-nuts-leave@bigbrie.com
> >
> > --
> > -Gary
> >
> > It is a simple thing to make things complex,
> > a complex thing to make things simple.
> >
> >
> >
> > _______________________________________________
> > Lug-nuts mailing list -- lug-nuts@bigbrie.com
> > To unsubscribe send an email to lug-nuts-leave@bigbrie.com
>
> --
> Brian Lavender
> https://www.brie.com/brian/
>
> "There are two ways of constructing a software design. One way is to
> make it so simple that there are obviously no deficiencies. And the other
> way is to make it so complicated that there are no obvious deficiencies."
>
> Professor C. A. R. Hoare
> The 1980 Turing award lecture
> _______________________________________________
> Lug-nuts mailing list -- lug-nuts@bigbrie.com
> To unsubscribe send an email to lug-nuts-leave@bigbrie.com

--
-Gary

It is a simple thing to make things complex,
a complex thing to make things simple.

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