[PATCH v2] mm/mremap: fix regression in vrm->new_addr check

[Carlos Llamas <cmllamas@google.com>]

Commit 3215eaceca87 ("mm/mremap: refactor initial parameter sanity
checks") moved the sanity check for vrm->new_addr from mremap_to() to
check_mremap_params().

However, this caused a regression as vrm->new_addr is now checked even
when MREMAP_FIXED and MREMAP_DONTUNMAP flags are not specified. In this
case, vrm->new_addr can be garbage and create unexpected failures.

Fix this by moving the new_addr check after the vrm_implies_new_addr()
guard. This ensures that the new_addr is only checked when the user has
specified one explicitly.

Cc: stable@vger.kernel.org
Fixes: 3215eaceca87 ("mm/mremap: refactor initial parameter sanity checks")
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
v2:
 - split out vrm->new_len into individual checks
 - cc stable, collect tags

v1:
https://lore.kernel.org/all/20250828032653.521314-1-cmllamas@google.com/

 mm/mremap.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index e618a706aff5..35de0a7b910e 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1774,15 +1774,18 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
 	if (!vrm->new_len)
 		return -EINVAL;
 
-	/* Is the new length or address silly? */
-	if (vrm->new_len > TASK_SIZE ||
-	    vrm->new_addr > TASK_SIZE - vrm->new_len)
+	/* Is the new length silly? */
+	if (vrm->new_len > TASK_SIZE)
 		return -EINVAL;
 
 	/* Remainder of checks are for cases with specific new_addr. */
 	if (!vrm_implies_new_addr(vrm))
 		return 0;
 
+	/* Is the new address silly? */
+	if (vrm->new_addr > TASK_SIZE - vrm->new_len)
+		return -EINVAL;
+
 	/* The new address must be page-aligned. */
 	if (offset_in_page(vrm->new_addr))
 		return -EINVAL;
-- 
2.51.0.268.g9569e192d0-goog