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

[Brian Gerst]

On Tue, Jul 18, 2023 at 10:49 AM Mika Penttilä <mpenttil@xxxxxxxxxx> wrote:
>
>
>
> 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

Original:
(64 - (47 + 1)) = (64 - 48) = 16

  c5:   48 c1 e1 10             shl    $0x10,%rcx
  c9:   48 c1 f9 10             sar    $0x10,%rcx

New:
(64 - 47 - 1) = (17 - 1) = 16

 18b:   b9 10 00 00 00          mov    $0x10,%ecx
 193:   48 d3 e2                shl    %cl,%rdx
 196:   48 d3 fa                sar    %cl,%rdx

Anyways, I'll switch it back to the original formula.  I'm not going
to argue any more about basic math.

Brian Gerst