Re: [PATCH 04/15] mm: add vm_ops->mapped hook

Usama Arif
2026-03-13T11:03:03Z

On Thu, 12 Mar 2026 20:27:19 +0000 "Lorenzo Stoakes (Oracle)" <ljs@kernel.org> wrote:

> Previously, when a driver needed to do something like establish a reference
> count, it could do so in the mmap hook in the knowledge that the mapping
> would succeed.
>
> With the introduction of f_op->mmap_prepare this is no longer the case, as
> it is invoked prior to actually establishing the mapping.
>
> To take this into account, introduce a new vm_ops->mapped callback which is
> invoked when the VMA is first mapped (though notably - not when it is
> merged - which is correct and mirrors existing mmap/open/close behaviour).
>
> We do better that vm_ops->open() here, as this callback can return an
> error, at which point the VMA will be unmapped.
>
> Note that vm_ops->mapped() is invoked after any mmap action is
> complete (such as I/O remapping).
>
> We intentionally do not expose the VMA at this point, exposing only the
> fields that could be used, and an output parameter in case the operation
> needs to update the vma->vm_private_data field.
>
> In order to deal with stacked filesystems which invoke inner filesystem's
> mmap() invocations, add __compat_vma_mapped() and invoke it on
> vfs_mmap() (via compat_vma_mmap()) to ensure that the mapped callback is
> handled when an mmap() caller invokes a nested filesystem's mmap_prepare()
> callback.
>
> We can now also remove call_action_complete() and invoke
> mmap_action_complete() directly, as we separate out the rmap lock logic to
> be called in __mmap_region() instead via maybe_drop_file_rmap_lock().
>
> We also abstract unmapping of a VMA on mmap action completion into its own
> helper function, unmap_vma_locked().
>
> Additionally, update VMA userland test headers to reflect the change.
>
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> ---
> include/linux/fs.h | 9 +++-
> include/linux/mm.h | 17 +++++++
> mm/internal.h | 10 ++++
> mm/util.c | 86 ++++++++++++++++++++++++---------
> mm/vma.c | 41 +++++++++++-----
> tools/testing/vma/include/dup.h | 34 ++++++++++++-
> 6 files changed, 158 insertions(+), 39 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index a2628a12bd2b..c390f5c667e3 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2059,13 +2059,20 @@ static inline bool can_mmap_file(struct file *file)
> }
>
> int compat_vma_mmap(struct file *file, struct vm_area_struct *vma);
> +int __vma_check_mmap_hook(struct vm_area_struct *vma);
>
> static inline int vfs_mmap(struct file *file, struct vm_area_struct *vma)
> {
> + int err;
> +
> if (file->f_op->mmap_prepare)
> return compat_vma_mmap(file, vma);
>
> - return file->f_op->mmap(file, vma);
> + err = file->f_op->mmap(file, vma);
> + if (err)
> + return err;
> +
> + return __vma_check_mmap_hook(vma);
> }
>
> static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 12a0b4c63736..7333d5db1221 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -759,6 +759,23 @@ struct vm_operations_struct {
> * Context: User context. May sleep. Caller holds mmap_lock.
> */
> void (*close)(struct vm_area_struct *vma);
> + /**
> + * @mapped: Called when the VMA is first mapped in the MM. Not called if
> + * the new VMA is merged with an adjacent VMA.
> + *
> + * The @vm_private_data field is an output field allowing the user to
> + * modify vma->vm_private_data as necessary.
> + *
> + * ONLY valid if set from f_op->mmap_prepare. Will result in an error if
> + * set from f_op->mmap.
> + *
> + * Returns %0 on success, or an error otherwise. On error, the VMA will
> + * be unmapped.
> + *
> + * Context: User context. May sleep. Caller holds mmap_lock.
> + */
> + int (*mapped)(unsigned long start, unsigned long end, pgoff_t pgoff,
> + const struct file *file, void **vm_private_data);
> /* Called any time before splitting to check if it's allowed */
> int (*may_split)(struct vm_area_struct *vma, unsigned long addr);
> int (*mremap)(struct vm_area_struct *vma);
> diff --git a/mm/internal.h b/mm/internal.h
> index 7bfa85b5e78b..f0f2cf1caa36 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -158,6 +158,8 @@ static inline void *folio_raw_mapping(const struct folio *folio)
> * mmap hook and safely handle error conditions. On error, VMA hooks will be
> * mutated.
> *
> + * IMPORTANT: f_op->mmap() is deprecated, prefer f_op->mmap_prepare().
> + *
> * @file: File which backs the mapping.
> * @vma: VMA which we are mapping.
> *
> @@ -201,6 +203,14 @@ static inline void vma_close(struct vm_area_struct *vma)
> /* unmap_vmas is in mm/memory.c */
> void unmap_vmas(struct mmu_gather *tlb, struct unmap_desc *unmap);
>
> +static inline void unmap_vma_locked(struct vm_area_struct *vma)
> +{
> + const size_t len = vma_pages(vma) << PAGE_SHIFT;
> +
> + mmap_assert_locked(vma->vm_mm);
> + do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
> +}
> +
> #ifdef CONFIG_MMU
>
> static inline void get_anon_vma(struct anon_vma *anon_vma)
> diff --git a/mm/util.c b/mm/util.c
> index dba1191725b6..2b0ed54008d6 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1163,6 +1163,55 @@ void flush_dcache_folio(struct folio *folio)
> EXPORT_SYMBOL(flush_dcache_folio);
> #endif
>
> +static int __compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + struct vm_area_desc desc = {
> + .mm = vma->vm_mm,
> + .file = file,
> + .start = vma->vm_start,
> + .end = vma->vm_end,
> +
> + .pgoff = vma->vm_pgoff,
> + .vm_file = vma->vm_file,
> + .vma_flags = vma->flags,
> + .page_prot = vma->vm_page_prot,
> +
> + .action.type = MMAP_NOTHING, /* Default */
> + };
> + int err;
> +
> + err = vfs_mmap_prepare(file, &desc);
> + if (err)
> + return err;
> +
> + err = mmap_action_prepare(&desc, &desc.action);
> + if (err)
> + return err;
> +
> + set_vma_from_desc(vma, &desc);
> + return mmap_action_complete(vma, &desc.action);
> +}
> +
> +static int __compat_vma_mapped(struct file *file, struct vm_area_struct *vma)
> +{
> + const struct vm_operations_struct *vm_ops = vma->vm_ops;
> + void *vm_private_data = vma->vm_private_data;
> + int err;
> +
> + if (!vm_ops->mapped)
> + return 0;
> +

Hello!

Can vm_ops be NULL here? __compat_vma_mapped() is called from
compat_vma_mmap(), which is reached when a filesystem provides
mmap_prepare. If the mmap_prepare hook does not set desc->vm_ops,
vma->vm_ops will be NULL and this dereferences a NULL pointer.

For e.g. drivers/char/mem.c, mmap_zero_prepare() would trigger
a NULL pointer dereference here.

Would need to do
if (!vm_ops || !vm_ops->mapped)
return 0;

here

> + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff, file,
> + &vm_private_data);
> + if (err)
> + unmap_vma_locked(vma);

when mapped() returns an error, unmap_vma_locked(vma) is called
but execution continues into the vm_private_data update below. After
unmap_vma_locked() the VMA may be freed (do_munmap can remove the VMA
entirely), so accessing vma->vm_private_data after that is a
use-after-free.

Probably need to do:
if (err) {
unmap_vma_locked(vma);
return err;
}

> + /* Update private data if changed. */
> + if (vm_private_data != vma->vm_private_data)
> + vma->vm_private_data = vm_private_data;
> +
> + return err;
> +}
> +
> /**
> * compat_vma_mmap() - Apply the file's .mmap_prepare() hook to an
> * existing VMA and execute any requested actions.
> @@ -1191,34 +1240,26 @@ EXPORT_SYMBOL(flush_dcache_folio);
> */
> int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
> {
> - struct vm_area_desc desc = {
> - .mm = vma->vm_mm,
> - .file = file,
> - .start = vma->vm_start,
> - .end = vma->vm_end,
> -
> - .pgoff = vma->vm_pgoff,
> - .vm_file = vma->vm_file,
> - .vma_flags = vma->flags,
> - .page_prot = vma->vm_page_prot,
> -
> - .action.type = MMAP_NOTHING, /* Default */
> - };
> int err;
>
> - err = vfs_mmap_prepare(file, &desc);
> - if (err)
> - return err;
> -
> - err = mmap_action_prepare(&desc, &desc.action);
> + err = __compat_vma_mmap(file, vma);
> if (err)
> return err;
>
> - set_vma_from_desc(vma, &desc);
> - return mmap_action_complete(vma, &desc.action);
> + return __compat_vma_mapped(file, vma);
> }
> EXPORT_SYMBOL(compat_vma_mmap);
>
> +int __vma_check_mmap_hook(struct vm_area_struct *vma)
> +{
> + /* vm_ops->mapped is not valid if mmap() is specified. */
> + if (WARN_ON_ONCE(vma->vm_ops->mapped))
> + return -EINVAL;

I think vma->vm_ops can be NULL here. Should be:

if (vma->vm_ops && WARN_ON_ONCE(vma->vm_ops->mapped))
return -EINVAL;

> +
> + return 0;
> +}
> +EXPORT_SYMBOL(__vma_check_mmap_hook);
> +
> static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
> const struct page *page)
> {
> @@ -1316,10 +1357,7 @@ static int mmap_action_finish(struct vm_area_struct *vma,
> * invoked if we do NOT merge, so we only clean up the VMA we created.
> */
> if (err) {
> - const size_t len = vma_pages(vma) << PAGE_SHIFT;
> -
> - do_munmap(current->mm, vma->vm_start, len, NULL);
> -
> + unmap_vma_locked(vma);
> if (action->error_hook) {
> /* We may want to filter the error. */
> err = action->error_hook(err);
> diff --git a/mm/vma.c b/mm/vma.c
> index 054cf1d262fb..ef9f5a5365d1 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2705,21 +2705,35 @@ static bool can_set_ksm_flags_early(struct mmap_state *map)
> return false;
> }
>
> -static int call_action_complete(struct mmap_state *map,
> - struct mmap_action *action,
> - struct vm_area_struct *vma)
> +static int call_mapped_hook(struct vm_area_struct *vma)
> {
> - int ret;
> + const struct vm_operations_struct *vm_ops = vma->vm_ops;
> + void *vm_private_data = vma->vm_private_data;
> + int err;
>
> - ret = mmap_action_complete(vma, action);
> + if (!vm_ops || !vm_ops->mapped)
> + return 0;
> + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff,
> + vma->vm_file, &vm_private_data);
> + if (err) {
> + unmap_vma_locked(vma);
> + return err;
> + }
> + /* Update private data if changed. */
> + if (vm_private_data != vma->vm_private_data)
> + vma->vm_private_data = vm_private_data;
> + return 0;
> +}
>
> - /* If we held the file rmap we need to release it. */
> - if (map->hold_file_rmap_lock) {
> - struct file *file = vma->vm_file;
> +static void maybe_drop_file_rmap_lock(struct mmap_state *map,
> + struct vm_area_struct *vma)
> +{
> + struct file *file;
>
> - i_mmap_unlock_write(file->f_mapping);
> - }
> - return ret;
> + if (!map->hold_file_rmap_lock)
> + return;
> + file = vma->vm_file;
> + i_mmap_unlock_write(file->f_mapping);
> }
>
> static unsigned long __mmap_region(struct file *file, unsigned long addr,
> @@ -2773,8 +2787,11 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> __mmap_complete(&map, vma);
>
> if (have_mmap_prepare && allocated_new) {
> - error = call_action_complete(&map, &desc.action, vma);
> + error = mmap_action_complete(vma, &desc.action);
> + if (!error)
> + error = call_mapped_hook(vma);
>
> + maybe_drop_file_rmap_lock(&map, vma);
> if (error)
> return error;
> }
> diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> index 908beb263307..47d8db809f31 100644
> --- a/tools/testing/vma/include/dup.h
> +++ b/tools/testing/vma/include/dup.h
> @@ -606,12 +606,34 @@ struct vm_area_struct {
> } __randomize_layout;
>
> struct vm_operations_struct {
> - void (*open)(struct vm_area_struct * area);
> + /**
> + * @open: Called when a VMA is remapped or split. Not called upon first
> + * mapping a VMA.
> + * Context: User context. May sleep. Caller holds mmap_lock.
> + */
> + void (*open)(struct vm_area_struct *vma);
> /**
> * @close: Called when the VMA is being removed from the MM.
> * Context: User context. May sleep. Caller holds mmap_lock.
> */
> - void (*close)(struct vm_area_struct * area);
> + void (*close)(struct vm_area_struct *vma);
> + /**
> + * @mapped: Called when the VMA is first mapped in the MM. Not called if
> + * the new VMA is merged with an adjacent VMA.
> + *
> + * The @vm_private_data field is an output field allowing the user to
> + * modify vma->vm_private_data as necessary.
> + *
> + * ONLY valid if set from f_op->mmap_prepare. Will result in an error if
> + * set from f_op->mmap.
> + *
> + * Returns %0 on success, or an error otherwise. On error, the VMA will
> + * be unmapped.
> + *
> + * Context: User context. May sleep. Caller holds mmap_lock.
> + */
> + int (*mapped)(unsigned long start, unsigned long end, pgoff_t pgoff,
> + const struct file *file, void **vm_private_data);
> /* Called any time before splitting to check if it's allowed */
> int (*may_split)(struct vm_area_struct *area, unsigned long addr);
> int (*mremap)(struct vm_area_struct *area);
> @@ -1345,3 +1367,11 @@ static inline void vma_set_file(struct vm_area_struct *vma, struct file *file)
> swap(vma->vm_file, file);
> fput(file);
> }
> +
> +static inline void unmap_vma_locked(struct vm_area_struct *vma)
> +{
> + const size_t len = vma_pages(vma) << PAGE_SHIFT;
> +
> + mmap_assert_locked(vma->vm_mm);
> + do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
> +}
> --
> 2.53.0
>
>