[PATCH] NFS: Avoid cross-structure casting

[PATCH] NFS: Avoid cross-structure casting

[Kees Cook]

When the call to nfs_devname() fails, the error path attempts to retain
the error via the mnt variable, but this requires a cast across very
different types (char * to struct vfsmount), which the upcoming structure
layout randomization plugin flags as being potentially dangerous in the
face of randomization. This is a false positive, but what this code
actually wants to do is retain the error value, so this patch explicitly
sets it, instead of using what seems to be an unexpected cast.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/nfs/namespace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index 786f17580582..89f50bf12f52 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -259,9 +259,10 @@ struct vfsmount *nfs_do_submount(struct dentry *dentry, struct nfs_fh *fh,
 	if (page == NULL)
 		goto out;
 	devname = nfs_devname(dentry, page, PAGE_SIZE);
-	mnt = (struct vfsmount *)devname;
-	if (IS_ERR(devname))
+	if (IS_ERR(devname)) {
+		mnt = ERR_PTR(PTR_ERR(devname));
 		goto free_page;
+	}
 	mnt = nfs_do_clone_mount(NFS_SB(dentry->d_sb), devname, &mountdata);
 free_page:
 	free_page((unsigned long)page);
-- 
2.7.4


-- 
Kees Cook
Pixel Security

Re: [PATCH] NFS: Avoid cross-structure casting

[NeilBrown]

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On Tue, Apr 04 2017, Kees Cook wrote:

> When the call to nfs_devname() fails, the error path attempts to retain
> the error via the mnt variable, but this requires a cast across very
> different types (char * to struct vfsmount), which the upcoming structure
> layout randomization plugin flags as being potentially dangerous in the
> face of randomization. This is a false positive, but what this code
> actually wants to do is retain the error value, so this patch explicitly
> sets it, instead of using what seems to be an unexpected cast.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  fs/nfs/namespace.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
> index 786f17580582..89f50bf12f52 100644
> --- a/fs/nfs/namespace.c
> +++ b/fs/nfs/namespace.c
> @@ -259,9 +259,10 @@ struct vfsmount *nfs_do_submount(struct dentry *dentry, struct nfs_fh *fh,
>  	if (page == NULL)
>  		goto out;
>  	devname = nfs_devname(dentry, page, PAGE_SIZE);
> -	mnt = (struct vfsmount *)devname;
> -	if (IS_ERR(devname))
> +	if (IS_ERR(devname)) {
> +		mnt = ERR_PTR(PTR_ERR(devname));

Allow me to introduce you to ERR_CAST()
/**
 * ERR_CAST - Explicitly cast an error-valued pointer to another pointer type
 * @ptr: The pointer to cast.
 *
 * Explicitly cast an error-valued pointer to another pointer type in such a
 * way as to make it clear that's what's going on.
 */

So:
 +		mnt = ERR_CAST(devname);

NeilBrown

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Re: [PATCH] NFS: Avoid cross-structure casting

[Kees Cook]

On Tue, Apr 4, 2017 at 6:09 PM, NeilBrown <neilb@suse.com> wrote:
> On Tue, Apr 04 2017, Kees Cook wrote:
>
>> When the call to nfs_devname() fails, the error path attempts to retain
>> the error via the mnt variable, but this requires a cast across very
>> different types (char * to struct vfsmount), which the upcoming structure
>> layout randomization plugin flags as being potentially dangerous in the
>> face of randomization. This is a false positive, but what this code
>> actually wants to do is retain the error value, so this patch explicitly
>> sets it, instead of using what seems to be an unexpected cast.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>  fs/nfs/namespace.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
>> index 786f17580582..89f50bf12f52 100644
>> --- a/fs/nfs/namespace.c
>> +++ b/fs/nfs/namespace.c
>> @@ -259,9 +259,10 @@ struct vfsmount *nfs_do_submount(struct dentry *dentry, struct nfs_fh *fh,
>>       if (page == NULL)
>>               goto out;
>>       devname = nfs_devname(dentry, page, PAGE_SIZE);
>> -     mnt = (struct vfsmount *)devname;
>> -     if (IS_ERR(devname))
>> +     if (IS_ERR(devname)) {
>> +             mnt = ERR_PTR(PTR_ERR(devname));
>
> Allow me to introduce you to ERR_CAST()

Ah! Thank you, yes, that's precisely what I wanted. And only 10 lines
away from where I was reading in err.h trying to find the right
approach. *sigh*

> /**
>  * ERR_CAST - Explicitly cast an error-valued pointer to another pointer type
>  * @ptr: The pointer to cast.
>  *
>  * Explicitly cast an error-valued pointer to another pointer type in such a
>  * way as to make it clear that's what's going on.
>  */
>
> So:
>  +              mnt = ERR_CAST(devname);

I'll send a v2. :)

-Kees

-- 
Kees Cook
Pixel Security