mm/munmap: replace can_modify_mm with can_modify_vma

We were doing an extra mmap tree traversal just to check if the entire
range is modifiable.  This can be done when we iterate through the VMAs
instead.

Link: https://lkml.kernel.org/r/20240817-mseal-depessimize-v3-2-d8d2e037df30@gmail.comSigned-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
LGTM, Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -1740,16 +1740,7 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		unsigned long start, unsigned long end, struct list_head *uf,
 		bool unlock)
 {
-	struct mm_struct *mm = vma->vm_mm;
-
-	/*
-	 * Check if memory is sealed, prevent unmapping a sealed VMA.
-	 * can_modify_mm assumes we have acquired the lock on MM.
-	 */
-	if (unlikely(!can_modify_mm(mm, start, end)))
-		return -EPERM;
-
-	return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
+	return do_vmi_align_munmap(vmi, vma, vma->vm_mm, start, end, uf, unlock);
 }
 
 /*

mm/vma.c

@@ -712,6 +712,12 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
 			goto map_count_exceeded;
 
+		/* Don't bother splitting the VMA if we can't unmap it anyway */
+		if (!can_modify_vma(vma)) {
+			error = -EPERM;
+			goto start_split_failed;
+		}
+
 		error = __split_vma(vmi, vma, start, 1);
 		if (error)
 			goto start_split_failed;
@@ -723,6 +729,11 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	 */
 	next = vma;
 	do {
+		if (!can_modify_vma(next)) {
+			error = -EPERM;
+			goto modify_vma_failed;
+		}
+
 		/* Does it split the end? */
 		if (next->vm_end > end) {
 			error = __split_vma(vmi, next, end, 0);
@@ -815,6 +826,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	__mt_destroy(&mt_detach);
 	return 0;
 
+modify_vma_failed:
 clear_tree_failed:
 userfaultfd_error:
 munmap_gather_failed:
@@ -860,13 +872,6 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
 	if (end == start)
 		return -EINVAL;
 
-	/*
-	 * Check if memory is sealed, prevent unmapping a sealed VMA.
-	 * can_modify_mm assumes we have acquired the lock on MM.
-	 */
-	if (unlikely(!can_modify_mm(mm, start, end)))
-		return -EPERM;
-
 	/* Find the first overlapping VMA */
 	vma = vma_find(vmi, end);
 	if (!vma) {