Re: [PATCH] ovl: fix mmap denywrite

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, Jun 23, 2021 at 01:41:02PM +0200, Christian König wrote:
> 
> 
> Am 22.06.21 um 17:10 schrieb Miklos Szeredi:
> > On Tue, 22 Jun 2021 at 14:43, Christian König <christian.koenig@amd.com> wrote:
> > > Am 22.06.21 um 14:30 schrieb Miklos Szeredi:
> > > > Overlayfs did not honor positive i_writecount on realfile for VM_DENYWRITE
> > > > mappings.  Similarly negative i_mmap_writable counts were ignored for
> > > > VM_SHARED mappings.
> > > > 
> > > > Fix by making vma_set_file() switch the temporary counts obtained and
> > > > released by mmap_region().
> > > Mhm, I don't fully understand the background but that looks like
> > > something specific to overlayfs to me.
> > > 
> > > So why are you changing the common helper?
> > Need to hold the temporary counts until the final ones are obtained in
> > vma_link(), which is out of overlayfs' scope.
> 
> Ah! So basically we need to move the denial counts which mmap_region() added
> to the original file to the new one as well. That's indeed a rather good
> point.
> 
> Can you rather change the vma_set_file() function to return the error and
> add a __must_check?
> 
> I can take care fixing the users in DMA-buf and DRM subsystem.

Okay, but changing to __must_check has to be the last step to avoid compile
errors.  This v2 is with __must_check commented out.

Thanks,
Miklos
---

From: Miklos Szeredi <mszeredi@redhat.com>
Subject: [PATCH v2] ovl: fix mmap denywrite

Overlayfs did not honor positive i_writecount on realfile for VM_DENYWRITE
mappings.  Similarly negative i_mmap_writable counts were ignored for
VM_SHARED mappings.

Fix by making vma_set_file() switch the temporary counts obtained and
released by mmap_region().

Reported-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/overlayfs/file.c |    4 +++-
 include/linux/mm.h  |    2 +-
 mm/mmap.c           |    2 +-
 mm/util.c           |   27 ++++++++++++++++++++++++++-
 4 files changed, 31 insertions(+), 4 deletions(-)

--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -430,7 +430,9 @@ static int ovl_mmap(struct file *file, s
 	if (WARN_ON(file != vma->vm_file))
 		return -EIO;
 
-	vma_set_file(vma, realfile);
+	ret = vma_set_file(vma, realfile);
+	if (ret)
+		return ret;
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = call_mmap(vma->vm_file, vma);
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2776,7 +2776,7 @@ static inline void vma_set_page_prot(str
 }
 #endif
 
-void vma_set_file(struct vm_area_struct *vma, struct file *file);
+int /* __must_check */ vma_set_file(struct vm_area_struct *vma, struct file *file);
 
 #ifdef CONFIG_NUMA_BALANCING
 unsigned long change_prot_numa(struct vm_area_struct *vma,
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1807,6 +1807,7 @@ unsigned long mmap_region(struct file *f
 		 */
 		vma->vm_file = get_file(file);
 		error = call_mmap(file, vma);
+		file = vma->vm_file;
 		if (error)
 			goto unmap_and_free_vma;
 
@@ -1868,7 +1869,6 @@ unsigned long mmap_region(struct file *f
 		if (vm_flags & VM_DENYWRITE)
 			allow_write_access(file);
 	}
-	file = vma->vm_file;
 out:
 	perf_event_mmap(vma);
 
--- a/mm/util.c
+++ b/mm/util.c
@@ -314,12 +314,37 @@ int vma_is_stack_for_current(struct vm_a
 /*
  * Change backing file, only valid to use during initial VMA setup.
  */
-void vma_set_file(struct vm_area_struct *vma, struct file *file)
+int vma_set_file(struct vm_area_struct *vma, struct file *file)
 {
+	vm_flags_t vm_flags = vma->vm_flags;
+	int err = 0;
+
 	/* Changing an anonymous vma with this is illegal */
 	get_file(file);
+
+	/* Get temporary denial counts on replacement */
+	if (vm_flags & VM_DENYWRITE) {
+		err = deny_write_access(file);
+		if (err)
+			goto out_put;
+	}
+	if (vm_flags & VM_SHARED) {
+		err = mapping_map_writable(file->f_mapping);
+		if (err)
+			goto out_allow;
+	}
+
 	swap(vma->vm_file, file);
+
+	/* Undo temporary denial counts on replaced */
+	if (vm_flags & VM_SHARED)
+		mapping_unmap_writable(file->f_mapping);
+out_allow:
+	if (vm_flags & VM_DENYWRITE)
+		allow_write_access(file);
+out_put:
 	fput(file);
+	return err;
 }
 EXPORT_SYMBOL(vma_set_file);