Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which
happens if the dma_buf_release() is called while the userspace is
accessing the dma_buf pseudo fs's dmabuffs_dname() in another process,
and dma_buf_release() releases the dmabuf object when the last reference
to the struct file goes away.
I discussed with Arnd Bergmann, and he suggested that rather than tying
the dma_buf_release() to the file_operations' release(), we can tie it to
the dentry_operations' d_release(), which will be called when the last ref
to the dentry is removed.
The path exercised by __fput() calls f_op->release() first, and then calls
dput, which eventually calls d_op->d_release().
In the 'normal' case, when no userspace access is happening via dma_buf
pseudo fs, there should be exactly one fd, file, dentry and inode, so
closing the fd will kill of everything right away.
In the presented case, the dentry's d_release() will be called only when
the dentry's last ref is released.
Therefore, lets move dma_buf_release() from fops->release() to
d_ops->d_release().
Many thanks to Arnd for his FS insights :)
[1]: https://lore.kernel.org/patchwork/patch/1238278/
Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
Reported-by: [email protected]
Cc: <[email protected]> [5.3+]
Cc: Arnd Bergmann <[email protected]>
Reported-by: Charan Teja Reddy <[email protected]>
Signed-off-by: Sumit Semwal <[email protected]>
---
drivers/dma-buf/dma-buf.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 01ce125f8e8d..92ba4b6ef3e7 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -54,8 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
dentry->d_name.name, ret > 0 ? name : "");
}
+static void dma_buf_release(struct dentry *dentry);
+
static const struct dentry_operations dma_buf_dentry_ops = {
.d_dname = dmabuffs_dname,
+ .d_release = dma_buf_release,
};
static struct vfsmount *dma_buf_mnt;
@@ -77,14 +80,14 @@ static struct file_system_type dma_buf_fs_type = {
.kill_sb = kill_anon_super,
};
-static int dma_buf_release(struct inode *inode, struct file *file)
+static void dma_buf_release(struct dentry *dentry)
{
struct dma_buf *dmabuf;
- if (!is_dma_buf_file(file))
- return -EINVAL;
+ if (dentry->d_op != &dma_buf_dentry_ops)
+ return;
- dmabuf = file->private_data;
+ dmabuf = dentry->d_fsdata;
BUG_ON(dmabuf->vmapping_counter);
@@ -110,7 +113,6 @@ static int dma_buf_release(struct inode *inode, struct file *file)
module_put(dmabuf->owner);
kfree(dmabuf->name);
kfree(dmabuf);
- return 0;
}
static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
@@ -412,7 +414,6 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
}
static const struct file_operations dma_buf_fops = {
- .release = dma_buf_release,
.mmap = dma_buf_mmap_internal,
.llseek = dma_buf_llseek,
.poll = dma_buf_poll,
--
2.27.0
On Wed, Jun 10, 2020 at 10:33 AM Sumit Semwal <[email protected]> wrote:
>
> Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which
> happens if the dma_buf_release() is called while the userspace is
> accessing the dma_buf pseudo fs's dmabuffs_dname() in another process,
> and dma_buf_release() releases the dmabuf object when the last reference
> to the struct file goes away.
>
> I discussed with Arnd Bergmann, and he suggested that rather than tying
> the dma_buf_release() to the file_operations' release(), we can tie it to
> the dentry_operations' d_release(), which will be called when the last ref
> to the dentry is removed.
>
> The path exercised by __fput() calls f_op->release() first, and then calls
> dput, which eventually calls d_op->d_release().
>
> In the 'normal' case, when no userspace access is happening via dma_buf
> pseudo fs, there should be exactly one fd, file, dentry and inode, so
> closing the fd will kill of everything right away.
>
> In the presented case, the dentry's d_release() will be called only when
> the dentry's last ref is released.
>
> Therefore, lets move dma_buf_release() from fops->release() to
> d_ops->d_release().
>
> Many thanks to Arnd for his FS insights :)
>
> [1]: https://lore.kernel.org/patchwork/patch/1238278/
>
> Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
> Reported-by: [email protected]
> Cc: <[email protected]> [5.3+]
> Cc: Arnd Bergmann <[email protected]>
> Reported-by: Charan Teja Reddy <[email protected]>
> Signed-off-by: Sumit Semwal <[email protected]>
The patch looks correct to me.
Reviewed-by: Arnd Bergmann <[email protected]>
Obviously this should still be verified against the original report if possible.
> drivers/dma-buf/dma-buf.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 01ce125f8e8d..92ba4b6ef3e7 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -54,8 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
> dentry->d_name.name, ret > 0 ? name : "");
> }
>
> +static void dma_buf_release(struct dentry *dentry);
> +
> static const struct dentry_operations dma_buf_dentry_ops = {
> .d_dname = dmabuffs_dname,
> + .d_release = dma_buf_release,
> };
I'd suggest rearranging the file to avoid the forward declaration, even
if it makes it a little harder to review the change, the resulting code
will remain organized more logically.
> static struct vfsmount *dma_buf_mnt;
> @@ -77,14 +80,14 @@ static struct file_system_type dma_buf_fs_type = {
> .kill_sb = kill_anon_super,
> };
>
> -static int dma_buf_release(struct inode *inode, struct file *file)
> +static void dma_buf_release(struct dentry *dentry)
> {
> struct dma_buf *dmabuf;
>
> - if (!is_dma_buf_file(file))
> - return -EINVAL;
> + if (dentry->d_op != &dma_buf_dentry_ops)
> + return;
I think the check here is redundant and it's clearer without it.
Arnd
On Wed, 10 Jun 2020 at 14:57, Arnd Bergmann <[email protected]> wrote:
>
> On Wed, Jun 10, 2020 at 10:33 AM Sumit Semwal <[email protected]> wrote:
> >
> > Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which
> > happens if the dma_buf_release() is called while the userspace is
> > accessing the dma_buf pseudo fs's dmabuffs_dname() in another process,
> > and dma_buf_release() releases the dmabuf object when the last reference
> > to the struct file goes away.
> >
> > I discussed with Arnd Bergmann, and he suggested that rather than tying
> > the dma_buf_release() to the file_operations' release(), we can tie it to
> > the dentry_operations' d_release(), which will be called when the last ref
> > to the dentry is removed.
> >
> > The path exercised by __fput() calls f_op->release() first, and then calls
> > dput, which eventually calls d_op->d_release().
> >
> > In the 'normal' case, when no userspace access is happening via dma_buf
> > pseudo fs, there should be exactly one fd, file, dentry and inode, so
> > closing the fd will kill of everything right away.
> >
> > In the presented case, the dentry's d_release() will be called only when
> > the dentry's last ref is released.
> >
> > Therefore, lets move dma_buf_release() from fops->release() to
> > d_ops->d_release().
> >
> > Many thanks to Arnd for his FS insights :)
> >
> > [1]: https://lore.kernel.org/patchwork/patch/1238278/
> >
> > Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
> > Reported-by: [email protected]
> > Cc: <[email protected]> [5.3+]
> > Cc: Arnd Bergmann <[email protected]>
> > Reported-by: Charan Teja Reddy <[email protected]>
> > Signed-off-by: Sumit Semwal <[email protected]>
>
> The patch looks correct to me.
>
> Reviewed-by: Arnd Bergmann <[email protected]>
>
> Obviously this should still be verified against the original report if possible.
Thanks, Arnd!
>
> > drivers/dma-buf/dma-buf.c | 13 +++++++------
> > 1 file changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 01ce125f8e8d..92ba4b6ef3e7 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -54,8 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
> > dentry->d_name.name, ret > 0 ? name : "");
> > }
> >
> > +static void dma_buf_release(struct dentry *dentry);
> > +
> > static const struct dentry_operations dma_buf_dentry_ops = {
> > .d_dname = dmabuffs_dname,
> > + .d_release = dma_buf_release,
> > };
>
> I'd suggest rearranging the file to avoid the forward declaration, even
> if it makes it a little harder to review the change, the resulting code
> will remain organized more logically.
Got it, will update it in v2.
>
> > static struct vfsmount *dma_buf_mnt;
> > @@ -77,14 +80,14 @@ static struct file_system_type dma_buf_fs_type = {
> > .kill_sb = kill_anon_super,
> > };
> >
> > -static int dma_buf_release(struct inode *inode, struct file *file)
> > +static void dma_buf_release(struct dentry *dentry)
> > {
> > struct dma_buf *dmabuf;
> >
> > - if (!is_dma_buf_file(file))
> > - return -EINVAL;
> > + if (dentry->d_op != &dma_buf_dentry_ops)
> > + return;
>
> I think the check here is redundant and it's clearer without it.
Ok, will remove.
>
> Arnd
Best,
Sumit.