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Date:   Sat, 25 Nov 2017 07:00:36 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 15/43] x86/entry/64: Create a percpu SYSCALL entry trampoline

On Sat, Nov 25, 2017 at 3:40 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Fri, Nov 24, 2017 at 06:23:43PM +0100, Ingo Molnar wrote:
>> From: Andy Lutomirski <luto@...nel.org>
>>
>> Handling SYSCALL is tricky: the SYSCALL handler is entered with every
>> single register (except FLAGS), including RSP, live.  It somehow needs
>> to set RSP to point to a valid stack, which means it needs to save the
>> user RSP somewhere and find its own stack pointer.  The canonical way
>> to do this is with SWAPGS, which lets us access percpu data using the
>> %gs prefix.
>>
>> With KAISER-like pagetable switching, this is problematic.  Without a
>> scratch register, switching CR3 is impossible, so %gs-based percpu
>> memory would need to be mapped in the user pagetables.  Doing that
>> without information leaks is difficult or impossible.
>>
>> Instead, use a different sneaky trick.  Map a copy of the first part
>> of the SYSCALL asm at a different address for each CPU.  Now RIP
>> varies depending on the CPU, so we can use RIP-relative memory access
>> to access percpu memory.  By putting the relevant information (one
>> scratch slot and the stack address) at a constant offset relative to
>> RIP, we can make SYSCALL work without relying on %gs.
>>
>> A nice thing about this approach is that we can easily switch it on
>> and off if we want pagetable switching to be configurable.
>>
>> The compat variant of SYSCALL doesn't have this problem in the first
>> place -- there are plenty of scratch registers, since we don't care
>> about preserving r8-r15.  This patch therefore doesn't touch SYSCALL32
>> at all.
>>
>> XXX: Whenever we settle how KAISER gets turned on and off, we should do
>> the same to this.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Borislav Petkov <bpetkov@...e.de>
>> Cc: Brian Gerst <brgerst@...il.com>
>> Cc: Dave Hansen <dave.hansen@...el.com>
>> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Link: https://lkml.kernel.org/r/b95ccae0a5a2f090c901e49fce7c9e8ff6acd40d.1511497875.git.luto@kernel.org
>> Signed-off-by: Ingo Molnar <mingo@...nel.org>
>> ---
>>  arch/x86/entry/entry_64.S     | 48 +++++++++++++++++++++++++++++++++++++++++++
>>  arch/x86/include/asm/fixmap.h |  2 ++
>>  arch/x86/kernel/asm-offsets.c |  1 +
>>  arch/x86/kernel/cpu/common.c  | 12 ++++++++++-
>>  arch/x86/kernel/vmlinux.lds.S | 10 +++++++++
>>  5 files changed, 72 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 426b8c669d6a..0cde243b7542 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -140,6 +140,54 @@ END(native_usergs_sysret64)
>>   * with them due to bugs in both AMD and Intel CPUs.
>>   */
>>
>> +     .pushsection .entry_trampoline, "ax"
>> +
>> +/*
>> + * The code in here gets remapped into cpu_entry_area's trampoline.  This means
>> + * that the assembler and linker have the wrong idea as to where this code
>> + * lives (and, in fact, it's mapped more than once, so it's not even at a
>> + * fixed address).  So we can't reference any symbols outside the entry
>> + * trampoline and expect it to work.
>> + *
>> + * Instead, we carefully abuse %rip-relative addressing.
>> + * .Lentry_trampoline(%rip) refers to the start of the remapped) entry
>
> _entry_trampoline(%rip) I'd guess.

Indeed.  It used to be .L, but then I put it in the linker script the
obvious way and it wasn't any more.

>
>> + * trampoline.  We can thus find cpu_entry_area with this macro:
>
> Uuh, fun. :)

That's what I thought!

>
>> + */
>> +
>> +#define CPU_ENTRY_AREA \
>> +     _entry_trampoline - CPU_ENTRY_AREA_entry_trampoline(%rip)
>
> So this generates
>
>         _entry_trampoline - 16384(%rip)
>
> here. IINM, the layout looks like this
>
> [ GDT | TSS | TSS | TSS | trampoline ]
>
> where each section is a page, and we have 4 pages per CPU. Just for my
> own understanding...

Indeed.

>
>> +
>> +/* The top word of the SYSENTER stack is hot and is usable as scratch space. */
>> +#define RSP_SCRATCH CPU_ENTRY_AREA_tss + CPU_TSS_SYSENTER_stack + \
>> +     SIZEOF_SYSENTER_stack - 8 + CPU_ENTRY_AREA
>
> I'm wondering if it would be easier to make SYSENTER_stack part of
> struct cpu_entry_area and thus simplify that wild offset math here :)

It's like that with the last patch that I haven't send out applied, actually :)

>
> Also, pls align:
>
> #define RSP_SCRATCH CPU_ENTRY_AREA_tss + CPU_TSS_SYSENTER_stack + \
>                     SIZEOF_SYSENTER_stack - 8 + CPU_ENTRY_AREA

Done.

>
>> +
>> +ENTRY(entry_SYSCALL_64_trampoline)
>> +     UNWIND_HINT_EMPTY
>> +     swapgs
>> +
>> +     /* Stash the user RSP. */
>> +     movq    %rsp, RSP_SCRATCH
>> +
>> +     /* Load the top of the task stack into RSP */
>> +     movq    CPU_ENTRY_AREA_tss + TSS_sp1 + CPU_ENTRY_AREA, %rsp
>
> Yeah, let's put CPU_ENTRY_AREA first because then it reads easier:
>
> movq    CPU_ENTRY_AREA + CPU_ENTRY_AREA_tss + TSS_sp1, %rsp

Nope, it won't build.  That would expand like 0x1(%rip) + 0x2, which
isn't valid.

>
> i.e., pointer to cpu_entry_area, offset to tss within said
> cpu_entry_area and then inside that, sp1.
>
> Ditto for above.
>
> ...
>
>> @@ -1417,10 +1424,13 @@ static DEFINE_PER_CPU_PAGE_ALIGNED(char, exception_stacks
>>  /* May not be marked __init: used by software suspend */
>>  void syscall_init(void)
>>  {
>> +     extern char _entry_trampoline[];
>> +     extern char entry_SYSCALL_64_trampoline[];
>> +
>>       int cpu = smp_processor_id();
>>
>>       wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
>> -     wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
>> +     wrmsrl(MSR_LSTAR, (unsigned long)get_cpu_entry_area(cpu)->entry_trampoline + (entry_SYSCALL_64_trampoline - _entry_trampoline));
>
> Definitely a local var.

Done.

>
>>  #ifdef CONFIG_IA32_EMULATION
>>       wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
>> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
>> index a4009fb9be87..2738cfb6c8c8 100644
>> --- a/arch/x86/kernel/vmlinux.lds.S
>> +++ b/arch/x86/kernel/vmlinux.lds.S
>> @@ -107,6 +107,16 @@ SECTIONS
>>               SOFTIRQENTRY_TEXT
>>               *(.fixup)
>>               *(.gnu.warning)
>> +
>> +#ifdef CONFIG_X86_64
>> +             /* Entry trampoline */
>
> No need for that comment - variable and section names are enough. :)
>

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