ACK(ish): [Trusty][Xenial][PATCH 1/1] v3: CVE-2015-1350 fs: Avoid premature clearing of capabilities

[Stefan Bader]

On 23.01.2018 12:32, Stefan Bader wrote:
> On 13.12.2017 03:15, Khalid Elmously wrote:
>> From: Jan Kara <jack at suse.cz>
>>
>>
>> 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.
>>
>>
>> (backported from commit 030b533c4fd4d2ec3402363323de4bb2983c9cee)
>> [kmously: This is a re-implementation of the upstream commit that doesn't include all the refactoring that was done in upstream)
>>
>> CVE-2015-1350
>>
>>
>> References: CVE-2015-1350
>> Reviewed-by: Christoph Hellwig <hch at lst.de>
>> Signed-off-by: Jan Kara <jack at suse.cz>
>> Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
> Acked-by: Stefan Bader <stefan.bader at canonical.com>
> 
>>
>>
>>
>> ---
> 
> Based on the suggestions from Thadeu this looks ok. Personally I am not 100%
> sure whether the usage of d_obtain_alias() is valid or not.

I did not want to object to this submission. Just to state that I am looking for
someone else (possibly with better understanding of fs code) to give some feedback.

Anybody? Mueller?

> 
> -Stefan
> 
>>  fs/attr.c | 24 ++++++++++++++++++------
>>  1 file changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/attr.c b/fs/attr.c
>> index df05bc167360..870d45103f29 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 +95,20 @@ 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;
>> +
>> +		dentry = d_obtain_alias(inode);
>> +		if (!IS_ERR(dentry)) {
>> +			error = security_inode_killpriv(dentry);
>> +			dput(dentry);
>> +			return error;
>> +		}
>> +	}
>> +
>>  	return 0;
>>  }
>>  EXPORT_SYMBOL(inode_change_ok);
>> @@ -250,13 +264,11 @@ 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;
>>  	}
>>  
>>  	/*
>>
> 
> 
> 
> 


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