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@xxxxxxx> 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@xxxxxxxxxx>
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@xxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
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);
