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Message-ID: <CAMzpN2ggqbwkfNjHtYg3KU5oeerU6NPa0AqzG7PFtJuWy3dyKA@mail.gmail.com>
Date: Mon, 27 Jun 2016 11:12:41 -0400
From: Brian Gerst <brgerst@...il.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: "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 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.
--
Brian Gerst
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