Re: [PATCH v3 2/2] riscv: Enable per-task stack canaries

From: Guo Ren
Date: Mon Jul 13 2020 - 00:05:49 EST

Hi Kees,

On Mon, Jul 13, 2020 at 10:40 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Fri, Jul 10, 2020 at 04:19:58PM +0000, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > This enables the use of per-task stack canary values if GCC has
> > support for emitting the stack canary reference relative to the
> > value of tp, which holds the task struct pointer in the riscv
> > kernel.
> >
> > After compare arm64 and x86 implementations, seems arm64's is more
> > flexible and readable. The key point is how gcc get the offset of
> > stack_canary from gs/el0_sp.
> >
> > x86: Use a fix offset from gs, not flexible.
> >
> > struct fixed_percpu_data {
> > /*
> > * GCC hardcodes the stack canary as %gs:40. Since the
> > * irq_stack is the object at %gs:0, we reserve the bottom
> > * 48 bytes of the irq stack for the canary.
> > */
> > char gs_base[40]; // :(
> > unsigned long stack_canary;
> > };
> >
> > arm64: Use -mstack-protector-guard-offset & guard-reg
> > gcc options:
> > -mstack-protector-guard=sysreg
> > -mstack-protector-guard-reg=sp_el0
> > -mstack-protector-guard-offset=xxx
> >
> > riscv: Use -mstack-protector-guard-offset & guard-reg
> > gcc options:
> > -mstack-protector-guard=tls
> > -mstack-protector-guard-reg=tp
> > -mstack-protector-guard-offset=xxx
> >
> > Here is riscv gcc's work [1].
> >
> > [1] https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549583.html
> >
> > In the end, these codes are inserted by gcc before return:
> >
> > * 0xffffffe00020b396 <+120>: ld a5,1008(tp) # 0x3f0
> > * 0xffffffe00020b39a <+124>: xor a5,a5,a4
> > * 0xffffffe00020b39c <+126>: mv a0,s5
> > * 0xffffffe00020b39e <+128>: bnez a5,0xffffffe00020b61c <_do_fork+766>
> > 0xffffffe00020b3a2 <+132>: ld ra,136(sp)
> > 0xffffffe00020b3a4 <+134>: ld s0,128(sp)
> > 0xffffffe00020b3a6 <+136>: ld s1,120(sp)
> > 0xffffffe00020b3a8 <+138>: ld s2,112(sp)
> > 0xffffffe00020b3aa <+140>: ld s3,104(sp)
> > 0xffffffe00020b3ac <+142>: ld s4,96(sp)
> > 0xffffffe00020b3ae <+144>: ld s5,88(sp)
> > 0xffffffe00020b3b0 <+146>: ld s6,80(sp)
> > 0xffffffe00020b3b2 <+148>: ld s7,72(sp)
> > 0xffffffe00020b3b4 <+150>: addi sp,sp,144
> > 0xffffffe00020b3b6 <+152>: ret
> > ...
> > * 0xffffffe00020b61c <+766>: auipc ra,0x7f8
> > * 0xffffffe00020b620 <+770>: jalr -1764(ra) # 0xffffffe000a02f38 <__stack_chk_fail>
> >
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: cooper <cooper.qu@xxxxxxxxxxxxxxxxx>
> > Cc: cooper <cooper.qu@xxxxxxxxxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> > Change v2:
> > - Change to -mstack-protector-guard=tls for gcc final define
> > - Solve compile error by changing position of KBUILD_CFLAGS in
> > Makefile
> >
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > ---
> > arch/riscv/Kconfig | 7 +++++++
> > arch/riscv/Makefile | 10 ++++++++++
> > arch/riscv/include/asm/stackprotector.h | 3 ++-
> > arch/riscv/kernel/asm-offsets.c | 3 +++
> > arch/riscv/kernel/process.c | 2 +-
> > 5 files changed, 23 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 4b0e308..d98ce29 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -394,6 +394,13 @@ config CMDLINE_FORCE
> >
> > endchoice
> >
> > +config CC_HAVE_STACKPROTECTOR_TLS
> > + def_bool $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=tp -mstack-protector-guard-offset=0)
> > +
> > +config STACKPROTECTOR_PER_TASK
> > + def_bool y
> > + depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_TLS
> > +
> > endmenu
> >
> > config BUILTIN_DTB
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index fb6e37d..f5f8ee9 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -68,6 +68,16 @@ KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-relax)
> > # architectures. It's faster to have GCC emit only aligned accesses.
> > KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
> >
> > +ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> > +prepare: stack_protector_prepare
> > +stack_protector_prepare: prepare0
> > + $(eval KBUILD_CFLAGS += -mstack-protector-guard=tls \
> > + -mstack-protector-guard-reg=tp \
> > + -mstack-protector-guard-offset=$(shell \
> > + awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
> > + include/generated/asm-offsets.h))
> > +endif
> > +
> > # arch specific predefines for sparse
> > CHECKFLAGS += -D__riscv -D__riscv_xlen=$(BITS)
> >
> > diff --git a/arch/riscv/include/asm/stackprotector.h b/arch/riscv/include/asm/stackprotector.h
> > index d95f7b2..a895e07 100644
> > --- a/arch/riscv/include/asm/stackprotector.h
> > +++ b/arch/riscv/include/asm/stackprotector.h
> > @@ -28,6 +28,7 @@ static __always_inline void boot_init_stack_canary(void)
> > canary &= CANARY_MASK;
> >
> > current->stack_canary = canary;
> > - __stack_chk_guard = current->stack_canary;
> > + if (!IS_ENABLED(CONFIG_STACKPROTECTOR_PER_TASK))
> > + __stack_chk_guard = current->stack_canary;
> > }
> > #endif /* _ASM_RISCV_STACKPROTECTOR_H */
> > diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> > index 07cb9c1..999b465 100644
> > --- a/arch/riscv/kernel/asm-offsets.c
> > +++ b/arch/riscv/kernel/asm-offsets.c
> > @@ -29,6 +29,9 @@ void asm_offsets(void)
> > OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
> > OFFSET(TASK_THREAD_SP, task_struct, thread.sp);
> > OFFSET(TASK_STACK, task_struct, stack);
> > +#ifdef CONFIG_STACKPROTECTOR
>
> Should this be CONFIG_STACKPROTECTOR_PER_TASK ?
Yes, it's more accurate, Thx. I also send a patch [1] to arm64, and
let's see how they reply?

[1] https://lore.kernel.org/linux-csky/1594613013-13059-1-git-send-email-guoren@xxxxxxxxxx/T/#u

--
Best Regards
Guo Ren

ML: https://lore.kernel.org/linux-csky/