NFS: Avoid cross-structure casting

PATCH Superseded Kees Cook 2 participants 3 messages Apr 5, 2017

Subsystems (2)

NFS client NFS server

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Kees Cook

author

2017-04-05 00:12 UTC #0 lore raw

PATCH 1/1 NFS: Avoid cross-structure casting

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 <[email protected]>

---
 fs/nfs/namespace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

fs/nfs/namespace.c +3 -2

index 786f17580582..89f50bf12f52 100644
@@ -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);

Signature · 6 lines

2.7.4


-- 
Kees Cook
Pixel Security

NeilBrown

2017-04-05 01:10 UTC #1 lore raw

Quoted 18 lines · 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 <[email protected]>
---
 fs/nfs/namespace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
fs/nfs/namespace.c +0 -0
index 786f17580582..89f50bf12f52 100644

fs/nfs/namespace.c struct vfsmount *nfs_do_submount(struct dentry *dentry, struct nfs_fh *fh, · line 263

Quoted 7 lines

fs/nfs/namespace.c +2 -2

@@ -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

Kees Cook

author

2017-04-05 15:26 UTC #2 lore raw

Quoted 15 lines · NeilBrown

Quoted 13 lines · 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 <[email protected]>
---
 fs/nfs/namespace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Quoted 14 lines

fs/nfs/namespace.c struct vfsmount *nfs_do_submount(struct dentry *dentry, struct nfs_fh *fh, · line 263

Quoted 12 lines

fs/nfs/namespace.c +2 -2

index 786f17580582..89f50bf12f52 100644
@@ -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*

Quoted 10 lines

/**
* 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

Signature · 2 lines

Kees Cook
Pixel Security