NACK: [X] [CVE-2015-1350] [PATCH] fs: Avoid premature clearing of capabilities

[Stefan Bader]

On 20.08.2018 08:58, Kleber Souza wrote:
> On 08/16/18 13:41, José Pekkarinen wrote:
>> CVE-2015-1350
>>
>> Buglink: https://launchpad.net/bugs/1415636
>>
>> Currently, notify_change() clears capabilities or IMA attributes by
>> calling security_inode_killpriv() before calling into ->setattr. Thus it
>> happens before any other permission checks in inode_change_ok() and user
>> is thus allowed to trigger clearing of capabilities or IMA attributes
>> for any file he can look up e.g. by calling chown for that file. This is
>> unexpected and can lead to user DoSing a system.
>>
>> Fix the problem by calling security_inode_killpriv() at the end of
>> inode_change_ok() instead of from notify_change(). At that moment we are
>> sure user has permissions to do the requested change.
>>
> 
> As Stefan mentioned, the complete commit message should be kept. So the
> line below shouldn't be removed.
> 
> References: CVE-2015-1350
>> Reviewed-by: Christoph Hellwig <hch at lst.de>
>> Signed-off-by: Jan Kara <jack at suse.cz>
> 
> As Stefan also mentioned, please add the original SHA1 of the patch
> mentioning if it's a clean cherry pick "(cherry picked from ...)" or a
> backport "(backported from ...)" as
> 
> (backported from 030b533c4fd4d2ec3402363323de4bb2983c9cee)
>> Signed-off-by: José Pekkarinen <jose.pekkarinen at canonical.com>
> 
> I'm not NAK'ing or ACK'ing the patch, if the patch is fine we can
> include those fixes when applying it if others agree as well.
> 
> 
> Thanks,
> Kleber
> 
>>
>> ---
>>  fs/attr.c | 23 +++++++++++++++++------
>>  1 file changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/attr.c b/fs/attr.c
>> index df05bc1..870e4b6 100644
>> --- a/fs/attr.c
>> +++ b/fs/attr.c
>> @@ -68,7 +68,7 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
>>  
>>  	/* If force is set do it anyway. */
>>  	if (ia_valid & ATTR_FORCE)
>> -		return 0;
>> +		goto kill_priv;
>>  
>>  	/* Make sure a caller can chown. */
>>  	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
>> @@ -95,6 +95,19 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
>>  			return -EPERM;
>>  	}
>>  
>> +kill_priv:
>> +	/* User has permission for the change */
>> +	if (ia_valid & ATTR_KILL_PRIV) {
>> +		int error;
>> +		struct dentry * dentry = d_find_alias(inode);
>> +
>> +		if (dentry) {
>> +			error = security_inode_killpriv(dentry);
>> +			if (error)
>> +				return error;
>> +		}
>> +	}
>> +
>>  	return 0;

Additionally to above reasons, this is basically a submission Khaled has made in
the past and Theadeu had proposed the use of d_obtain_alias, but _also_ said a
matching dput() would be needed. Plus, if one really reads the code, it is
obvious that the return value of d_obtain_alias() is nevel NULL, hence that part
should look like in Khaled's v3 submission:

+kill_priv:
+	/* User has permission for the change */
+	if (ia_valid & ATTR_KILL_PRIV) {
+		int error;
+		struct dentry *dentry;
+
+		dentry = d_obtain_alias(inode);
+		if (!IS_ERR(dentry)) {
+			error = security_inode_killpriv(dentry);
+			dput(dentry);
+			return error;
+		}
+	}
+

-Stefan

PS: Same reply for the Trusty patch which could be made a single patch
submission with the note that the first hunk needs to accept a fuzz 1.

>>  }
>>  EXPORT_SYMBOL(inode_change_ok);
>> @@ -250,13 +263,11 @@ int notify_change(struct dentry * dentry, struct iattr * attr, struct inode **de
>>  	if (!(ia_valid & ATTR_MTIME_SET))
>>  		attr->ia_mtime = now;
>>  	if (ia_valid & ATTR_KILL_PRIV) {
>> -		attr->ia_valid &= ~ATTR_KILL_PRIV;
>> -		ia_valid &= ~ATTR_KILL_PRIV;
>>  		error = security_inode_need_killpriv(dentry);
>> -		if (error > 0)
>> -			error = security_inode_killpriv(dentry);
>> -		if (error)
>> +		if (error < 0)
>>  			return error;
>> +		if (error == 0)
>> +			ia_valid = attr->ia_valid &= ~ATTR_KILL_PRIV;
>>  	}
>>  
>>  	/*
>>
> 
> 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: OpenPGP digital signature
URL: <https://lists.ubuntu.com/archives/kernel-team/attachments/20180820/3bd7d4ae/attachment.sig>