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Message-ID: <f25d597b-a0e5-95db-5538-79893b8d37b2@redhat.com>
Date: Tue, 18 Jul 2023 17:49:36 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Brian Gerst <brgerst@...il.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
"H . Peter Anvin" <hpa@...or.com>,
Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 2/6] x86/entry/64: Convert SYSRET validation tests to C
On 18.7.2023 17.25, Brian Gerst wrote:
> On Tue, Jul 18, 2023 at 10:17 AM Mika Penttilä <mpenttil@...hat.com> wrote:
>>
>> Hi,
>>
>>
>> On 18.7.2023 16.44, Brian Gerst wrote:
>>> Signed-off-by: Brian Gerst <brgerst@...il.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
>
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