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

[Deepak Gupta <debug@rivosinc.com>]

On Mon, Sep 02, 2024 at 08:08:15PM +0100, 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.
>
>Architectures which do not have this feature will define VM_SHADOW_STACK
>to VM_NONE and hence be unaffected.
>
>Suggested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>Signed-off-by: Mark Brown <broonie@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;
> }
>
>+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);
>
> 	/*
>

lgtm

Reviewed-by: Deepak Gupta <debug@rivosinc.com>

>-- 
>2.39.2
>