On Fri, Oct 09, 2020 at 12:53:01PM +0100, Andrew Cooper wrote: > Despite appearing to be a deliberate design choice of early PV64, the > resulting behaviour for unregistered SYSCALL callbacks creates an untenable > testability problem for Xen. Furthermore, the behaviour is undocumented, > bizarre, and inconsistent with related behaviour in Xen, and very liable > introduce a security vulnerability into a PV guest if the author hasn't > studied Xen's assembly code in detail. > > There are two different bugs here. > > 1) The current logic confuses the registered entrypoints, and may deliver a > SYSCALL from 32bit userspace to the 64bit entry, when only a 64bit > entrypoint is registered. > > This has been the case ever since 2007 (c/s cd75d47348b) but up until > 2018 (c/s dba899de14) the wrong selectors would be handed to the guest for > a 32bit SYSCALL entry, making it appear as if it a 64bit entry all along. > > Xen would malfunction under these circumstances, if it were a PV guest. > Linux would as well, but PVOps has always registered both entrypoints and > discarded the Xen-provided selectors. NetBSD really does malfunction as a > consequence (benignly now, but a VM DoS before the 2018 Xen selector fix). > > 2) In the case that neither SYSCALL callbacks are registered, the guest will > be crashed when userspace executes a SYSCALL instruction, which is a > userspace => kernel DoS. > > 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 corner cases of a PV > guest kernel registering neither callback, or not registering the 32bit > callback when running on AMD/Hygon hardware. > > It brings the behaviour in line with PV32 SYSCALL/SYSENTER, and PV64 > SYSENTER (safe by default, until explicitly enabled), as well as native > hardware (always delivered to the single applicable callback). > > Most importantly however, and the primary reason for the change, is that it > lets us sensibly test the fast system call entrypoints under all states a PV > guest can construct, to prove correct behaviour. > > Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
Acked-by: Roger Pau Monné <roger....@citrix.com> > --- > CC: Jan Beulich <jbeul...@suse.com> > CC: Roger Pau Monné <roger....@citrix.com> > CC: Wei Liu <w...@xen.org> > CC: Andy Lutomirski <l...@kernel.org> > CC: Manuel Bouyer <bou...@antioche.eu.org> > > v2: > * Drop unnecessary instruction suffixes > * Don't truncate #UD entrypoint to 32 bits > > Manuel: This will result in a corner case change for NetBSD. > > At the moment on native, 32bit userspace on 64bit NetBSD will get #UD (Intel, > etc), or an explicit -ENOSYS (AMD, etc) when trying to execute a 32bit SYSCALL > instruction. > > After this change, a 64bit PV VM will consistently see #UD (like on Intel, etc > hardware) even when running on AMD/Hygon hardware (as Xsyscall32 isn't > registered with Xen), rather than following Xsyscall into the proper system > call path. > --- > xen/arch/x86/x86_64/entry.S | 26 +++++++++++++++++++------- > 1 file changed, 19 insertions(+), 7 deletions(-) > > diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S > index 000eb9722b..aaf8402f93 100644 > --- a/xen/arch/x86/x86_64/entry.S > +++ b/xen/arch/x86/x86_64/entry.S > @@ -26,18 +26,30 @@ > /* %rbx: struct vcpu */ > ENTRY(switch_to_kernel) > leaq VCPU_trap_bounce(%rbx),%rdx > - /* TB_eip = (32-bit syscall && syscall32_addr) ? > - * syscall32_addr : syscall_addr */ > - xor %eax,%eax > + > + /* TB_eip = 32-bit syscall ? syscall32_addr : syscall_addr */ > + mov VCPU_syscall32_addr(%rbx), %ecx This being an unsigned long field, shouldn't you use %rcx here? Roger.
