[PATCH v2] userfaultfd: validate the whole range before changing write protection

[Umang Pokhriyal]

mwriteprotect_range() checks each VMA as it walks the range.  When a later
VMA is not registered for write protection, or is a hugetlb VMA the range
is not aligned to, the call fails after the earlier VMAs were already
changed.  When unprotecting, userfaultfd_writeprotect() then skips
wake_userfault(), so threads blocked on those pages are not woken although
the pages are writable again.

An unprivileged process can trigger this with a range that spans an
unregistered VMA.  It hasn't been seen in practice; it was found while
reviewing the userfaultfd range ioctls with an LLM.

Validate the whole range first, as mrwprotect_range() already does.

Fixes: a1b92a3f1498 ("mm/userfaultfd: support WP on multiple VMAs")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Umang Pokhriyal <umangpokhriyall@xxxxxxxxx>
---
v2:
- drop the redundant error assignments in the validation loop (Mike)
- drop the selftest (Mike)

v1: https://lore.kernel.org/all/20261102103621.156928-1-umangpokhriyall@xxxxxxxxx/

 mm/userfaultfd.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 87da92526..1b7320d0a 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1105,21 +1105,26 @@ static int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
 	if (atomic_read(&ctx->mmap_changing))
 		goto out_unlock;
 
+	/*
+	 * Validate every spanned VMA before changing any of them, so an
+	 * invalid range is rejected without changing part of it.
+	 */
 	err = -ENOENT;
 	for_each_vma_range(vmi, dst_vma, end) {
-
-		if (!userfaultfd_wp(dst_vma)) {
-			err = -ENOENT;
-			break;
-		}
+		if (!userfaultfd_wp(dst_vma))
+			goto out_unlock;
 
 		if (vma_is_hugetlb(dst_vma)) {
-			err = -EINVAL;
 			page_mask = vma_kernel_pagesize(dst_vma) - 1;
-			if ((start & page_mask) || (len & page_mask))
-				break;
+			if ((start & page_mask) || (len & page_mask)) {
+				err = -EINVAL;
+				goto out_unlock;
+			}
 		}
+	}
 
+	vma_iter_set(&vmi, start);
+	for_each_vma_range(vmi, dst_vma, end) {
 		_start = max(dst_vma->vm_start, start);
 		_end = min(dst_vma->vm_end, end);
 

base-commit: 74d73ffcffe03804f97e9fb82301dadf75671282
-- 
2.53.0