[v4,12/14] x86/mm: Implement HAVE_ARCH_UNMAPPED_AREA_VMFLAGS

Commit Message

Rick Edgecombe March 26, 2024, 2:16 a.m. UTC

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.

Add x86 arch implementations of arch_get_unmapped_area_vmflags/_topdown()
so future changes can allow the guard gap of type of vma being placed to
be taken into account. This will be used for shadow stack memory.

Signed-off-by: Rick Edgecombe <[e-mail]>
---
v3:
 - Commit log grammar

v2:
 - Remove unnecessary added extern
---
 arch/x86/include/asm/pgtable_64.h |  1 +
 arch/x86/kernel/sys_x86_64.c      | 25 ++++++++++++++++++++-----
 2 files changed, 21 insertions(+), 5 deletions(-)

Patch

diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
index 7e9db77231ac..3c4407271d08 100644
--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -245,6 +245,7 @@  extern void cleanup_highmap(void);
 
 #define HAVE_ARCH_UNMAPPED_AREA
 #define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
+#define HAVE_ARCH_UNMAPPED_AREA_VMFLAGS
 
 #define PAGE_AGP    PAGE_KERNEL_NOCACHE
 #define HAVE_PAGE_AGP 1
diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index 96b9d29aead0..75966afb6251 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -113,8 +113,8 @@  static void find_start_end(unsigned long addr, unsigned long flags,
 }
 
 unsigned long
-arch_get_unmapped_area(struct file *filp, unsigned long addr,
-		unsigned long len, unsigned long pgoff, unsigned long flags)
+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)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
@@ -149,9 +149,9 @@  arch_get_unmapped_area(struct file *filp, unsigned long addr,
 }
 
 unsigned long
-arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
-			  const unsigned long len, const unsigned long pgoff,
-			  const unsigned long flags)
+arch_get_unmapped_area_topdown_vmflags(struct file *filp, unsigned long addr0,
+			  unsigned long len, unsigned long pgoff,
+			  unsigned long flags, vm_flags_t vm_flags)
 {
 	struct vm_area_struct *vma;
 	struct mm_struct *mm = current->mm;
@@ -220,3 +220,18 @@  arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
 	 */
 	return arch_get_unmapped_area(filp, addr0, len, pgoff, flags);
 }
+
+unsigned long
+arch_get_unmapped_area(struct file *filp, unsigned long addr,
+		unsigned long len, unsigned long pgoff, unsigned long flags)
+{
+	return arch_get_unmapped_area_vmflags(filp, addr, len, pgoff, flags, 0);
+}
+
+unsigned long
+arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr,
+			  const unsigned long len, const unsigned long pgoff,
+			  const unsigned long flags)
+{
+	return arch_get_unmapped_area_topdown_vmflags(filp, addr, len, pgoff, flags, 0);
+}