[PATCH 6.17 2/3] mm/mremap: catch invalid multi VMA moves earlier

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

In remap_move() we must account for both a single VMA case, where we are
permitted to move a single VMA regardless of multi-VMA move eligiblity, and
multiple VMAs which, of course, must be eligible for such an operation.

We determine this via vma_multi_allowed().

Currently, if the first VMA is not eligible, but others are, we will move
the first then return an error. This is not ideal, as we are performing an
operation which we don't need to do which has an impact on the memory
mapping.

We can very easily determine if this is a multi VMA move prior to the move
of the first VMA, by checking vma->vm_end vs. the specified end address.

Therefore this patch does so, and as a result eliminates unnecessary logic
around tracking whether the first VMA was permitted or not.

This is most useful for cases where a user attempts to erroneously move
mutliple VMAs which are not eligible for non-transient reasons - for
instance, UFFD-armed VMAs, or file-backed VMAs backed by a file system or
driver which specifies a custom f_op->get_unmapped_area.

In the less likely instance of a failure due to transient issues such as
out of memory or mapping limits being hit, the issue is already likely
fatal and so the fact the operation may be partially complete is
acceptable.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/mremap.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 46f9f3160dff..f61a9ea0b244 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1816,10 +1816,11 @@ 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);
 
 	/*
@@ -1833,9 +1834,6 @@ static unsigned long remap_move(struct vma_remap_struct *vrm)
 		unsigned long len = min(end, vma->vm_end) - addr;
 		unsigned long offset, res_vma;
 
-		if (!allowed)
-			return -EFAULT;
-
 		/* No gap permitted at the start of the range. */
 		if (!seen_vma && start < vma->vm_start)
 			return -EFAULT;
@@ -1863,9 +1861,14 @@ 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;
+		if (!vma_multi_allowed(vma)) {
+			/* 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)
@@ -1874,7 +1877,8 @@ 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(vma_multi_allowed(vma) &&
+					res_vma != new_addr);
 			res = res_vma;
 		}
 
-- 
2.50.1