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Date: Wed, 21 Feb 2018 14:24:43 +0100
From: Borislav Petkov <bp@...en8.de>
To: Alexander Popov <alex.popov@...ux.com>
Cc: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>,
PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>,
Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
Tycho Andersen <tycho@...ho.ws>, Laura Abbott <labbott@...hat.com>,
Mark Rutland <mark.rutland@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>,
"H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
"Dmitry V . Levin" <ldv@...linux.org>, x86@...nel.org
Subject: Re: [PATCH RFC v8 1/6] x86/entry: Add STACKLEAK erasing the kernel
stack at the end of syscalls
On Fri, Feb 16, 2018 at 09:10:52PM +0300, Alexander Popov wrote:
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 63bf349..62748bd 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -119,6 +119,7 @@ config X86
> select HAVE_ARCH_COMPAT_MMAP_BASES if MMU && COMPAT
> select HAVE_ARCH_SECCOMP_FILTER
> select HAVE_ARCH_THREAD_STRUCT_WHITELIST
> + select HAVE_ARCH_STACKLEAK
> select HAVE_ARCH_TRACEHOOK
> select HAVE_ARCH_TRANSPARENT_HUGEPAGE
> select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 16c2c02..4a7365a 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -77,6 +77,90 @@
> #endif
> .endm
>
> +.macro erase_kstack
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> + call erase_kstack
Hmm, why do we need a macro *and* a function of the same name? Why not do
call erase_kstack
everywhere we need to?
And then do
ENTRY(erase_kstack)
#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
...
to tone down the ifdeffery.
> +#endif
> +.endm
> +
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +ENTRY(erase_kstack)
> + pushl %edi
> + pushl %ecx
> + pushl %eax
> + pushl %ebp
> +
> + movl PER_CPU_VAR(current_task), %ebp
> + mov TASK_lowest_stack(%ebp), %edi
So instead of TASK_lowest_stack and adding all the ifdeffery around, why
not do this computation:
(unsigned long)task_stack_page(p) + 2 * sizeof(unsigned long);
in asm?
You only need task->stack as an offset variable. And other code might
need it too, anyway so you could just as well use it instead of adding
another var to thread_struct.
/me reads further.
Ah. So this lowest_stack thing changes at the end:
"Set the lowest_stack value to the top_of_stack - 256"
Why?
So that magic needs more explanation for slow people like me.
> + mov $STACKLEAK_POISON, %eax
> + std
> +
> + /*
> + * Let's search for the poison value in the stack.
> + * Start from the lowest_stack and go to the bottom (see std above).
Let's write insns capitalized: "see STD above".
...
> + /*
> + * Check that some further qwords contain poison. If so, the part
> + * of the stack below the address in %rdi is likely to be poisoned.
> + * Otherwise we need to search deeper.
> + */
> + mov $STACKLEAK_POISON_CHECK_DEPTH / 8, %ecx
> + repe scasq
> + jecxz 2f /* Poison the upper part of the stack */
> + jne 1b /* Search deeper */
> +
> +2:
You could name that label
.Lpoison
and make the code more readable:
jb .Lpoison
and so on.
> + /*
> + * Two qwords at the bottom of the thread stack are reserved and
> + * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
> + */
> + or $2 * 8, %rdi
> +
> + /*
> + * Check whether we are on the thread stack to prepare the counter
> + * for stack poisoning.
> + */
> + mov PER_CPU_VAR(cpu_current_top_of_stack), %rcx
> + sub %rsp, %rcx
> + cmp $THREAD_SIZE_asm, %rcx
> + jb 3f
> +
> + /*
> + * We are not on the thread stack, so we can write poison between
> + * the address in %rdi and the stack top.
> + */
> + mov PER_CPU_VAR(cpu_current_top_of_stack), %rcx
> + sub %rdi, %rcx
> + jmp 4f
> +
> +3:
> + /*
> + * We are on the thread stack, so we can write poison between the
> + * address in %rdi and the address in %rsp (not included, used memory).
> + */
> + mov %rsp, %rcx
> + sub %rdi, %rcx
> +
> +4:
> + /* Check that the counter value is sane */
> + cmp $THREAD_SIZE_asm, %rcx
> + jb 5f
> + ud2
> +
> +5:
> + /*
> + * So let's write the poison value to the kernel stack. Start from the
> + * address in %rdi and move up (see cld).
> + */
> + cld
> + shr $3, %ecx
> + rep stosq
Hohumm, that's >2K loops per syscall here and stack is
0xffffc90000008010: 0xffffffffffff4111 0xffffffffffff4111
0xffffc90000008020: 0xffffffffffff4111 0xffffffffffff4111
0xffffc90000008030: 0xffffffffffff4111 0xffffffffffff4111
0xffffc90000008040: 0xffffffffffff4111 0xffffffffffff4111
0xffffc90000008050: 0xffffffffffff4111 0xffffffffffff4111
0xffffc90000008060: 0xffffffffffff4111 0xffffffffffff4111
...
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 9eb448c..6dc55f6 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -281,6 +281,11 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
> p->thread.sp = (unsigned long) fork_frame;
> p->thread.io_bitmap_ptr = NULL;
>
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> + p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
> + 2 * sizeof(unsigned long);
> +#endif
> +
> savesegment(gs, p->thread.gsindex);
> p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
> savesegment(fs, p->thread.fsindex);
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index e835fc0..fe04f60 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -337,4 +337,10 @@ unsigned long read_word_at_a_time(const void *addr)
> compiletime_assert(__native_word(t), \
> "Need native word sized stores/loads for atomicity.")
>
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +/* Poison value points to the unused hole in the virtual memory map */
There are a bunch of those there now. Please document it in
Documentation/x86/x86_64/mm.txt
> +# define STACKLEAK_POISON -0xBEEF
> +# define STACKLEAK_POISON_CHECK_DEPTH 128
> +#endif
> +
> #endif /* __LINUX_COMPILER_H */
> --
> 2.7.4
>
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
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