[PATCH v2 3/3] mm: Care about shadow stack guard gap when getting an unmapped area

[Mark Brown]

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. x86 uses a single page guard, this is also
sufficient for arm64 where we either do single word pops and pushes or
unconstrained writes.

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@xxxxxxxxx>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
 mm/mmap.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index b06ba847c96e..050c5ae2f80f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1753,6 +1753,18 @@ static unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
 	return gap;
 }
 
+/*
+ * Determine if the allocation needs to ensure that there is no
+ * existing mapping within it's guard gaps, for use as start_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 +1826,7 @@ 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 +1876,7 @@ 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