[PATCH 03/46] mm/memfd: check for real shmem inodes before accessing shmem state

[David Hildenbrand (Arm)]

shmem_file() only checks a file's backing mapping. That mapping can belong
to another file, as with a Coda file backed by a tmpfs container after
mmap(). It does not establish that file_inode(file) is a shmem inode.

Seal handling and memfd preservation use this check before accessing
shmem inode state, so they can access an inode of the wrong type.

Add file_inode_is_shmem() to check the inode's file operations in both
paths. This identifies regular shmem inodes, including those used by
memfds and ordinary tmpfs files, independently of wrapper file operations.

Fixes: c49fcfcda8b5 ("mm/shmem: update file sealing comments and file checking")
Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
Cc: stable at vger.kernel.org
Reported-by: Chris Mason <mason at kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 include/linux/shmem_fs.h | 6 ++++++
 mm/memfd.c               | 8 +++++---
 mm/memfd_luo.c           | 2 +-
 mm/shmem.c               | 5 +++++
 4 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index a7c7a96a7cbf9..2ceca7993e4f5 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -117,8 +117,14 @@ extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts);
 #ifdef CONFIG_SHMEM
+bool file_inode_is_shmem(const struct inode *inode);
 bool shmem_mapping(const struct address_space *mapping);
 #else
+static inline bool file_inode_is_shmem(const struct inode *inode)
+{
+	return false;
+}
+
 static inline bool shmem_mapping(const struct address_space *mapping)
 {
 	return false;
diff --git a/mm/memfd.c b/mm/memfd.c
index 0f6fff004f5e4..7d2da23157003 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -211,12 +211,14 @@ static int memfd_wait_for_pins(struct address_space *mapping)
 
 static unsigned int *memfd_file_seals_ptr(struct file *file)
 {
-	if (shmem_file(file))
-		return &SHMEM_I(file_inode(file))->seals;
+	struct inode *inode = file_inode(file);
+
+	if (file_inode_is_shmem(inode))
+		return &SHMEM_I(inode)->seals;
 
 #ifdef CONFIG_HUGETLBFS
 	if (is_file_hugepages(file))
-		return &HUGETLBFS_I(file_inode(file))->seals;
+		return &HUGETLBFS_I(inode)->seals;
 #endif
 
 	return NULL;
diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
index 59de210bee5f9..c0811407ce397 100644
--- a/mm/memfd_luo.c
+++ b/mm/memfd_luo.c
@@ -582,7 +582,7 @@ static bool memfd_luo_can_preserve(struct liveupdate_file_handler *handler,
 {
 	struct inode *inode = file_inode(file);
 
-	return shmem_file(file) && !inode->i_nlink;
+	return file_inode_is_shmem(inode) && !inode->i_nlink;
 }
 
 static unsigned long memfd_luo_get_id(struct file *file)
diff --git a/mm/shmem.c b/mm/shmem.c
index fadf23f9e21ed..0d41aac125df9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -275,6 +275,11 @@ static const struct vm_operations_struct shmem_vm_ops;
 static const struct vm_operations_struct shmem_anon_vm_ops;
 static struct file_system_type shmem_fs_type;
 
+bool file_inode_is_shmem(const struct inode *inode)
+{
+	return inode->i_fop == &shmem_file_operations;
+}
+
 bool shmem_mapping(const struct address_space *mapping)
 {
 	return mapping->a_ops == &shmem_aops;

-- 
2.43.0