[PATCH] mm: account for MTE in arm64 on mmap_region() operation

[Lorenzo Stoakes]

Correctly account for MTE on mmap_region(). We need to check this ahead of
the operation, the shmem mmap hook was doing it, but this is at a point
where a failure would mean we'd have to tear down a partially installed
VMA.

Avoid all this by adding a function to specifically handle this case.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
 mm/mmap.c  | 20 ++++++++++++++++++++
 mm/shmem.c |  3 ---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 8462de1ee583..83afa1ebfd75 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1575,6 +1575,24 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 	return error;
 }

+/*
+ * We check VMA flag validity early in the mmap() process, however this can
+ * cause issues for arm64 when using MTE, which requires that it be used with
+ * shmem and in this instance and only then is VM_MTE_ALLOWED set permitting
+ * this operation.
+ *
+ * To avoid having to tear down a partially complete mapping we do this ahead of
+ * time.
+ */
+static vm_flags_t arch_adjust_flags(struct file *file, vm_flags_t vm_flags)
+{
+	if (!IS_ENABLED(CONFIG_ARM64))
+		return vm_flags;
+
+	if (shmem_file(file))
+		return vm_flags | VM_MTE_ALLOWED;
+}
+
 unsigned long mmap_region(struct file *file, unsigned long addr,
 			  unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
 			  struct list_head *uf)
@@ -1586,6 +1604,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	if (map_deny_write_exec(vm_flags, vm_flags))
 		return -EACCES;

+	vm_flags = arch_adjust_flags(file, vm_flags);
+
 	/* Allow architectures to sanity-check the vm_flags. */
 	if (!arch_validate_flags(vm_flags))
 		return -EINVAL;
diff --git a/mm/shmem.c b/mm/shmem.c
index 4ba1d00fabda..e87f5d6799a7 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2733,9 +2733,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 	if (ret)
 		return ret;

-	/* arm64 - allow memory tagging on RAM-based files */
-	vm_flags_set(vma, VM_MTE_ALLOWED);
-
 	file_accessed(file);
 	/* This is anonymous shared memory if it is unlinked at the time of mmap */
 	if (inode->i_nlink)
--