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Date: Mon, 27 Jun 2016 08:22:30 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Brian Gerst <brgerst@...il.com>
Cc: Andy Lutomirski <luto@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>, Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Linus Torvalds <torvalds@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Jann Horn <jann@...jh.net>, Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 15/29] x86/mm/64: Enable vmapped stacks
On Mon, Jun 27, 2016 at 8:12 AM, Brian Gerst <brgerst@...il.com> wrote:
> On Mon, Jun 27, 2016 at 11:01 AM, Brian Gerst <brgerst@...il.com> wrote:
>> On Sun, Jun 26, 2016 at 5:55 PM, Andy Lutomirski <luto@...nel.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@...nel.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.
>
> I guess it doesn't really matter, since the fault is fatal either way.
> The error message might be incorrect though.
>
It's at least worth a comment, though. Maybe I should check if
regs->rsp is within 40 bytes of the bottom of the stack, too, such
that delivery of an inner fault would have double-faulted assuming the
inner fault didn't use an IST vector.
--Andy
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