Re: [PATCH v4 15/29] x86/mm/64: Enable vmapped stacks

[Brian Gerst <brgerst@gmail.com>]

On Sun, Jun 26, 2016 at 5:55 PM, Andy Lutomirski <luto@kernel.org> wrote:
> This allows x86_64 kernels to enable vmapped stacks.  There are a
> couple of interesting bits.
>
> First, x86 lazily faults in top-level paging entries for the vmalloc
> area.  This won't work if we get a page fault while trying to access
> the stack: the CPU will promote it to a double-fault and we'll die.
> To avoid this problem, probe the new stack when switching stacks and
> forcibly populate the pgd entry for the stack when switching mms.
>
> Second, once we have guard pages around the stack, we'll want to
> detect and handle stack overflow.
>
> I didn't enable it on x86_32.  We'd need to rework the double-fault
> code a bit and I'm concerned about running out of vmalloc virtual
> addresses under some workloads.
>
> This patch, by itself, will behave somewhat erratically when the
> stack overflows while RSP is still more than a few tens of bytes
> above the bottom of the stack.  Specifically, we'll get #PF and make
> it to no_context and an oops without triggering a double-fault, and
> no_context doesn't know about stack overflows.  The next patch will
> improve that case.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/Kconfig                 |  1 +
>  arch/x86/include/asm/switch_to.h | 28 +++++++++++++++++++++++++++-
>  arch/x86/kernel/traps.c          | 32 ++++++++++++++++++++++++++++++++
>  arch/x86/mm/tlb.c                | 15 +++++++++++++++
>  4 files changed, 75 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index d9a94da0c29f..afdcf96ef109 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -92,6 +92,7 @@ config X86
>         select HAVE_ARCH_TRACEHOOK
>         select HAVE_ARCH_TRANSPARENT_HUGEPAGE
>         select HAVE_EBPF_JIT                    if X86_64
> +       select HAVE_ARCH_VMAP_STACK             if X86_64
>         select HAVE_CC_STACKPROTECTOR
>         select HAVE_CMPXCHG_DOUBLE
>         select HAVE_CMPXCHG_LOCAL
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index 8f321a1b03a1..14e4b20f0aaf 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -8,6 +8,28 @@ struct tss_struct;
>  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>                       struct tss_struct *tss);
>
> +/* This runs runs on the previous thread's stack. */
> +static inline void prepare_switch_to(struct task_struct *prev,
> +                                    struct task_struct *next)
> +{
> +#ifdef CONFIG_VMAP_STACK
> +       /*
> +        * If we switch to a stack that has a top-level paging entry
> +        * that is not present in the current mm, the resulting #PF will
> +        * will be promoted to a double-fault and we'll panic.  Probe
> +        * the new stack now so that vmalloc_fault can fix up the page
> +        * tables if needed.  This can only happen if we use a stack
> +        * in vmap space.
> +        *
> +        * We assume that the stack is aligned so that it never spans
> +        * more than one top-level paging entry.
> +        *
> +        * To minimize cache pollution, just follow the stack pointer.
> +        */
> +       READ_ONCE(*(unsigned char *)next->thread.sp);
> +#endif
> +}
> +
>  #ifdef CONFIG_X86_32
>
>  #ifdef CONFIG_CC_STACKPROTECTOR
> @@ -39,6 +61,8 @@ do {                                                                  \
>          */                                                             \
>         unsigned long ebx, ecx, edx, esi, edi;                          \
>                                                                         \
> +       prepare_switch_to(prev, next);                                  \
> +                                                                       \
>         asm volatile("pushl %%ebp\n\t"          /* save    EBP   */     \
>                      "movl %%esp,%[prev_sp]\n\t"        /* save    ESP   */ \
>                      "movl %[next_sp],%%esp\n\t"        /* restore ESP   */ \
> @@ -103,7 +127,9 @@ do {                                                                        \
>   * clean in kernel mode, with the possible exception of IOPL.  Kernel IOPL
>   * has no effect.
>   */
> -#define switch_to(prev, next, last) \
> +#define switch_to(prev, next, last)                                      \
> +       prepare_switch_to(prev, next);                                    \
> +                                                                         \
>         asm volatile(SAVE_CONTEXT                                         \
>              "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */       \
>              "movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */    \
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 00f03d82e69a..9cb7ea781176 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -292,12 +292,30 @@ DO_ERROR(X86_TRAP_NP,     SIGBUS,  "segment not present", segment_not_present)
>  DO_ERROR(X86_TRAP_SS,     SIGBUS,  "stack segment",            stack_segment)
>  DO_ERROR(X86_TRAP_AC,     SIGBUS,  "alignment check",          alignment_check)
>
> +#ifdef CONFIG_VMAP_STACK
> +static void __noreturn handle_stack_overflow(const char *message,
> +                                            struct pt_regs *regs,
> +                                            unsigned long fault_address)
> +{
> +       printk(KERN_EMERG "BUG: stack guard page was hit at %p (stack is %p..%p)\n",
> +                (void *)fault_address, current->stack,
> +                (char *)current->stack + THREAD_SIZE - 1);
> +       die(message, regs, 0);
> +
> +       /* Be absolutely certain we don't return. */
> +       panic(message);
> +}
> +#endif
> +
>  #ifdef CONFIG_X86_64
>  /* Runs on IST stack */
>  dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>  {
>         static const char str[] = "double fault";
>         struct task_struct *tsk = current;
> +#ifdef CONFIG_VMAP_STACK
> +       unsigned long cr2;
> +#endif
>
>  #ifdef CONFIG_X86_ESPFIX64
>         extern unsigned char native_irq_return_iret[];
> @@ -332,6 +350,20 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>         tsk->thread.error_code = error_code;
>         tsk->thread.trap_nr = X86_TRAP_DF;
>
> +#ifdef CONFIG_VMAP_STACK
> +       /*
> +        * If we overflow the stack into a guard page, the CPU will fail
> +        * to deliver #PF and will send #DF instead.  CR2 will contain
> +        * the linear address of the second fault, which will be in the
> +        * guard page below the bottom of the stack.
> +        */
> +       cr2 = read_cr2();
> +       if ((unsigned long)tsk->stack - 1 - cr2 < PAGE_SIZE)
> +               handle_stack_overflow(
> +                       "kernel stack overflow (double-fault)",
> +                       regs, cr2);
> +#endif

Is there any other way to tell if this was from a page fault?  If it
wasn't a page fault then CR2 is undefined.

--
Brian Gerst