Re: [PATCH -next] evm: Use __vfs_setxattr() to update security.evm

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2023-01-30 at 09:53 +0800, Guozihua (Scott) wrote:
> On 2023/1/19 5:45, Mimi Zohar wrote:
> > On Wed, 2022-12-28 at 11:02 +0800, Xiu Jianfeng wrote:
> >> Currently it uses __vfs_setxattr_noperm() to update "security.evm",
> >> however there are two lsm hooks(inode_post_setxattr and inode_setsecurity)
> >> being called inside this function, which don't make any sense for xattr
> >> "security.evm", because the handlers of these two hooks, such as selinux
> >> and smack, only care about their own xattr.
> > 
> > Updating the security.ima hash triggers re-calculating and writing the
> > security.evm HMAC.  Refer to evm_inode_post_setxattr().
> 
> Hi Mimi,
> 
> I believe what Jianfeng is trying to do is to avoid re-triggering
> security_inode_post_setxattr if we are updating security.evm. I can't
> think of any other xattr that could "absorb" security.evm.

I understand.  Comments below ...
> > 
> >>
> >> On the other hand, there is a literally rather than actually cyclical
> >> callchain as follows:
> >> security_inode_post_setxattr
> >>   ->evm_inode_post_setxattr
> >>     ->evm_update_evmxattr
> >>       ->__vfs_setxattr_noperm
> >>         ->security_inode_post_setxattr
> >>
> >> So use __vfs_setxattr() to update "security.evm".
> >>
> >> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
> >> ---
> >>  security/integrity/evm/evm_crypto.c   | 7 +++----
> >>  security/integrity/ima/ima_appraise.c | 8 ++++----
> >>  2 files changed, 7 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> >> index fa5ff13fa8c9..d8275dfa49ef 100644
> >> --- a/security/integrity/evm/evm_crypto.c
> >> +++ b/security/integrity/evm/evm_crypto.c
> >> @@ -376,10 +376,9 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
> >>  			   xattr_value_len, &data);
> >>  	if (rc == 0) {
> >>  		data.hdr.xattr.sha1.type = EVM_XATTR_HMAC;
> >> -		rc = __vfs_setxattr_noperm(&init_user_ns, dentry,
> >> -					   XATTR_NAME_EVM,
> >> -					   &data.hdr.xattr.data[1],
> >> -					   SHA1_DIGEST_SIZE + 1, 0);
> >> +		rc = __vfs_setxattr(&init_user_ns, dentry, d_inode(dentry),
> >> +				    XATTR_NAME_EVM, &data.hdr.xattr.data[1],
> >> +				    SHA1_DIGEST_SIZE + 1, 0);

Although __vfs_setxattr_noperm() doesn't do any permission checking, it
does other things - make sure the filesystem supports writing xattrs,
calls fsnotify_xattr().

> >>  	} else if (rc == -ENODATA && (inode->i_opflags & IOP_XATTR)) {
> >>  		rc = __vfs_removexattr(&init_user_ns, dentry, XATTR_NAME_EVM);
> >>  	}
> >> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> >> index ee6f7e237f2e..d2de9dc6c345 100644
> >> --- a/security/integrity/ima/ima_appraise.c
> >> +++ b/security/integrity/ima/ima_appraise.c
> >> @@ -98,10 +98,10 @@ static int ima_fix_xattr(struct dentry *dentry,
> >>  		iint->ima_hash->xattr.ng.type = IMA_XATTR_DIGEST_NG;
> >>  		iint->ima_hash->xattr.ng.algo = algo;
> >>  	}
> >> -	rc = __vfs_setxattr_noperm(&init_user_ns, dentry, XATTR_NAME_IMA,
> >> -				   &iint->ima_hash->xattr.data[offset],
> >> -				   (sizeof(iint->ima_hash->xattr) - offset) +
> >> -				   iint->ima_hash->length, 0);
> >> +	rc = __vfs_setxattr(&init_user_ns, dentry, d_inode(dentry),
> >> +			    XATTR_NAME_IMA, &iint->ima_hash->xattr.data[offset],
> >> +			    (sizeof(iint->ima_hash->xattr) - offset) +
> >> +			    iint->ima_hash->length, 0);

To clarify, ima_fix_xattr() is either directly called when in "fix"
mode or from ima_update_xattr().  With this change, the recalculated
file hash would be written to security.ima, but security.evm would not
be updated.

> >>  	return rc;
> >>  }

-- 
thanks,

Mimi