On Sun, Jun 26, 2016 at 5:55 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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
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