Message-ID: <20250902104533.222730-1-lorenzo.stoakes@oracle.com>
Date: Tue,  2 Sep 2025 11:45:33 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: [PATCH] mm: do not assume file == vma->vm_file in compat_vma_mmap_prepare()

In commit bb666b7c2707 ("mm: add mmap_prepare() compatibility layer for
nested file systems") we introduced the ability for 'nested' drivers and
file systems to correctly invoke the f_op->mmap_prepare() handler from an
f_op->mmap() handler via a compatibility layer implemented in
compat_vma_mmap_prepare().

This invokes vma_to_desc() to populate vm_area_desc fields according to
those found in the (not yet fully initialised) VMA passed to f_op->mmap().

However this function implicitly assumes that the struct file which we are
operating upon is equal to vma->vm_file. This is not a safe assumption in
all cases.

This is not an issue currently, as so far we have only implemented
f_op->mmap_prepare() handlers for some file systems and internal mm uses,
and the only nested f_op->mmap() operations that can be performed upon
these are those in backing_file_mmap() and coda_file_mmap(), both of which
use vma->vm_file.

However, moving forward, as we convert drivers to using
f_op->mmap_prepare(), this will become a problem.

Resolve this issue by explicitly setting desc->file to the provided file
parameter and update callers accordingly.

We also need to adjust set_vma_from_desc() to account for this fact, and
only update the vma->vm_file field if the f_op->mmap_prepare() caller
reassigns it.

We may in future wish to add a new field to struct vm_area_desc to account
for this 'nested mmap invocation' case, but for now it seems unnecessary.

While we are here, also provide a variant of compat_vma_mmap_prepare() that
operates against a pointer to any file_operations struct and does not
assume that the file_operations struct we are interested in is file->f_op.

This function is __compat_vma_mmap_prepare() and we invoke it from
compat_vma_mmap_prepare() so that we share code between the two functions.

This is important, because some drivers provide hooks in a separate struct,
for instance struct drm_device provides an fops field for this purpose.

Also update the VMA selftests accordingly.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 include/linux/fs.h               |  2 ++
 mm/util.c                        | 33 +++++++++++++++++++++++---------
 mm/vma.h                         | 14 ++++++++++----
 tools/testing/vma/vma_internal.h | 19 +++++++++++-------
 4 files changed, 48 insertions(+), 20 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index d7ab4f96d705..3e7160415066 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2279,6 +2279,8 @@ static inline bool can_mmap_file(struct file *file)
 	return true;
 }
 
+int __compat_vma_mmap_prepare(const struct file_operations *f_op,
+		struct file *file, struct vm_area_struct *vma);
 int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma);
 
 static inline int vfs_mmap(struct file *file, struct vm_area_struct *vma)
diff --git a/mm/util.c b/mm/util.c
index bb4b47cd6709..83fe15e4483a 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1133,6 +1133,29 @@ void flush_dcache_folio(struct folio *folio)
 EXPORT_SYMBOL(flush_dcache_folio);
 #endif
 
+/**
+ * __compat_vma_mmap_prepare() - See description for compat_vma_mmap_prepare()
+ * for details. This is the same operation, only with a specific file operations
+ * struct which may or may not be the same as vma->vm_file->f_op.
+ * @f_op - The file operations whose .mmap_prepare() hook is specified.
+ * @vma: The VMA to apply the .mmap_prepare() hook to.
+ * Returns: 0 on success or error.
+ */
+int __compat_vma_mmap_prepare(const struct file_operations *f_op,
+		struct file *file, struct vm_area_struct *vma)
+{
+	struct vm_area_desc desc;
+	int err;
+
+	err = f_op->mmap_prepare(vma_to_desc(vma, file, &desc));
+	if (err)
+		return err;
+	set_vma_from_desc(vma, file, &desc);
+
+	return 0;
+}
+EXPORT_SYMBOL(__compat_vma_mmap_prepare);
+
 /**
  * compat_vma_mmap_prepare() - Apply the file's .mmap_prepare() hook to an
  * existing VMA
@@ -1161,15 +1184,7 @@ EXPORT_SYMBOL(flush_dcache_folio);
  */
 int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
 {
-	struct vm_area_desc desc;
-	int err;
-
-	err = file->f_op->mmap_prepare(vma_to_desc(vma, &desc));
-	if (err)
-		return err;
-	set_vma_from_desc(vma, &desc);
-
-	return 0;
+	return __compat_vma_mmap_prepare(file->f_op, file, vma);
 }
 EXPORT_SYMBOL(compat_vma_mmap_prepare);
 
diff --git a/mm/vma.h b/mm/vma.h
index bcdc261c5b15..9b21d47ba630 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -230,14 +230,14 @@ static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
  */
 
 static inline struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
-		struct vm_area_desc *desc)
+		struct file *file, struct vm_area_desc *desc)
 {
 	desc->mm = vma->vm_mm;
 	desc->start = vma->vm_start;
 	desc->end = vma->vm_end;
 
 	desc->pgoff = vma->vm_pgoff;
-	desc->file = vma->vm_file;
+	desc->file = file;
 	desc->vm_flags = vma->vm_flags;
 	desc->page_prot = vma->vm_page_prot;
 
@@ -248,7 +248,7 @@ static inline struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
 }
 
 static inline void set_vma_from_desc(struct vm_area_struct *vma,
-		struct vm_area_desc *desc)
+		struct file *orig_file, struct vm_area_desc *desc)
 {
 	/*
 	 * Since we're invoking .mmap_prepare() despite having a partially
@@ -258,7 +258,13 @@ static inline void set_vma_from_desc(struct vm_area_struct *vma,
 
 	/* Mutable fields. Populated with initial state. */
 	vma->vm_pgoff = desc->pgoff;
-	if (vma->vm_file != desc->file)
+	/*
+	 * The desc->file may not be the same as vma->vm_file, but if the
+	 * f_op->mmap_prepare() handler is setting this parameter to something
+	 * different, it indicates that it wishes the VMA to have its file
+	 * assigned to this.
+	 */
+	if (orig_file != desc->file && vma->vm_file != desc->file)
 		vma_set_file(vma, desc->file);
 	if (vma->vm_flags != desc->vm_flags)
 		vm_flags_set(vma, desc->vm_flags);
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index 6f95ec14974f..4ceb4284b6b9 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -1411,25 +1411,30 @@ static inline void free_anon_vma_name(struct vm_area_struct *vma)
 
 /* Declared in vma.h. */
 static inline void set_vma_from_desc(struct vm_area_struct *vma,
-		struct vm_area_desc *desc);
-
+		struct file *orig_file, struct vm_area_desc *desc);
 static inline struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
-		struct vm_area_desc *desc);
+		struct file *file, struct vm_area_desc *desc);
 
-static int compat_vma_mmap_prepare(struct file *file,
-		struct vm_area_struct *vma)
+static inline int __compat_vma_mmap_prepare(const struct file_operations *f_op,
+		struct file *file, struct vm_area_struct *vma)
 {
 	struct vm_area_desc desc;
 	int err;
 
-	err = file->f_op->mmap_prepare(vma_to_desc(vma, &desc));
+	err = f_op->mmap_prepare(vma_to_desc(vma, file, &desc));
 	if (err)
 		return err;
-	set_vma_from_desc(vma, &desc);
+	set_vma_from_desc(vma, file, &desc);
 
 	return 0;
 }
 
+static inline int compat_vma_mmap_prepare(struct file *file,
+		struct vm_area_struct *vma)
+{
+	return __compat_vma_mmap_prepare(file->f_op, file, vma);
+}
+
 /* Did the driver provide valid mmap hook configuration? */
 static inline bool can_mmap_file(struct file *file)
 {
-- 
2.50.1

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