Re: [PATCH v3] x86/pv: inject #UD for entirely missing SYSCALL callbacks

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.10.2020 10:40, Jan Beulich wrote:

And of course this should have

From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

right here, sorry.

Jan

> In the case that no 64-bit SYSCALL callback is registered, the guest
> will be crashed when 64-bit userspace executes a SYSCALL instruction,
> which would be a userspace => kernel DoS.  Similarly for 32-bit
> userspace when no 32-bit SYSCALL callback was registered either.
> 
> This has been the case ever since the introduction of 64bit PV support,
> but behaves unlike all other SYSCALL/SYSENTER callbacks in Xen, which
> yield #GP/#UD in userspace before the callback is registered, and are
> therefore safe by default.
> 
> This change does constitute a change in the PV ABI, for the corner case
> of a PV guest kernel not registering a 64-bit callback (which has to be
> considered a defacto requirement of the unwritten PV ABI, considering
> there is no PV equivalent of EFER.SCE).
> 
> It brings the behaviour in line with PV32 SYSCALL/SYSENTER, and PV64
> SYSENTER (safe by default, until explicitly enabled).
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <JBeulich@xxxxxxxx>