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

["Mika Penttilä" <mpenttil@redhat.com>]

On 18.7.2023 17.25, Brian Gerst wrote:
> On Tue, Jul 18, 2023 at 10:17 AM Mika Penttilä <mpenttil@redhat.com> wrote:
>>
>> Hi,
>>
>>
>> On 18.7.2023 16.44, Brian Gerst wrote:
>>> Signed-off-by: Brian Gerst <brgerst@gmail.com>
>>> ---
>>>    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
>