[PATCH 6.6.y] mm/vma: fix expansion of VMA_MIXEDMAP_BIT VMAs

[Sasha Levin <sashal@kernel.org>]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

[ Upstream commit 8146c6478224d8f6395c6b08a19c344a56d79867 ]

Commit ca3d76b0aa80 ("mm: add merging after mremap resize") combined
merging with expansion of mremap()'d VMAs, however it also unintentionally
disallowed the expansion of unmergeable VMAs.

Previously it was possible to expand a mixed mapping which mremap() had
already ensured was a legal expansion.

This is clearly seen in the mremap() logic, as it checks
vma_is_fixed_mapping() to determine legality of an expansion rather than
vma_can_merge().

Additionally, copy_vma() allows for a failed merge to result in a new VMA
being instantiated if MREMAP_MAYMOVE is specified.

However, this is not attempted, as a failure in vma_merge_extend() is
assumed to be an -ENOMEM failure.

Since vma_merge_extend() is used exclusively by mremap() to perform a merge
and/or expansion, resolve the issue by invoking vma_expand() for
unmergeable VMAs there.

Explicitly assert that the VMA is both non-fixed and a mixed mapping.
This provides double insurance against broken changes in future.

Note that khugepaged_enter_vma() (which vma_merge_new_range() runs) is
redundant in this case, as a mixed mapping is not subject to khugepaged.

Also update the VMA userland tests to add a required predicate.

Link: https://lore.kernel.org/20261023-vma-expansion-fix-v1-1-6142baf1f46b@kernel.org
Fixes: ca3d76b0aa80 ("mm: add merging after mremap resize")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Luiz Capitulino <luizcap@redhat.com>
Closes: https://lore.kernel.org/all/20261023023106.515365-1-luizcap@redhat.com/
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jakub Matěna <matenajakub@gmail.com>
Cc: <stable@vger.kernel.org>
[ sashal: Adapted vma_merge_extend() changes to the inline mremap() implementation using the older vma_expand() interface. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/mremap.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 0739a31b54eed..a37bcfda29261 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1120,10 +1120,21 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 			 * the next vma if it becomes adjacent to the expanded
 			 * vma and  otherwise compatible.
 			 */
-			vma = vma_merge(&vmi, mm, vma, extension_start,
-				extension_end, vma->vm_flags, vma->anon_vma,
-				vma->vm_file, extension_pgoff, vma_policy(vma),
-				vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+			if (!(vma->vm_flags & (VM_PFNMAP | VM_DONTEXPAND |
+					      VM_MIXEDMAP))) {
+				vma = vma_merge(&vmi, mm, vma, extension_start,
+					extension_end, vma->vm_flags, vma->anon_vma,
+					vma->vm_file, extension_pgoff, vma_policy(vma),
+					vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+			} else {
+				/* Mixed mappings are permitted to be expanded, not merged. */
+				VM_WARN_ON_ONCE(vma->vm_flags & (VM_PFNMAP | VM_DONTEXPAND));
+				VM_WARN_ON_ONCE(!(vma->vm_flags & VM_MIXEDMAP));
+				vma_prev(&vmi); /* Reposition to start of VMA. */
+				if (vma_expand(&vmi, vma, vma->vm_start,
+					       extension_end, vma->vm_pgoff, NULL))
+					vma = NULL;
+			}
 			if (!vma) {
 				vm_unacct_memory(pages);
 				ret = -ENOMEM;
-- 
2.53.0