[PATCH] arm64: Make TSK_STACK_CANARY more accurate defined

[Guo Ren]

BTW, Jim found a GCC security leak in arm64, and would you want to
have a look at it?
-------
I notice in the epilogue I get
    ld a4, 8(sp)
    ld a5, 100(t6)
    xor a5, a4, a5
    bne a5,zero,.L4
This looks like a security leak that the canary value is left in a4.
The i386 implementation operates directly on memory without loading
into registers.  The rs6000 implementation is careful to load 0 into
the other register in the stack_protector_test code after the xor.  I
think this is a bug in the aarch64 code that it isn't clearing the
other register.  And I think it is a bug in your code too.  If we
don't need to clear the canary from the two registers, then you should
eliminate the xor and just use "bne a5,a4,.L4".  But I think the way
you have it is right, you just need to clear the a4 register after the
xor.
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[1] https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549910.html

On Tue, Jul 14, 2020 at 4:37 PM Will Deacon <will at kernel.org> wrote:
>
> On Mon, Jul 13, 2020 at 04:03:33AM +0000, guoren at kernel.org wrote:
> > From: Guo Ren <guoren at linux.alibaba.com>
> >
> > TSK_STACK_CANARY only used in arm64/Makefile with
> > CONFIG_STACKPROTECTOR_PER_TASK wrap. So use the same policy in
> > asm-offset.c.
> >
> > Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> > Co-developed-by: Kees Cook <keescook at chromium.org>
> > Cc: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: Will Deacon <will at kernel.org>
> > ---
> >  arch/arm64/kernel/asm-offsets.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> > index 0577e21..37d5d3d 100644
> > --- a/arch/arm64/kernel/asm-offsets.c
> > +++ b/arch/arm64/kernel/asm-offsets.c
> > @@ -39,7 +39,7 @@ int main(void)
> >    DEFINE(TSK_TI_SCS_SP,              offsetof(struct task_struct, thread_info.scs_sp));
> >  #endif
> >    DEFINE(TSK_STACK,          offsetof(struct task_struct, stack));
> > -#ifdef CONFIG_STACKPROTECTOR
> > +#ifdef CONFIG_STACKPROTECTOR_PER_TASK
> >    DEFINE(TSK_STACK_CANARY,   offsetof(struct task_struct, stack_canary));
> >  #endif
>
> I don't think this really makese much sense. The 'stack_canary' field in
> 'struct task_struct' is defined as:
>
> #ifdef CONFIG_STACKPROTECTOR
>         /* Canary value for the -fstack-protector GCC feature: */
>         unsigned long                   stack_canary;
> #endif
>
> so I think it makes sense to follow that in asm-offsets.c
>
> Does the current code actually cause a problem?
>
> Will



--
Best Regards
 Guo Ren

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