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

[Brian Gerst]

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.

Brian Gerst