Re: [PATCH 15/43] x86/entry/64: Create a percpu SYSCALL entry trampoline

[Borislav Petkov]

On Fri, Nov 24, 2017 at 06:23:43PM +0100, Ingo Molnar wrote:
> From: Andy Lutomirski <luto@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bpetkov@xxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/b95ccae0a5a2f090c901e49fce7c9e8ff6acd40d.1511497875.git.luto@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  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.

> + * trampoline.  We can thus find cpu_entry_area with this macro:

Uuh, fun. :)

> + */
> +
> +#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...

> +
> +/* 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 :)

Also, pls align:

#define RSP_SCRATCH CPU_ENTRY_AREA_tss + CPU_TSS_SYSENTER_stack + \
                    SIZEOF_SYSENTER_stack - 8 + CPU_ENTRY_AREA

> +
> +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

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.

>  #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. :)

> +		. = ALIGN(PAGE_SIZE);
> +		_entry_trampoline = .;
> +		*(.entry_trampoline)
> +		. = ALIGN(PAGE_SIZE);
> +		ASSERT(. - _entry_trampoline == PAGE_SIZE, "entry trampoline is too big");
> +#endif
> +
>  		/* End of text section */
>  		_etext = .;
>  	} :text = 0x9090
> -- 

-- 
Regards/Gruss,
    Boris.

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