[RFC PATCH 7/8] x86/mm: Care about shadow stack guard gap during placement

[Rick Edgecombe]

When memory is being placed, mmap() will take care to respect the guard
gaps of certain types of memory (VM_SHADOWSTACK, VM_GROWSUP and
VM_GROWSDOWN). In order to ensure guard gaps between mappings, mmap()
needs to consider two things:
 1. That the new mapping isn’t placed in an any existing mappings guard
    gaps.
 2. That the new mapping isn’t placed such that any existing mappings
    are not in *its* guard gaps.

The long standing behavior of mmap() is to ensure 1, but not take any care
around 2. So for example, if there is a PAGE_SIZE free area, and a
mmap() with a PAGE_SIZE size, and a type that has a guard gap is being
placed, mmap() may place the shadow stack in the PAGE_SIZE free area. Then
the mapping that is supposed to have a guard gap will not have a gap to
the adjacent VMA.

Now that the vm_flags is passed into the arch get_unmapped_area()'s, and
vm_unmapped_area() is ready to consider it, have VM_SHADOW_STACK's get
guard gap consideration for scenario 2.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---
 arch/x86/kernel/sys_x86_64.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index f92780cf9662..3b78fdc235fc 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -119,6 +119,14 @@ static void find_start_end(unsigned long addr, unsigned long flags,
 		*end = task_size_64bit(addr > DEFAULT_MAP_WINDOW);
 }
 
+static inline unsigned long stack_guard_placement(vm_flags_t vm_flags)
+{
+	if (vm_flags & VM_SHADOW_STACK)
+		return PAGE_SIZE;
+
+	return 0;
+}
+
 extern unsigned long
 arch_get_unmapped_area_vmflags(struct file *filp, unsigned long addr, unsigned long len,
 		       unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
@@ -144,12 +152,13 @@ arch_get_unmapped_area_vmflags(struct file *filp, unsigned long addr, unsigned l
 			return addr;
 	}
 
-	info.flags = 0;
+	info.flags = VM_UNMAPPED_START_GAP_SET;
 	info.length = len;
 	info.low_limit = begin;
 	info.high_limit = end;
 	info.align_mask = 0;
 	info.align_offset = pgoff << PAGE_SHIFT;
+	info.start_gap = stack_guard_placement(vm_flags);
 	if (filp) {
 		info.align_mask = get_align_mask();
 		info.align_offset += get_align_bits();
@@ -191,7 +200,7 @@ arch_get_unmapped_area_topdown_vmflags(struct file *filp, unsigned long addr0,
 	}
 get_unmapped_area:
 
-	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
+	info.flags = VM_UNMAPPED_AREA_TOPDOWN | VM_UNMAPPED_START_GAP_SET;
 	info.length = len;
 	if (!in_32bit_syscall() && (flags & MAP_ABOVE4G))
 		info.low_limit = SZ_4G;
@@ -199,6 +208,7 @@ arch_get_unmapped_area_topdown_vmflags(struct file *filp, unsigned long addr0,
 		info.low_limit = PAGE_SIZE;
 
 	info.high_limit = get_mmap_base(0);
+	info.start_gap = stack_guard_placement(vm_flags);
 
 	/*
 	 * If hint address is above DEFAULT_MAP_WINDOW, look for unmapped area
-- 
2.34.1