[PATCH 3/3] mm: Care about shadow stack guard gap when getting an unmapped area

[Lorenzo Stoakes]

On Mon, Sep 02, 2024 at 08:08:15PM GMT, Mark Brown wrote:
> As covered in the commit log for c44357c2e76b ("x86/mm: care about shadow
> stack guard gap during placement") our current mmap() implementation does
> not take care to ensure that a new mapping isn't placed with existing
> mappings inside it's own guard gaps. This is particularly important for
> shadow stacks since if two shadow stacks end up getting placed adjacent to
> each other then they can overflow into each other which weakens the
> protection offered by the feature.
>
> On x86 there is a custom arch_get_unmapped_area() which was updated by the
> above commit to cover this case by specifying a start_gap for allocations
> with VM_SHADOW_STACK. Both arm64 and RISC-V have equivalent features and
> use the generic implementation of arch_get_unmapped_area() so let's make
> the equivalent change there so they also don't get shadow stack pages
> placed without guard pages.

Don't you need to unwind that change in x86 now you're doing it in generic code?

>
> Architectures which do not have this feature will define VM_SHADOW_STACK
> to VM_NONE and hence be unaffected.

Nice.

>
> Suggested-by: Rick Edgecombe <rick.p.edgecombe at intel.com>
> Signed-off-by: Mark Brown <broonie at kernel.org>
> ---
>  mm/mmap.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b06ba847c96e..902c482b6084 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1753,6 +1753,14 @@ static unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
>  	return gap;
>  }
>

Would be nice to take some of the context in commit message as a short comment
describing the function. I mean it's kinda trivially self-documenting obviously,
but it's useful context for somebody wanting to understand _why_ we are doing
this at a glance.

> +static inline unsigned long stack_guard_placement(vm_flags_t vm_flags)
> +{
> +	if (vm_flags & VM_SHADOW_STACK)
> +		return PAGE_SIZE;
> +
> +	return 0;
> +}
> +
>  /*
>   * Search for an unmapped address range.
>   *
> @@ -1814,6 +1822,7 @@ generic_get_unmapped_area(struct file *filp, unsigned long addr,
>  	info.length = len;
>  	info.low_limit = mm->mmap_base;
>  	info.high_limit = mmap_end;
> +	info.start_gap = stack_guard_placement(vm_flags);
>  	return vm_unmapped_area(&info);
>  }
>
> @@ -1863,6 +1872,7 @@ generic_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
>  	info.length = len;
>  	info.low_limit = PAGE_SIZE;
>  	info.high_limit = arch_get_mmap_base(addr, mm->mmap_base);
> +	info.start_gap = stack_guard_placement(vm_flags);
>  	addr = vm_unmapped_area(&info);
>
>  	/*
>
> --
> 2.39.2
>