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    SubjectRe: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)
    On Fri, Oct 18, 2019 at 09:10:21AM -0700, Sami Tolvanen wrote:
    > This change adds generic support for Clang's Shadow Call Stack, which
    > uses a shadow stack to protect return addresses from being overwritten
    > by an attacker. Details are available here:
    >
    > https://clang.llvm.org/docs/ShadowCallStack.html
    >
    > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
    > ---
    > Makefile | 6 ++
    > arch/Kconfig | 39 ++++++++
    > include/linux/compiler-clang.h | 2 +
    > include/linux/compiler_types.h | 4 +
    > include/linux/scs.h | 88 ++++++++++++++++++
    > init/init_task.c | 6 ++
    > init/main.c | 3 +
    > kernel/Makefile | 1 +
    > kernel/fork.c | 9 ++
    > kernel/sched/core.c | 2 +
    > kernel/sched/sched.h | 1 +
    > kernel/scs.c | 162 +++++++++++++++++++++++++++++++++
    > 12 files changed, 323 insertions(+)
    > create mode 100644 include/linux/scs.h
    > create mode 100644 kernel/scs.c
    >
    > diff --git a/Makefile b/Makefile
    > index ffd7a912fc46..e401fa500f62 100644
    > --- a/Makefile
    > +++ b/Makefile
    > @@ -846,6 +846,12 @@ ifdef CONFIG_LIVEPATCH
    > KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
    > endif
    >
    > +ifdef CONFIG_SHADOW_CALL_STACK
    > +KBUILD_CFLAGS += -fsanitize=shadow-call-stack
    > +DISABLE_SCS := -fno-sanitize=shadow-call-stack
    > +export DISABLE_SCS
    > +endif

    I think it would be preferable to follow the example of CC_FLAGS_FTRACE
    so that this can be filtered out, e.g.

    ifdef CONFIG_SHADOW_CALL_STACK
    CFLAGS_SCS := -fsanitize=shadow-call-stack
    KBUILD_CFLAGS += $(CFLAGS_SCS)
    export CC_FLAGS_SCS
    endif

    ... with removal being:

    CFLAGS_REMOVE := $(CC_FLAGS_SCS)

    ... or:

    CFLAGS_REMOVE_obj.o := $(CC_FLAGS_SCS)

    That way you only need to define the flags once, so the enable and
    disable falgs remain in sync by construction.

    [...]

    > +config ARCH_SUPPORTS_SHADOW_CALL_STACK
    > + bool
    > + help
    > + An architecture should select this if it supports Clang's Shadow
    > + Call Stack, has asm/scs.h, and implements runtime support for shadow
    > + stack switching.
    > +
    > +config SHADOW_CALL_STACK_VMAP
    > + def_bool n

    A bool is default n, so you can just say bool here.

    > + depends on SHADOW_CALL_STACK
    > + help
    > + Use virtually mapped shadow call stacks. Selecting this option
    > + provides better stack exhaustion protection, but increases per-thread
    > + memory consumption as a full page is allocated for each shadow stack.
    > +
    > +choice
    > + prompt "Return-oriented programming (ROP) protection"
    > + default ROP_PROTECTION_NONE
    > + help
    > + This option controls kernel protections against return-oriented
    > + programming (ROP) attacks.

    Are we expecting more options here in future?

    > +config ROP_PROTECTION_NONE
    > + bool "None"

    IIUC this is for the benefit of the kretprobes Kconfig.

    I think it would be better to make that depend on !SHADOW_CALL_STACK, as
    it's plausible that we can add a different ROP protection mechanism that
    is compatible with kretprobes.

    > +config SHADOW_CALL_STACK
    > + bool "Clang Shadow Call Stack"
    > + depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
    > + depends on CC_IS_CLANG && CLANG_VERSION >= 70000

    Is there a reason for an explicit version check rather than a
    CC_HAS_<feature> check? e.g. was this available but broken in prior
    versions of clang?

    [...]

    > +#define SCS_GFP (GFP_KERNEL | __GFP_ZERO)

    Normally GFP_ is a prefix. For consistency, GFP_SCS would be preferable.

    > +extern unsigned long init_shadow_call_stack[];

    Do we need this exposed here? IIUC this is only assigned by assembly in
    arch code.

    [...]

    > +void scs_set_init_magic(struct task_struct *tsk)
    > +{
    > + scs_save(tsk);
    > + scs_set_magic(tsk);
    > + scs_load(tsk);
    > +}

    Can we initialize this at compile time instead?

    Thanks,
    Mark.

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