Re: [Bug 218339] New: kernel goes unresponsive if single-stepping over an instruction which writes to an address for which a hardware read/write watchpoint has been set

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 04, 2024, bugzilla-daemon@kernel.org wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=218339
> 
>             Bug ID: 218339
>            Summary: kernel goes unresponsive if single-stepping over an
>                     instruction which writes to an address for which a
>                     hardware read/write watchpoint has been set
>            Product: Virtualization
>            Version: unspecified
>           Hardware: All
>                 OS: Linux
>             Status: NEW
>           Severity: normal
>           Priority: P3
>          Component: kvm
>           Assignee: virtualization_kvm@kernel-bugs.osdl.org
>           Reporter: anthony.louis.eden@gmail.com
>         Regression: No
> 
> In a debian QEMU/KVM virtual machine, run `gdb` on any executable (e.g.
> `/usr/bin/ls`). Run the program by typing `starti`. Proceed to `_dl_start`
> (i.e. `break _dl_start`, `continue`). When you get there disassemble the
> function (i.e. `disas`). Find an instruction that's going to be executed for
> which you can compute the address in memory it will write to. Run the program
> to that instruction (i.e. `break *0xINSN`, `continue`). When you're on that
> instruction, set a read/write watchpoint on the address it will write to, then
> single-step (i.e. `stepi`) and the kernel will go unresponsive.

By "the kernel", I assume you mean the guest kernel?

> >(gdb) x/1i $pc
> >=> 0x7ffff7fe6510 <_dl_start+48>:      mov    %rdi,-0x88(%rbp)
> >(gdb) x/1wx $rbp-0x88
> >0x7fffffffec28:        0x00000000
> >(gdb) awatch *0x7fffffffec28
> >Hardware access (read/write) watchpoint 2: *0x7fffffffec28
> >(gdb) stepi
> 
> 
> Looking with `journalctl`, I cannot find anything printed to dmesg.
> 
> The kernel of the guest inside the virtual machine is Debian 6.1.0-15-amd64.
> The kernel of the host running qemu-system-x86_64 is Archlinux 6.6.7-arch1-1.
> gdb is version 13.1.

Is this a regression or something that has always been broken?  I.e. did this work
on previous host kernels?