mm: use get_unmapped_area_vmflags()

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.

Use mm_get_unmapped_area_vmflags() in the do_mmap() so future changes can
cause shadow stack mappings to be placed with a guard gap.  Also use the
THP variant that takes vm_flags, such that THP shadow stack can get the
same treatment.  Adjust the vm_flags calculation to happen earlier so that
the vm_flags can be passed into __get_unmapped_area().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Rick Edgecombe <[email protected]>
Reviewed-by: Christophe Leroy <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Borislav Petkov (AMD) <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Deepak Gupta <[email protected]>
Cc: Guo Ren <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: H. Peter Anvin (Intel) <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Naveen N. Rao <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: rpedgeco

include/linux/mm.h

@@ -3413,7 +3413,16 @@ extern int install_special_mapping(struct mm_struct *mm,
 unsigned long randomize_stack_top(unsigned long stack_top);
 unsigned long randomize_page(unsigned long start, unsigned long range);
 
-extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
+unsigned long
+__get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
+		    unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags);
+
+static inline unsigned long
+get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
+		  unsigned long pgoff, unsigned long flags)
+{
+	return __get_unmapped_area(file, addr, len, pgoff, flags, 0);
+}
 
 extern unsigned long mmap_region(struct file *file, unsigned long addr,
 	unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,

mm/mmap.c

@@ -1255,18 +1255,6 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 	if (mm->map_count > sysctl_max_map_count)
 		return -ENOMEM;
 
-	/* Obtain the address to map to. we verify (or select) it and ensure
-	 * that it represents a valid section of the address space.
-	 */
-	addr = get_unmapped_area(file, addr, len, pgoff, flags);
-	if (IS_ERR_VALUE(addr))
-		return addr;
-
-	if (flags & MAP_FIXED_NOREPLACE) {
-		if (find_vma_intersection(mm, addr, addr + len))
-			return -EEXIST;
-	}
-
 	if (prot == PROT_EXEC) {
 		pkey = execute_only_pkey(mm);
 		if (pkey < 0)
@@ -1280,6 +1268,18 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 	vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
 			mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
 
+	/* Obtain the address to map to. we verify (or select) it and ensure
+	 * that it represents a valid section of the address space.
+	 */
+	addr = __get_unmapped_area(file, addr, len, pgoff, flags, vm_flags);
+	if (IS_ERR_VALUE(addr))
+		return addr;
+
+	if (flags & MAP_FIXED_NOREPLACE) {
+		if (find_vma_intersection(mm, addr, addr + len))
+			return -EEXIST;
+	}
+
 	if (flags & MAP_LOCKED)
 		if (!can_do_mlock())
 			return -EPERM;
@@ -1836,8 +1836,8 @@ unsigned long mm_get_unmapped_area_vmflags(struct mm_struct *mm, struct file *fi
 }
 
 unsigned long
-get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
-		unsigned long pgoff, unsigned long flags)
+__get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
+		unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
 {
 	unsigned long (*get_area)(struct file *, unsigned long,
 				  unsigned long, unsigned long, unsigned long)
@@ -1872,8 +1872,8 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
 	if (get_area)
 		addr = get_area(file, addr, len, pgoff, flags);
 	else
-		addr = mm_get_unmapped_area(current->mm, file, addr, len,
-					    pgoff, flags);
+		addr = mm_get_unmapped_area_vmflags(current->mm, file, addr, len,
+						    pgoff, flags, vm_flags);
 	if (IS_ERR_VALUE(addr))
 		return addr;