[PATCH v5 1/5] selinux: try security xattr after genfs for kernfs filesystems

[Stephen Smalley]

On 2/5/19 6:06 AM, Ondrej Mosnacek wrote:
> Since kernfs supports the security xattr handlers, we can simply use
> these to determine the inode's context, dropping the need to update it
> from kernfs explicitly using a security_inode_notifysecctx() call.
> 
> We achieve this by setting a new sbsec flag SE_SBGENFS_XATTR to all
> mounts that are known to use kernfs under the hood and then fetching the
> xattrs after determining the fallback genfs sid in
> inode_doinit_with_dentry() when this flag is set.
> 
> This will allow implementing full security xattr support in kernfs and
> removing the ...notifysecctx() call in a subsequent patch.
> 
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> ---
>   security/selinux/hooks.c            | 159 +++++++++++++++-------------
>   security/selinux/include/security.h |   1 +
>   2 files changed, 88 insertions(+), 72 deletions(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 81e012c66d95..758a99d1086e 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -793,11 +793,13 @@ static int selinux_set_mnt_opts(struct super_block *sb,
>   
>   	if (!strcmp(sb->s_type->name, "debugfs") ||
>   	    !strcmp(sb->s_type->name, "tracefs") ||
> -	    !strcmp(sb->s_type->name, "sysfs") ||
> -	    !strcmp(sb->s_type->name, "pstore") ||
> +	    !strcmp(sb->s_type->name, "pstore"))
> +		sbsec->flags |= SE_SBGENFS;
> +
> +	if (!strcmp(sb->s_type->name, "sysfs") ||
>   	    !strcmp(sb->s_type->name, "cgroup") ||
>   	    !strcmp(sb->s_type->name, "cgroup2"))
> -		sbsec->flags |= SE_SBGENFS;
> +		sbsec->flags |= SE_SBGENFS | SE_SBGENFS_XATTR;
>   
>   	if (!sbsec->behavior) {
>   		/*
> @@ -1392,6 +1394,71 @@ static int selinux_genfs_get_sid(struct dentry *dentry,
>   	return rc;
>   }
>   
> +static int inode_doinit_use_xattr(struct inode *inode, struct dentry *dentry,
> +				  u32 def_sid, u32 *sid)
> +{
> +#define INITCONTEXTLEN 255
> +	char *context = NULL;
> +	unsigned int len = 0;
> +	int rc;
> +
> +	*sid = def_sid;
> +
> +	if (!(inode->i_opflags & IOP_XATTR))
> +		return 0;
> +
> +	len = INITCONTEXTLEN;
> +	context = kmalloc(len + 1, GFP_NOFS);
> +	if (!context)
> +		return -ENOMEM;
> +
> +	context[len] = '\0';
> +	rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, context, len);
> +	if (rc == -ERANGE) {
> +		kfree(context);
> +
> +		/* Need a larger buffer.  Query for the right size. */
> +		rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, NULL, 0);
> +		if (rc < 0)
> +			return rc;
> +
> +		len = rc;
> +		context = kmalloc(len + 1, GFP_NOFS);
> +		if (!context)
> +			return -ENOMEM;
> +
> +		context[len] = '\0';
> +		rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX,
> +				    context, len);
> +	}
> +	if (rc < 0) {
> +		kfree(context);
> +		if (rc != -ENODATA) {
> +			pr_warn("SELinux: %s:  getxattr returned %d for dev=%s ino=%ld\n",
> +				__func__, -rc, inode->i_sb->s_id, inode->i_ino);
> +			return rc;
> +		}
> +		return 0;
> +	}
> +
> +	rc = security_context_to_sid_default(&selinux_state, context, rc, sid,
> +					     def_sid, GFP_NOFS);
> +	if (rc) {
> +		char *dev = inode->i_sb->s_id;
> +		unsigned long ino = inode->i_ino;
> +
> +		if (rc == -EINVAL) {
> +			pr_notice_ratelimited("SELinux: inode=%lu on dev=%s was found to have an invalid context=%s.  This indicates you may need to relabel the inode or the filesystem in question.\n",
> +					      ino, dev, context);
> +		} else {
> +			pr_warn("SELinux: %s:  context_to_sid(%s) returned %d for dev=%s ino=%ld\n",
> +				__func__, context, -rc, dev, ino);
> +		}
> +	}
> +	kfree(context);
> +	return 0;
> +}
> +
>   /* The inode's security attributes must be initialized before first use. */
>   static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dentry)
>   {
> @@ -1401,8 +1468,6 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
>   	u16 sclass;
>   	struct dentry *dentry;
>   #define INITCONTEXTLEN 255

This definition can also be removed, having been moved up above.

> -	char *context = NULL;
> -	unsigned len = 0;
>   	int rc = 0;
>   
>   	if (isec->initialized == LABEL_INITIALIZED)
> @@ -1470,72 +1535,11 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
>   			goto out;
>   		}
>   
> -		len = INITCONTEXTLEN;
> -		context = kmalloc(len+1, GFP_NOFS);
> -		if (!context) {
> -			rc = -ENOMEM;
> -			dput(dentry);
> -			goto out;
> -		}
> -		context[len] = '\0';
> -		rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, context, len);
> -		if (rc == -ERANGE) {
> -			kfree(context);
> -
> -			/* Need a larger buffer.  Query for the right size. */
> -			rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, NULL, 0);
> -			if (rc < 0) {
> -				dput(dentry);
> -				goto out;
> -			}
> -			len = rc;
> -			context = kmalloc(len+1, GFP_NOFS);
> -			if (!context) {
> -				rc = -ENOMEM;
> -				dput(dentry);
> -				goto out;
> -			}
> -			context[len] = '\0';
> -			rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, context, len);
> -		}
> +		rc = inode_doinit_use_xattr(inode, dentry, sbsec->def_sid,
> +					    &sid);
>   		dput(dentry);
> -		if (rc < 0) {
> -			if (rc != -ENODATA) {
> -				pr_warn("SELinux: %s:  getxattr returned "
> -				       "%d for dev=%s ino=%ld\n", __func__,
> -				       -rc, inode->i_sb->s_id, inode->i_ino);
> -				kfree(context);
> -				goto out;
> -			}
> -			/* Map ENODATA to the default file SID */
> -			sid = sbsec->def_sid;
> -			rc = 0;
> -		} else {
> -			rc = security_context_to_sid_default(&selinux_state,
> -							     context, rc, &sid,
> -							     sbsec->def_sid,
> -							     GFP_NOFS);
> -			if (rc) {
> -				char *dev = inode->i_sb->s_id;
> -				unsigned long ino = inode->i_ino;
> -
> -				if (rc == -EINVAL) {
> -					if (printk_ratelimit())
> -						pr_notice("SELinux: inode=%lu on dev=%s was found to have an invalid "
> -							"context=%s.  This indicates you may need to relabel the inode or the "
> -							"filesystem in question.\n", ino, dev, context);
> -				} else {
> -					pr_warn("SELinux: %s:  context_to_sid(%s) "
> -					       "returned %d for dev=%s ino=%ld\n",
> -					       __func__, context, -rc, dev, ino);
> -				}
> -				kfree(context);
> -				/* Leave with the unlabeled SID */
> -				rc = 0;
> -				break;
> -			}
> -		}
> -		kfree(context);
> +		if (rc)
> +			goto out;
>   		break;
>   	case SECURITY_FS_USE_TASK:
>   		sid = task_sid;
> @@ -1586,9 +1590,20 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
>   				goto out;
>   			rc = selinux_genfs_get_sid(dentry, sclass,
>   						   sbsec->flags, &sid);
> -			dput(dentry);
> -			if (rc)
> +			if (rc) {
> +				dput(dentry);
>   				goto out;
> +			}
> +
> +			if (sbsec->flags & SE_SBGENFS_XATTR) {
> +				rc = inode_doinit_use_xattr(inode, dentry,
> +							    sid, &sid);
> +				if (rc) {
> +					dput(dentry);
> +					goto out;
> +				}
> +			}
> +			dput(dentry);
>   		}
>   		break;
>   	}
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index f68fb25b5702..6e5928f951da 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -58,6 +58,7 @@
>   #define SE_SBINITIALIZED	0x0100
>   #define SE_SBPROC		0x0200
>   #define SE_SBGENFS		0x0400
> +#define SE_SBGENFS_XATTR	0x0800
>   
>   #define CONTEXT_STR	"context="
>   #define FSCONTEXT_STR	"fscontext="
>