[PATCH] kernfs: fix false-positive WARN(nr_mmapped) in kernfs_drain_open_files

[PATCH] kernfs: fix false-positive WARN(nr_mmapped) in kernfs_drain_open_files

[Neel Natu]

Prior to this change 'on->nr_mmapped' tracked the total number of
mmaps across all of its associated open files via kernfs_fop_mmap().
Thus if the file descriptor associated with a kernfs_open_file was
mmapped 10 times then we would have: 'of->mmapped = true' and
'of_on(of)->nr_mmapped = 10'.

The problem is that closing or draining a 'of->mmapped' file would
only decrement one from the 'of_on(of)->nr_mmapped' counter.

For e.g. we have this from kernfs_unlink_open_file():
        if (of->mmapped)
                on->nr_mmapped--;

The WARN_ON_ONCE(on->nr_mmapped) in kernfs_drain_open_files() is
easy to reproduce by:
1. opening a (mmap-able) kernfs file.
2. mmap-ing that file more than once (mapping just once masks the issue).
3. trigger a drain of that kernfs file.

Modulo out-of-tree patches I was able to trigger this reliably by
identifying pci device nodes in sysfs that have resource regions
that are mmap-able and that don't have any driver attached to them
(steps 1 and 2). For step 3 we can "echo 1 > remove" to trigger a
kernfs_drain.

Signed-off-by: Neel Natu <neelnatu@google.com>
---
 fs/kernfs/file.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index ffa4565c275a..e9df2f87072c 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -483,9 +483,11 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
 		goto out_put;
 
 	rc = 0;
-	of->mmapped = true;
-	of_on(of)->nr_mmapped++;
-	of->vm_ops = vma->vm_ops;
+	if (!of->mmapped) {
+		of->mmapped = true;
+		of_on(of)->nr_mmapped++;
+		of->vm_ops = vma->vm_ops;
+	}
 	vma->vm_ops = &kernfs_vm_ops;
 out_put:
 	kernfs_put_active(of->kn);
-- 
2.43.0.429.g432eaa2c6b-goog

Re: [PATCH] kernfs: fix false-positive WARN(nr_mmapped) in kernfs_drain_open_files

[Greg Kroah-Hartman]

On Sat, Jan 27, 2024 at 03:46:36PM -0800, Neel Natu wrote:
> Prior to this change 'on->nr_mmapped' tracked the total number of
> mmaps across all of its associated open files via kernfs_fop_mmap().
> Thus if the file descriptor associated with a kernfs_open_file was
> mmapped 10 times then we would have: 'of->mmapped = true' and
> 'of_on(of)->nr_mmapped = 10'.
> 
> The problem is that closing or draining a 'of->mmapped' file would
> only decrement one from the 'of_on(of)->nr_mmapped' counter.
> 
> For e.g. we have this from kernfs_unlink_open_file():
>         if (of->mmapped)
>                 on->nr_mmapped--;
> 
> The WARN_ON_ONCE(on->nr_mmapped) in kernfs_drain_open_files() is
> easy to reproduce by:
> 1. opening a (mmap-able) kernfs file.
> 2. mmap-ing that file more than once (mapping just once masks the issue).
> 3. trigger a drain of that kernfs file.
> 
> Modulo out-of-tree patches I was able to trigger this reliably by
> identifying pci device nodes in sysfs that have resource regions
> that are mmap-able and that don't have any driver attached to them
> (steps 1 and 2). For step 3 we can "echo 1 > remove" to trigger a
> kernfs_drain.
> 
> Signed-off-by: Neel Natu <neelnatu@google.com>
> ---
>  fs/kernfs/file.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index ffa4565c275a..e9df2f87072c 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -483,9 +483,11 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
>  		goto out_put;
>  
>  	rc = 0;
> -	of->mmapped = true;
> -	of_on(of)->nr_mmapped++;
> -	of->vm_ops = vma->vm_ops;
> +	if (!of->mmapped) {
> +		of->mmapped = true;
> +		of_on(of)->nr_mmapped++;
> +		of->vm_ops = vma->vm_ops;
> +	}
>  	vma->vm_ops = &kernfs_vm_ops;
>  out_put:
>  	kernfs_put_active(of->kn);

What commit id does this fix?

thanks,

greg k-h

Re: [PATCH] kernfs: fix false-positive WARN(nr_mmapped) in kernfs_drain_open_files

[Neel Natu]

Hi,

On Sat, Jan 27, 2024 at 4:02 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Sat, Jan 27, 2024 at 03:46:36PM -0800, Neel Natu wrote:
> > Prior to this change 'on->nr_mmapped' tracked the total number of
> > mmaps across all of its associated open files via kernfs_fop_mmap().
> > Thus if the file descriptor associated with a kernfs_open_file was
> > mmapped 10 times then we would have: 'of->mmapped = true' and
> > 'of_on(of)->nr_mmapped = 10'.
> >
> > The problem is that closing or draining a 'of->mmapped' file would
> > only decrement one from the 'of_on(of)->nr_mmapped' counter.
> >
> > For e.g. we have this from kernfs_unlink_open_file():
> >         if (of->mmapped)
> >                 on->nr_mmapped--;
> >
> > The WARN_ON_ONCE(on->nr_mmapped) in kernfs_drain_open_files() is
> > easy to reproduce by:
> > 1. opening a (mmap-able) kernfs file.
> > 2. mmap-ing that file more than once (mapping just once masks the issue).
> > 3. trigger a drain of that kernfs file.
> >
> > Modulo out-of-tree patches I was able to trigger this reliably by
> > identifying pci device nodes in sysfs that have resource regions
> > that are mmap-able and that don't have any driver attached to them
> > (steps 1 and 2). For step 3 we can "echo 1 > remove" to trigger a
> > kernfs_drain.
> >
> > Signed-off-by: Neel Natu <neelnatu@google.com>
> > ---
> >  fs/kernfs/file.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> > index ffa4565c275a..e9df2f87072c 100644
> > --- a/fs/kernfs/file.c
> > +++ b/fs/kernfs/file.c
> > @@ -483,9 +483,11 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
> >               goto out_put;
> >
> >       rc = 0;
> > -     of->mmapped = true;
> > -     of_on(of)->nr_mmapped++;
> > -     of->vm_ops = vma->vm_ops;
> > +     if (!of->mmapped) {
> > +             of->mmapped = true;
> > +             of_on(of)->nr_mmapped++;
> > +             of->vm_ops = vma->vm_ops;
> > +     }
> >       vma->vm_ops = &kernfs_vm_ops;
> >  out_put:
> >       kernfs_put_active(of->kn);
>
> What commit id does this fix?

bdb2fd7fc56e197a63c0b0e7e07d25d5e20e7c72
kernfs: Skip kernfs_drain_open_files() more aggressively

best
Neel

>
> thanks,
>
> greg k-h