Re: [PATCH] ARM: fix randomized task_struct

[Russell King - ARM Linux]

On Fri, Jun 30, 2017 at 03:49:41PM +0000, Ard Biesheuvel wrote:
> 
> 
> > On 30 Jun 2017, at 15:34, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > 
> > With the new task struct randomization, we can run into a build
> > failure for certain random seeds:
> > 
> > arch/arm/kernel/entry-armv.S: Assembler messages:
> > arch/arm/kernel/entry-armv.S:803: Error: bad immediate value for offset (4096)
> > 
> > Only two constants in asm-offset.h are affected, and I'm changing
> > both of them here to work correctly in all configurations.
> > 
> > One more macro has the problem, but is currently unused, so this
> > removes it instead of adding complexity.
> > 
> > Suggested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > Fixes: c33d8b12fbbd ("task_struct: Allow randomized layout")
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> > arch/arm/kernel/entry-armv.S |  5 ++++-
> > arch/arm/mm/proc-macros.S    | 10 ++++------
> > 2 files changed, 8 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> > index 9f157e7c51e7..db6d22b23bd8 100644
> > --- a/arch/arm/kernel/entry-armv.S
> > +++ b/arch/arm/kernel/entry-armv.S
> > @@ -797,7 +797,10 @@ ENTRY(__switch_to)
> > #if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
> >    ldr    r7, [r2, #TI_TASK]
> >    ldr    r8, =__stack_chk_guard
> > -    ldr    r7, [r7, #TSK_STACK_CANARY]
> > +    .if (TSK_STACK_CANARY > PAGE_MASK)
> 
> Shouldn't this be ~PAGE_MASK?
> 
> I think 
> 
> .if (TSK_STACK_CANARY & PAGE_MASK) != 0
> 
> is better and clearer as well

It's not really that much clearer - what has any of this got to do with
the size of a page?  Just because a definition appears to be numerically
the same, it doesn't mean it should be used!

The LDR instruction takes a maximum of a 12-bit constant.  This 12-bit
constant has nothing to do with the page size; it's been that way since
the early ARMs that knew nothing about page tables.

Please instead create a LDR_IMM12_MASK or similar definition for this.

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