linux-kernel - Re: [PATCH 6/7] SELinux: The copy-up operation must have read permission on the lower file

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Date:	Wed, 05 Nov 2014 12:54:44 -0500
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	David Howells <dhowells@...hat.com>, linux-unionfs@...r.kernel.org,
	selinux@...ho.nsa.gov
CC:	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] SELinux: The copy-up operation must have read permission
 on the lower file

On 11/05/2014 11:43 AM, Stephen Smalley wrote:
> On 11/05/2014 10:43 AM, David Howells wrote:
>> The copy-up operation must have read permission on the lower file for the task
>> that caused the copy-up.  This helps prevent overlayfs from being used to
>> access something it shouldn't.
>>
>> Signed-off-by: David Howells <dhowells@...hat.com>
>> ---
>>
>>  security/selinux/hooks.c |    3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index f43f07fdc028..57f9c641779f 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -3144,7 +3144,8 @@ static void selinux_inode_getsecid(const struct inode *inode, u32 *secid)
>>  
>>  static int selinux_inode_copy_up(struct dentry *src, struct dentry *dst)
>>  {
>> -	return 0;
>> +	const struct cred *cred = current_cred();
>> +	return dentry_has_perm(cred, src, FILE__OPEN | FILE__READ);
>>  }
> 
> Won't this get checked anyway when overlayfs calls vfs helpers to open
> the source and those vfs helpers call the security hooks and apply the
> usual checks?
> 
> Or, if not, where do you check permissions for the destination?

BTW, a quick look at overlayfs suggests further problems with regard to
permission checking, e.g. ovl_copy_up_one() does:
        /*
         * CAP_SYS_ADMIN for copying up extended attributes
         * CAP_DAC_OVERRIDE for create
         * CAP_FOWNER for chmod, timestamp update
         * CAP_FSETID for chmod
         * CAP_CHOWN for chown
         * CAP_MKNOD for mknod
         */
        cap_raise(override_cred->cap_effective, CAP_SYS_ADMIN);
        cap_raise(override_cred->cap_effective, CAP_DAC_OVERRIDE);
        cap_raise(override_cred->cap_effective, CAP_FOWNER);
        cap_raise(override_cred->cap_effective, CAP_FSETID);
        cap_raise(override_cred->cap_effective, CAP_CHOWN);
        cap_raise(override_cred->cap_effective, CAP_MKNOD);
        old_cred = override_creds(override_cred);

This means that it expects to trigger those capability checks as part of
its subsequent actions.  Raising those capabilities temporarily in its
credentials will pass the capability module checks but won't address the
corresponding SELinux checks (both capability and file-based), so you'll
end up triggering an entire set of checks against the current process'
credentials.  This same pattern is repeated elsewhere in overlayfs.

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