Re: [PATCH 2/6] x86/entry/64: Convert SYSRET validation tests to C

[Mika Penttilä]

On 18.7.2023 17.25, Brian Gerst wrote:
> On Tue, Jul 18, 2023 at 10:17 AM Mika Penttilä <mpenttil@xxxxxxxxxx> wrote:
> > Hi,
> >
> >
> > On 18.7.2023 16.44, Brian Gerst wrote:
> > > Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
> > > ---
> > >    arch/x86/entry/common.c        | 50 ++++++++++++++++++++++++++++++-
> > >    arch/x86/entry/entry_64.S      | 55 ++--------------------------------
> > >    arch/x86/include/asm/syscall.h |  2 +-
> > >    3 files changed, 52 insertions(+), 55 deletions(-)
> > >
> > > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > > index 6c2826417b33..afe79c3f1c5b 100644
> > > --- a/arch/x86/entry/common.c
> > > +++ b/arch/x86/entry/common.c
> > > @@ -70,8 +70,12 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
> > >        return false;
> > >    }
> > >
> > > -__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
> > > +/* Returns true to return using SYSRET, or false to use IRET */
> > > +__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
> > >    {
> > > +     long rip;
> > > +     unsigned int shift_rip;
> > > +
> > >        add_random_kstack_offset();
> > >        nr = syscall_enter_from_user_mode(regs, nr);
> > >
> > > @@ -84,6 +88,50 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
> > >
> > >        instrumentation_end();
> > >        syscall_exit_to_user_mode(regs);
> > > +
> > > +     /*
> > > +      * Check that the register state is valid for using SYSRET to exit
> > > +      * to userspace.  Otherwise use the slower but fully capable IRET
> > > +      * exit path.
> > > +      */
> > > +
> > > +     /* XEN PV guests always use IRET path */
> > > +     if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > > +             return false;
> > > +
> > > +     /* SYSRET requires RCX == RIP and R11 == EFLAGS */
> > > +     if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
> > > +             return false;
> > > +
> > > +     /* CS and SS must match the values set in MSR_STAR */
> > > +     if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
> > > +             return false;
> > > +
> > > +     /*
> > > +      * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> > > +      * in kernel space.  This essentially lets the user take over
> > > +      * the kernel, since userspace controls RSP.
> > > +      *
> > > +      * Change top bits to match most significant bit (47th or 56th bit
> > > +      * depending on paging mode) in the address.
> > > +      */
> > > +     shift_rip = (64 - __VIRTUAL_MASK_SHIFT + 1);
> >
> > Should this be:
> >
> >          shift_rip = (64 - __VIRTUAL_MASK_SHIFT - 1);
> > ?
>
> I removed a set of parentheses, which switched the sign from -1 to +1.
> I could put it back if that's less confusing.

I mean isn't it supposed to be:
shift_rip = (64 - 48) for 4 level, now it's
shift_rip = (64 - 46)

__VIRTUAL_MASK_SHIFT == 47


> Brian Gerst