Re: integrity: nfsd imbalance bug fix

[Andrew Morton]

On Tue, 28 Apr 2009 14:32:14 -0400
Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:

> The number of calls to ima_path_check()/ima_file_free()
> should be balanced.  An extra call to fput(), indicates
> the file could have been accessed without first being
> measured.
> 
> An nfsd exported file is opened/closed by the kernel
> causing an integrity imbalance message.
> 
> - rename and export opencount_get to ima_opencount_get
> - replace ima_shm_check calls with ima_opencount_get
> - add call to increment opencount for files opened by nfsd.
> - add call to measure exported files in nfsd_permission().
> - export ima_path_check
> 

The patch looks fragile to me.  It sprinkles IMA-specific operations
over random unrelated subsystems.  There is hence a decent chance of
breakage in the future.

Suppose some other new piece of kernel code opens/closes a file.  The
developer didn't think to add the IMA hooks and whoops, we have a bug.

It would be really really better if we could add the IMA hooks in a
single place.  That might require the addition of a new function, and
that's fine.  d_instantiate_kernel() or init_file_kernel() or whatever
- that's fine.  It still has the risk that new code will forget to use
the in-kernel variant, but we have a better chance of detecting it.

> 
> Index: security-testing-2.6/fs/nfsd/vfs.c
> ===================================================================
> --- security-testing-2.6.orig/fs/nfsd/vfs.c
> +++ security-testing-2.6/fs/nfsd/vfs.c
> @@ -55,6 +55,7 @@
>  #include <linux/security.h>
>  #endif /* CONFIG_NFSD_V4 */
>  #include <linux/jhash.h>
> +#include <linux/ima.h>
>  
>  #include <asm/uaccess.h>
>  
> @@ -735,6 +736,8 @@ nfsd_open(struct svc_rqst *rqstp, struct
>  			    flags, cred);
>  	if (IS_ERR(*filp))
>  		host_err = PTR_ERR(*filp);
> +	else
> +		ima_opencount_get(*filp);

This suggests dentry_open_kernel().

>  out_nfserr:
>  	err = nfserrno(host_err);
>  out:
> @@ -2096,7 +2099,13 @@ nfsd_permission(struct svc_rqst *rqstp, 
>  	if (err == -EACCES && S_ISREG(inode->i_mode) &&
>  	    acc == (NFSD_MAY_READ | NFSD_MAY_OWNER_OVERRIDE))
>  		err = inode_permission(inode, MAY_EXEC);
> +	if (err)
> +		goto nfsd_out;
>  
> +	err = ima_path_check(&exp->ex_path,
> +			     acc & (MAY_READ | MAY_WRITE | MAY_EXEC));

hm, dunno what to do about that.

> +	return err;
> +nfsd_out:
>  	return err? nfserrno(err) : 0;
>  }
>  
> Index: security-testing-2.6/security/integrity/ima/ima_main.c
> ===================================================================
> --- security-testing-2.6.orig/security/integrity/ima/ima_main.c
> +++ security-testing-2.6/security/integrity/ima/ima_main.c
> @@ -206,6 +206,7 @@ out:
>  	kref_put(&iint->refcount, iint_free);
>  	return 0;
>  }
> +EXPORT_SYMBOL_GPL(ima_path_check);
>  
>  static int process_measurement(struct file *file, const unsigned char *filename,
>  			       int mask, int function)
> @@ -234,7 +235,16 @@ out:
>  	return rc;
>  }
>  
> -static void opencount_get(struct file *file)
> +/*
> + * ima_opencount_get - incr opencount for files opened by the kernel
> + *
> + * - IPC shm and shmat create/fput a file.
> + * - nfsd opens/closes exported files.
> + *
> + * Increment the opencount for these files to prevent unnecessary
> + * imbalance messages.
> + */
> +void ima_opencount_get(struct file *file)
>  {
>  	struct inode *inode = file->f_dentry->d_inode;
>  	struct ima_iint_cache *iint;
> @@ -248,6 +258,7 @@ static void opencount_get(struct file *f
>  	iint->opencount++;
>  	mutex_unlock(&iint->mutex);
>  }
> +EXPORT_SYMBOL_GPL(ima_opencount_get);
>  
>  /**
>   * ima_file_mmap - based on policy, collect/store measurement.
> @@ -272,18 +283,6 @@ int ima_file_mmap(struct file *file, uns
>  	return 0;
>  }
>  
> -/*
> - * ima_shm_check - IPC shm and shmat create/fput a file
> - *
> - * Maintain the opencount for these files to prevent unnecessary
> - * imbalance messages.
> - */
> -void ima_shm_check(struct file *file)
> -{
> -	opencount_get(file);
> -	return;
> -}
> -
>  /**
>   * ima_bprm_check - based on policy, collect/store measurement.
>   * @bprm: contains the linux_binprm structure
> Index: security-testing-2.6/include/linux/ima.h
> ===================================================================
> --- security-testing-2.6.orig/include/linux/ima.h
> +++ security-testing-2.6/include/linux/ima.h
> @@ -20,7 +20,7 @@ extern void ima_inode_free(struct inode 
>  extern int ima_path_check(struct path *path, int mask);
>  extern void ima_file_free(struct file *file);
>  extern int ima_file_mmap(struct file *file, unsigned long prot);
> -extern void ima_shm_check(struct file *file);
> +extern void ima_opencount_get(struct file *file);
>  
>  #else
>  static inline int ima_bprm_check(struct linux_binprm *bprm)
> @@ -53,7 +53,7 @@ static inline int ima_file_mmap(struct f
>  	return 0;
>  }
>  
> -static inline void ima_shm_check(struct file *file)
> +static inline void ima_opencount_get(struct file *file)
>  {
>  	return;
>  }
> Index: security-testing-2.6/ipc/shm.c
> ===================================================================
> --- security-testing-2.6.orig/ipc/shm.c
> +++ security-testing-2.6/ipc/shm.c
> @@ -384,7 +384,7 @@ static int newseg(struct ipc_namespace *
>  	error = PTR_ERR(file);
>  	if (IS_ERR(file))
>  		goto no_file;
> -	ima_shm_check(file);
> +	ima_opencount_get(file);
>  
>  	id = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
>  	if (id < 0) {
> @@ -891,7 +891,7 @@ long do_shmat(int shmid, char __user *sh
>  	file = alloc_file(path.mnt, path.dentry, f_mode, &shm_file_operations);
>  	if (!file)
>  		goto out_free;
> -	ima_shm_check(file);
> +	ima_opencount_get(file);
>  
>  	file->private_data = sfd;
>  	file->f_mapping = shp->shm_file->f_mapping;
> Index: security-testing-2.6/mm/shmem.c
> ===================================================================
> --- security-testing-2.6.orig/mm/shmem.c
> +++ security-testing-2.6/mm/shmem.c
> @@ -2680,7 +2680,7 @@ int shmem_zero_setup(struct vm_area_stru
>  	if (IS_ERR(file))
>  		return PTR_ERR(file);
>  
> -	ima_shm_check(file);
> +	ima_opencount_get(file);
>  	if (vma->vm_file)
>  		fput(vma->vm_file);
>  	vma->vm_file = file;

Maybe do the IMA operations in (or under) shmem_file_setup() and
hugetlb_file_setup()?

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