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    SubjectRe: [PATCH 2/2] arm64: Clear the stack
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    Hello Laura and Kees,

    On 03.05.2018 02:07, Laura Abbott wrote:
    > On 05/02/2018 02:31 PM, Kees Cook wrote:
    >> On Wed, May 2, 2018 at 1:33 PM, Laura Abbott <labbott@redhat.com> wrote:
    >>>
    >>> Implementation of stackleak based heavily on the x86 version
    >>
    >> Awesome! Notes below for both you and Alexander, since I think we can
    >> create a common code base instead of having near-duplicates in the
    >> arch/ trees...

    Yes, sure.

    I will extract the common part and send v12 for x86. Then Laura will be able to
    add arm64 support in a separate patch series. Is it fine?

    >>> Signed-off-by: Laura Abbott <labbott@redhat.com>
    >>> ---
    >>> Now written in C instead of a bunch of assembly.
    >>> ---
    >>> arch/arm64/Kconfig | 1 +
    >>> arch/arm64/include/asm/processor.h | 6 ++++
    >>> arch/arm64/kernel/Makefile | 3 ++
    >>> arch/arm64/kernel/entry.S | 6 ++++
    >>> arch/arm64/kernel/erase.c | 55 +++++++++++++++++++++++++++++++++++
    >>> arch/arm64/kernel/process.c | 16 ++++++++++
    >>> drivers/firmware/efi/libstub/Makefile | 3 +-
    >>> scripts/Makefile.gcc-plugins | 5 +++-
    >>> 8 files changed, 93 insertions(+), 2 deletions(-)
    >>> create mode 100644 arch/arm64/kernel/erase.c
    >>>
    >>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
    >>> index eb2cf4938f6d..b0221db95dc9 100644
    >>> --- a/arch/arm64/Kconfig
    >>> +++ b/arch/arm64/Kconfig
    >>> @@ -92,6 +92,7 @@ config ARM64
    >>> select HAVE_ARCH_MMAP_RND_BITS
    >>> select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
    >>> select HAVE_ARCH_SECCOMP_FILTER
    >>> + select HAVE_ARCH_STACKLEAK
    >>> select HAVE_ARCH_THREAD_STRUCT_WHITELIST
    >>> select HAVE_ARCH_TRACEHOOK
    >>> select HAVE_ARCH_TRANSPARENT_HUGEPAGE
    >>> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
    >>> index 767598932549..d31ab80ff647 100644
    >>> --- a/arch/arm64/include/asm/processor.h
    >>> +++ b/arch/arm64/include/asm/processor.h
    >>> @@ -124,6 +124,12 @@ struct thread_struct {
    >>> unsigned long fault_address; /* fault info */
    >>> unsigned long fault_code; /* ESR_EL1 value */
    >>> struct debug_info debug; /* debugging */
    >>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
    >>> + unsigned long lowest_stack;
    >>> +#ifdef CONFIG_STACKLEAK_METRICS
    >>> + unsigned long prev_lowest_stack;
    >>> +#endif
    >>> +#endif
    >>
    >> I wonder if x86 and arm64 could include a common struct here that was
    >> empty when the plugin is disabled... it would keep the ifdefs in one
    >> place. Maybe include/linux/stackleak.h could be:
    >>
    >> ---start---
    >> /* Poison value points to the unused hole in the virtual memory map */
    >> #define STACKLEAK_POISON -0xBEEF
    >> #define STACKLEAK_POISON_CHECK_DEPTH 128
    >>
    >> struct stackleak {
    >> #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
    >> unsigned long lowest;
    >> #ifdef CONFIG_STACKLEAK_METRICS
    >> unsigned long prev_lowest;
    >> #endif
    >> #endif
    >> };
    >>
    >
    > Is this well defined across all compilers if the plugin is off?
    > This seems to compile with gcc at least but 0 sized structs
    > make me a little uneasy.

    Empty struct is not defined by C standard but is permitted by gcc
    https://gcc.gnu.org/onlinedocs/gcc/Empty-Structures.html#Empty-Structures

    Fast example:

    #include <stdio.h>

    int main(void)
    {
    struct a {};

    printf("size %zu\n", sizeof(struct a));

    return 0;
    }

    # gcc -pedantic t.c -o t
    t.c: In function ‘main’:
    t.c:5:9: warning: struct has no members [-Wpedantic]
    struct a {};
    ^

    # clang -Weverything t.c -o tc
    t.c:5:2: warning: empty struct has size 0 in C, size 1 in C++ [-Wc++-compat]
    struct a {};
    ^
    t.c:5:2: warning: empty struct is a GNU extension [-Wgnu-empty-struct]
    2 warnings generated.


    But both programs print "size 0". There are a lot of empty structs around the
    kernel, so I'll create another one.

    >> asmlinkage void erase_kstack(void);
    >> ---eof---
    >>
    >> and arch/*/include/asm/processor.h could do:
    >>
    >> @@ -124,6 +124,12 @@ struct thread_struct {
    >> unsigned long fault_address; /* fault info */
    >> unsigned long fault_code; /* ESR_EL1 value */
    >> struct debug_info debug; /* debugging */
    >> + struct stackleak stackleak;
    >>
    >> and arch/x86/entry/erase.c could move to maybe kernel/stackleak.c?
    >> (Oh, I notice this needs an SPDX line too.)

    Thanks, I'll add it.

    >>> static inline void arch_thread_struct_whitelist(unsigned long *offset,
    >>> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
    >>> index bf825f38d206..0ceea613c65b 100644
    >>> --- a/arch/arm64/kernel/Makefile
    >>> +++ b/arch/arm64/kernel/Makefile
    >>> @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
    >>> arm64-obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
    >>> arm64-obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o
    >>>
    >>> +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o
    >>> +KASAN_SANITIZE_erase.o := n
    >>> +
    >>> obj-y += $(arm64-obj-y) vdso/ probes/
    >>> obj-m += $(arm64-obj-m)
    >>> head-y := head.o
    >>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
    >>> index ec2ee720e33e..3144f1ebdc18 100644
    >>> --- a/arch/arm64/kernel/entry.S
    >>> +++ b/arch/arm64/kernel/entry.S
    >>> @@ -401,6 +401,11 @@ tsk .req x28 // current thread_info
    >>>
    >>> .text
    >>>
    >>> + .macro ERASE_KSTACK
    >>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
    >>> + bl erase_kstack
    >>> +#endif
    >>> + .endm
    >>> /*
    >>> * Exception vectors.
    >>> */
    >>> @@ -906,6 +911,7 @@ ret_to_user:
    >>> cbnz x2, work_pending
    >>> finish_ret_to_user:
    >>> enable_step_tsk x1, x2
    >>> + ERASE_KSTACK
    >>> kernel_exit 0
    >>> ENDPROC(ret_to_user)
    >>
    >> Nice. All of the return paths end up here (I went looking for
    >> ret_from_fork's path). :)
    >>
    >>>
    >>> diff --git a/arch/arm64/kernel/erase.c b/arch/arm64/kernel/erase.c
    >>> new file mode 100644
    >>> index 000000000000..b8b5648d893b
    >>> --- /dev/null
    >>> +++ b/arch/arm64/kernel/erase.c
    >>> @@ -0,0 +1,55 @@
    >>> +#include <linux/bug.h>
    >>> +#include <linux/sched.h>
    >>> +#include <asm/current.h>
    >>> +#include <asm/linkage.h>
    >>> +#include <asm/processor.h>
    >>> +
    >>> +asmlinkage void erase_kstack(void)
    >>> +{
    >>> + unsigned long p = current->thread.lowest_stack;
    >>> + unsigned long boundary = p & ~(THREAD_SIZE - 1);
    >>> + unsigned long poison = 0;
    >>> + const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
    >>> + sizeof(unsigned long);
    >>> +
    >>> + /*
    >>> + * Let's search for the poison value in the stack.
    >>> + * Start from the lowest_stack and go to the bottom.
    >>> + */
    >>> + while (p > boundary && poison <= check_depth) {
    >>> + if (*(unsigned long *)p == STACKLEAK_POISON)
    >>> + poison++;
    >>> + else
    >>> + poison = 0;
    >>> +
    >>> + p -= sizeof(unsigned long);
    >>> + }
    >>> +
    >>> + /*
    >>> + * One long int at the bottom of the thread stack is reserved and
    >>> + * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
    >>> + */
    >>> + if (p == boundary)
    >>> + p += sizeof(unsigned long);
    >>> +
    >>> +#ifdef CONFIG_STACKLEAK_METRICS
    >>> + current->thread.prev_lowest_stack = p;
    >>> +#endif
    >>> +
    >>> + /*
    >>> + * So let's write the poison value to the kernel stack.
    >>> + * Start from the address in p and move up till the new boundary.
    >>> + */
    >>> + boundary = current_stack_pointer;
    >> This is the only difference between x86 and arm64 in this code. What
    >> do you think about implementing on_thread_stack() to match x86:
    >>
    >> if (on_thread_stack())
    >> boundary = current_stack_pointer;
    >> else
    >> boundary = current_top_of_stack();
    >>
    >> then we could make this common code too instead of having two copies in arch/?
    >>
    >
    > The issue isn't on_thread_stack, it's current_top_of_stack which isn't
    > defined on arm64. I agree it would be good if the code would be common
    > but I'm not sure how much we want to start trying to force APIs.
    >
    >>> + BUG_ON(boundary - p >= THREAD_SIZE);
    >>> +
    >>> + while (p < boundary) {
    >>> + *(unsigned long *)p = STACKLEAK_POISON;
    >>> + p += sizeof(unsigned long);
    >>> + }
    >>> +
    >>> + /* Reset the lowest_stack value for the next syscall */
    >>> + current->thread.lowest_stack = current_stack_pointer;
    >>> +}
    >>> +
    >>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
    >>> index f08a2ed9db0d..156fa0a0da19 100644
    >>> --- a/arch/arm64/kernel/process.c
    >>> +++ b/arch/arm64/kernel/process.c
    >>> @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
    >>> p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
    >>> p->thread.cpu_context.sp = (unsigned long)childregs;
    >>>
    >>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
    >>> + p->thread.lowest_stack = (unsigned long)task_stack_page(p);
    >>> +#endif

    I think it should be (unsigned long)task_stack_page(p) + sizeof(unsigned long).

    >>> ptrace_hw_copy_thread(p);
    >>>
    >>> return 0;
    >>> @@ -493,3 +496,16 @@ void arch_setup_new_exec(void)
    >>> {
    >>> current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
    >>> }
    >>> +
    >>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
    >>> +void __used check_alloca(unsigned long size)
    >>> +{
    >>> + unsigned long sp, stack_left;
    >>> +
    >>> + sp = current_stack_pointer;
    >>> +
    >>> + stack_left = sp & (THREAD_SIZE - 1);
    >>> + BUG_ON(stack_left < 256 || size >= stack_left - 256);
    >>> +}
    >>> +EXPORT_SYMBOL(check_alloca);
    >>
    >> This is pretty different from x86. Is this just an artifact of ORC, or
    >> something else?
    >>
    >
    > This was based on the earlier version of x86. I'll confess to
    > not seeing how the current x86 version ended up with get_stack_info
    > but I suspect it's either related to ORC unwinding or it's best
    > practice.

    I've changed that in v4. Quote from the changelog:
    - Fixed the surplus and erroneous code for calculating stack_left in
    check_alloca() on x86_64. That code repeats the work which is already
    done in get_stack_info() and it misses the fact that different
    exception stacks on x86_64 have different size.

    http://www.openwall.com/lists/kernel-hardening/2017/10/04/68

    We can see that in arch/x86/kernel/dumpstack_64.c.

    Is it fine if check_alloca() would be arch-specific?

    >>> +#endif
    >>> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
    >>> index a34e9290a699..25dd2a14560d 100644
    >>> --- a/drivers/firmware/efi/libstub/Makefile
    >>> +++ b/drivers/firmware/efi/libstub/Makefile
    >>> @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB) += -I$(srctree)/scripts/dtc/libfdt
    >>> KBUILD_CFLAGS := $(cflags-y) -DDISABLE_BRANCH_PROFILING \
    >>> -D__NO_FORTIFY \
    >>> $(call cc-option,-ffreestanding) \
    >>> - $(call cc-option,-fno-stack-protector)
    >>> + $(call cc-option,-fno-stack-protector) \
    >>> + $(DISABLE_STACKLEAK_PLUGIN)
    >>>
    >>> GCOV_PROFILE := n
    >>> KASAN_SANITIZE := n
    >>> diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
    >>> index 8d6070fc538f..6cc0e35d3324 100644
    >>> --- a/scripts/Makefile.gcc-plugins
    >>> +++ b/scripts/Makefile.gcc-plugins
    >>> @@ -37,11 +37,14 @@ ifdef CONFIG_GCC_PLUGINS
    >>>
    >>> gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak_plugin.so
    >>> gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) += -DSTACKLEAK_PLUGIN -fplugin-arg-stackleak_plugin-track-min-size=$(CONFIG_STACKLEAK_TRACK_MIN_SIZE)
    >>> + ifdef CONFIG_GCC_PLUGIN_STACKLEAK
    >>> + DISABLE_STACKLEAK_PLUGIN += -fplugin-arg-stackleak_plugin-disable
    >>> + endif
    >>>
    >>> GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
    >>>
    >>> export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR
    >>> - export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN
    >>> + export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN DISABLE_STACKLEAK_PLUGIN
    >>>
    >>> ifneq ($(PLUGINCC),)
    >>> # SANCOV_PLUGIN can be only in CFLAGS_KCOV because avoid duplication.
    >>> --
    >>> 2.14.3
    >>>

    Best regards,
    Alexander

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