mm/mremap: catch invalid multi VMA moves earlier

Previously, any attempt to solely move a VMA would require that the
span specified reside within the span of that single VMA, with no gaps
before or afterwards.

After commit d23cb64 ("mm/mremap: permit mremap() move of multiple
VMAs"), the multi VMA move permitted a gap to exist only after VMAs. 
This was done to provide maximum flexibility.

However, We have consequently permitted this behaviour for the move of
a single VMA including those not eligible for multi VMA move.

The change introduced here means that we no longer permit non-eligible
VMAs from being moved in this way.

This is consistent, as it means all eligible VMA moves are treated the
same, and all non-eligible moves are treated as they were before.

This change does not break previous behaviour, which equally would have
disallowed such a move (only in all cases).

[lorenzo.stoakes@oracle.com: do not incorrectly reference invalid VMA in VM_WARN_ON_ONCE()]
  Link: https://lkml.kernel.org/r/b6dbda20-667e-4053-abae-8ed4fa84bb6c@lucifer.local
Link: https://lkml.kernel.org/r/2b5aad5681573be85b5b8fac61399af6fb6b68b6.1754218667.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ljskernel

mm/mremap.c

@@ -1820,10 +1820,11 @@ static unsigned long remap_move(struct vma_remap_struct *vrm)
 	unsigned long start = vrm->addr;
 	unsigned long end = vrm->addr + vrm->old_len;
 	unsigned long new_addr = vrm->new_addr;
-	bool allowed = true, seen_vma = false;
 	unsigned long target_addr = new_addr;
 	unsigned long res = -EFAULT;
 	unsigned long last_end;
+	bool seen_vma = false;
+
 	VMA_ITERATOR(vmi, current->mm, start);
 
 	/*
@@ -1836,9 +1837,7 @@ static unsigned long remap_move(struct vma_remap_struct *vrm)
 		unsigned long addr = max(vma->vm_start, start);
 		unsigned long len = min(end, vma->vm_end) - addr;
 		unsigned long offset, res_vma;
-
-		if (!allowed)
-			return -EFAULT;
+		bool multi_allowed;
 
 		/* No gap permitted at the start of the range. */
 		if (!seen_vma && start < vma->vm_start)
@@ -1867,9 +1866,15 @@ static unsigned long remap_move(struct vma_remap_struct *vrm)
 		vrm->new_addr = target_addr + offset;
 		vrm->old_len = vrm->new_len = len;
 
-		allowed = vma_multi_allowed(vma);
-		if (seen_vma && !allowed)
-			return -EFAULT;
+		multi_allowed = vma_multi_allowed(vma);
+		if (!multi_allowed) {
+			/* This is not the first VMA, abort immediately. */
+			if (seen_vma)
+				return -EFAULT;
+			/* This is the first, but there are more, abort. */
+			if (vma->vm_end < end)
+				return -EFAULT;
+		}
 
 		res_vma = check_prep_vma(vrm);
 		if (!res_vma)
@@ -1878,7 +1883,7 @@ static unsigned long remap_move(struct vma_remap_struct *vrm)
 			return res_vma;
 
 		if (!seen_vma) {
-			VM_WARN_ON_ONCE(allowed && res_vma != new_addr);
+			VM_WARN_ON_ONCE(multi_allowed && res_vma != new_addr);
 			res = res_vma;
 		}