[PATCH] ovl: filter of trusted xattr results in audit

[PATCH] ovl: filter of trusted xattr results in audit

[Mark Salyzyn]

When filtering xattr list for reading, presence of trusted xattr
results in a security audit log.  However, if there is other content
no errno will be set, and if there isn't, the errno will be -ENODATA
and not -EPERM as is usually associated with a lack of capability.
The check does not block the request to list the xattrs present.

Switch to has_capability_noaudit to reflect a more appropriate check.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: linux-security-module@vger.kernel.org
Cc: kernel-team@android.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org # v3.18
Fixes: upstream a082c6f680da ("ovl: filter trusted xattr for non-admin")
Fixes: 3.18 4bcc9b4b3a0a ("ovl: filter trusted xattr for non-admin")
---
Replaced ns_capable_noaudit with 3.18.y tree specific
has_capability_noaudit present in original submission to kernel.org
commit 5c2e9f346b815841f9bed6029ebcb06415caf640
("ovl: filter of trusted xattr results in audit")

 fs/overlayfs/inode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index a01ec1836a72..1175efa5e956 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -265,7 +265,8 @@ static bool ovl_can_list(const char *s)
 		return true;
 
 	/* Never list trusted.overlay, list other trusted for superuser only */
-	return !ovl_is_private_xattr(s) && capable(CAP_SYS_ADMIN);
+	return !ovl_is_private_xattr(s) &&
+	       has_capability_noaudit(current, CAP_SYS_ADMIN);
 }
 
 ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
-- 
2.23.0.581.g78d2f28ef7-goog

Re: [PATCH] ovl: filter of trusted xattr results in audit

[Greg Kroah-Hartman]

On Mon, Oct 07, 2019 at 09:09:16AM -0700, Mark Salyzyn wrote:
> When filtering xattr list for reading, presence of trusted xattr
> results in a security audit log.  However, if there is other content
> no errno will be set, and if there isn't, the errno will be -ENODATA
> and not -EPERM as is usually associated with a lack of capability.
> The check does not block the request to list the xattrs present.
> 
> Switch to has_capability_noaudit to reflect a more appropriate check.
> 
> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> Cc: linux-security-module@vger.kernel.org
> Cc: kernel-team@android.com
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: stable@vger.kernel.org # v3.18
> Fixes: upstream a082c6f680da ("ovl: filter trusted xattr for non-admin")
> Fixes: 3.18 4bcc9b4b3a0a ("ovl: filter trusted xattr for non-admin")
> ---
> Replaced ns_capable_noaudit with 3.18.y tree specific
> has_capability_noaudit present in original submission to kernel.org
> commit 5c2e9f346b815841f9bed6029ebcb06415caf640
> ("ovl: filter of trusted xattr results in audit")
> 
>  fs/overlayfs/inode.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index a01ec1836a72..1175efa5e956 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -265,7 +265,8 @@ static bool ovl_can_list(const char *s)
>  		return true;
>  
>  	/* Never list trusted.overlay, list other trusted for superuser only */
> -	return !ovl_is_private_xattr(s) && capable(CAP_SYS_ADMIN);
> +	return !ovl_is_private_xattr(s) &&
> +	       has_capability_noaudit(current, CAP_SYS_ADMIN);
>  }
>  
>  ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> -- 
> 2.23.0.581.g78d2f28ef7-goog
> 

Thanks for the backport, this one worked!

greg k-h

Re: [PATCH] ovl: filter of trusted xattr results in audit

[Greg Kroah-Hartman]

On Mon, Oct 07, 2019 at 06:16:16PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 07, 2019 at 09:09:16AM -0700, Mark Salyzyn wrote:
> > When filtering xattr list for reading, presence of trusted xattr
> > results in a security audit log.  However, if there is other content
> > no errno will be set, and if there isn't, the errno will be -ENODATA
> > and not -EPERM as is usually associated with a lack of capability.
> > The check does not block the request to list the xattrs present.
> > 
> > Switch to has_capability_noaudit to reflect a more appropriate check.
> > 
> > Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> > Cc: linux-security-module@vger.kernel.org
> > Cc: kernel-team@android.com
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: stable@vger.kernel.org # v3.18
> > Fixes: upstream a082c6f680da ("ovl: filter trusted xattr for non-admin")
> > Fixes: 3.18 4bcc9b4b3a0a ("ovl: filter trusted xattr for non-admin")
> > ---
> > Replaced ns_capable_noaudit with 3.18.y tree specific
> > has_capability_noaudit present in original submission to kernel.org
> > commit 5c2e9f346b815841f9bed6029ebcb06415caf640
> > ("ovl: filter of trusted xattr results in audit")
> > 
> >  fs/overlayfs/inode.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > index a01ec1836a72..1175efa5e956 100644
> > --- a/fs/overlayfs/inode.c
> > +++ b/fs/overlayfs/inode.c
> > @@ -265,7 +265,8 @@ static bool ovl_can_list(const char *s)
> >  		return true;
> >  
> >  	/* Never list trusted.overlay, list other trusted for superuser only */
> > -	return !ovl_is_private_xattr(s) && capable(CAP_SYS_ADMIN);
> > +	return !ovl_is_private_xattr(s) &&
> > +	       has_capability_noaudit(current, CAP_SYS_ADMIN);
> >  }
> >  
> >  ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> > -- 
> > 2.23.0.581.g78d2f28ef7-goog
> > 
> 
> Thanks for the backport, this one worked!

I spoke too soon:

ERROR: "has_capability_noaudit" [fs/overlayfs/overlay.ko] undefined!

That function isn't exported for modules :(

greg k-h

Re: [PATCH] ovl: filter of trusted xattr results in audit

[Greg Kroah-Hartman]

On Mon, Oct 07, 2019 at 06:17:25PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 07, 2019 at 06:16:16PM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Oct 07, 2019 at 09:09:16AM -0700, Mark Salyzyn wrote:
> > > When filtering xattr list for reading, presence of trusted xattr
> > > results in a security audit log.  However, if there is other content
> > > no errno will be set, and if there isn't, the errno will be -ENODATA
> > > and not -EPERM as is usually associated with a lack of capability.
> > > The check does not block the request to list the xattrs present.
> > > 
> > > Switch to has_capability_noaudit to reflect a more appropriate check.
> > > 
> > > Signed-off-by: Mark Salyzyn <salyzyn@android.com>
> > > Cc: linux-security-module@vger.kernel.org
> > > Cc: kernel-team@android.com
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Cc: stable@vger.kernel.org # v3.18
> > > Fixes: upstream a082c6f680da ("ovl: filter trusted xattr for non-admin")
> > > Fixes: 3.18 4bcc9b4b3a0a ("ovl: filter trusted xattr for non-admin")
> > > ---
> > > Replaced ns_capable_noaudit with 3.18.y tree specific
> > > has_capability_noaudit present in original submission to kernel.org
> > > commit 5c2e9f346b815841f9bed6029ebcb06415caf640
> > > ("ovl: filter of trusted xattr results in audit")
> > > 
> > >  fs/overlayfs/inode.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > > index a01ec1836a72..1175efa5e956 100644
> > > --- a/fs/overlayfs/inode.c
> > > +++ b/fs/overlayfs/inode.c
> > > @@ -265,7 +265,8 @@ static bool ovl_can_list(const char *s)
> > >  		return true;
> > >  
> > >  	/* Never list trusted.overlay, list other trusted for superuser only */
> > > -	return !ovl_is_private_xattr(s) && capable(CAP_SYS_ADMIN);
> > > +	return !ovl_is_private_xattr(s) &&
> > > +	       has_capability_noaudit(current, CAP_SYS_ADMIN);
> > >  }
> > >  
> > >  ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
> > > -- 
> > > 2.23.0.581.g78d2f28ef7-goog
> > > 
> > 
> > Thanks for the backport, this one worked!
> 
> I spoke too soon:
> 
> ERROR: "has_capability_noaudit" [fs/overlayfs/overlay.ko] undefined!
> 
> That function isn't exported for modules :(

But, if this really is needed, and it fixes the issue, I'll go export
that symbol with EXPORT_SYMBOL_GPL() to fix the problem.  Any
objections?

thanks,

greg k-h

Re: [PATCH] ovl: filter of trusted xattr results in audit

[Mark Salyzyn]

On 10/7/19 9:17 AM, Greg Kroah-Hartman wrote:
> On Mon, Oct 07, 2019 at 06:16:16PM +0200, Greg Kroah-Hartman wrote:
>> On Mon, Oct 07, 2019 at 09:09:16AM -0700, Mark Salyzyn wrote:
>>> When filtering xattr list for reading, presence of trusted xattr
>>> results in a security audit log.  However, if there is other content
>>> no errno will be set, and if there isn't, the errno will be -ENODATA
>>> and not -EPERM as is usually associated with a lack of capability.
>>> The check does not block the request to list the xattrs present.
>>>
>>> Switch to has_capability_noaudit to reflect a more appropriate check.
>>>
>>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>>> Cc: linux-security-module@vger.kernel.org
>>> Cc: kernel-team@android.com
>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>> Cc: stable@vger.kernel.org # v3.18
>>> Fixes: upstream a082c6f680da ("ovl: filter trusted xattr for non-admin")
>>> Fixes: 3.18 4bcc9b4b3a0a ("ovl: filter trusted xattr for non-admin")
>>> ---
>>> Replaced ns_capable_noaudit with 3.18.y tree specific
>>> has_capability_noaudit present in original submission to kernel.org
>>> commit 5c2e9f346b815841f9bed6029ebcb06415caf640
>>> ("ovl: filter of trusted xattr results in audit")
>>>
>>>   fs/overlayfs/inode.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
>>> index a01ec1836a72..1175efa5e956 100644
>>> --- a/fs/overlayfs/inode.c
>>> +++ b/fs/overlayfs/inode.c
>>> @@ -265,7 +265,8 @@ static bool ovl_can_list(const char *s)
>>>   		return true;
>>>   
>>>   	/* Never list trusted.overlay, list other trusted for superuser only */
>>> -	return !ovl_is_private_xattr(s) && capable(CAP_SYS_ADMIN);
>>> +	return !ovl_is_private_xattr(s) &&
>>> +	       has_capability_noaudit(current, CAP_SYS_ADMIN);
>>>   }
>>>   
>>>   ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>>> -- 
>>> 2.23.0.581.g78d2f28ef7-goog
>>>
>> Thanks for the backport, this one worked!
> I spoke too soon:
>
> ERROR: "has_capability_noaudit" [fs/overlayfs/overlay.ko] undefined!
>
> That function isn't exported for modules :(
>
> greg k-h

<sigh>

Now what is the playbook, we have three options in order of preference:

1) #ifdef MODULE use capable() to preserve API, add a short comment 
about the side effects if overlayfs is used as a module.

2) export has_capability_nodaudit (proc and oom_kill use it, and are 
both built-in only), but affect the 3.18 API at near EOL. AFAIK no one 
wants that?

3) Do nothing more. Make this a distro concern only. Leave this posted 
as a back-port for the record, but never merged, for those that are 
_interested_ and declare 3.18 stable as noisy for sepolicy and overlayfs 
under some usage patterns with few user space mitigation unless they 
explicitly take this back-port into their tree (eg: android common 
kernel) if used built-in. This way, in 3.18.y at least the module and 
built-in version behave the _same_ in stable.

Looking for feedback.

Sincerely -- Mark Salyzyn

Re: [PATCH] ovl: filter of trusted xattr results in audit

[Mark Salyzyn]

On 10/7/19 9:40 AM, Greg Kroah-Hartman wrote:
> On Mon, Oct 07, 2019 at 06:17:25PM +0200, Greg Kroah-Hartman wrote:
>> On Mon, Oct 07, 2019 at 06:16:16PM +0200, Greg Kroah-Hartman wrote:
>>> On Mon, Oct 07, 2019 at 09:09:16AM -0700, Mark Salyzyn wrote:
>>>> When filtering xattr list for reading, presence of trusted xattr
>>>> results in a security audit log.  However, if there is other content
>>>> no errno will be set, and if there isn't, the errno will be -ENODATA
>>>> and not -EPERM as is usually associated with a lack of capability.
>>>> The check does not block the request to list the xattrs present.
>>>>
>>>> Switch to has_capability_noaudit to reflect a more appropriate check.
>>>>
>>>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>>>> Cc: linux-security-module@vger.kernel.org
>>>> Cc: kernel-team@android.com
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Cc: stable@vger.kernel.org # v3.18
>>>> Fixes: upstream a082c6f680da ("ovl: filter trusted xattr for non-admin")
>>>> Fixes: 3.18 4bcc9b4b3a0a ("ovl: filter trusted xattr for non-admin")
>>>> ---
>>>> Replaced ns_capable_noaudit with 3.18.y tree specific
>>>> has_capability_noaudit present in original submission to kernel.org
>>>> commit 5c2e9f346b815841f9bed6029ebcb06415caf640
>>>> ("ovl: filter of trusted xattr results in audit")
>>>>
>>>>   fs/overlayfs/inode.c | 3 ++-
>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
>>>> index a01ec1836a72..1175efa5e956 100644
>>>> --- a/fs/overlayfs/inode.c
>>>> +++ b/fs/overlayfs/inode.c
>>>> @@ -265,7 +265,8 @@ static bool ovl_can_list(const char *s)
>>>>   		return true;
>>>>   
>>>>   	/* Never list trusted.overlay, list other trusted for superuser only */
>>>> -	return !ovl_is_private_xattr(s) && capable(CAP_SYS_ADMIN);
>>>> +	return !ovl_is_private_xattr(s) &&
>>>> +	       has_capability_noaudit(current, CAP_SYS_ADMIN);
>>>>   }
>>>>   
>>>>   ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>>>> -- 
>>>> 2.23.0.581.g78d2f28ef7-goog
>>>>
>>> Thanks for the backport, this one worked!
>> I spoke too soon:
>>
>> ERROR: "has_capability_noaudit" [fs/overlayfs/overlay.ko] undefined!
>>
>> That function isn't exported for modules :(
> But, if this really is needed, and it fixes the issue, I'll go export
> that symbol with EXPORT_SYMBOL_GPL() to fix the problem.  Any
> objections?
>
> thanks,
>
> greg k-h

Ok, you just answered my question in cross-emails. Yes, 
EXPORT_SYMBOL_GPL() (my option 2).


Thanks


-- Mark Salyzyn

Re: [PATCH] ovl: filter of trusted xattr results in audit

[Greg Kroah-Hartman]

On Mon, Oct 07, 2019 at 09:42:08AM -0700, Mark Salyzyn wrote:
> 
> <sigh>
> 
> Now what is the playbook, we have three options in order of preference:
> 
> 1) #ifdef MODULE use capable() to preserve API, add a short comment about
> the side effects if overlayfs is used as a module.
> 
> 2) export has_capability_nodaudit (proc and oom_kill use it, and are both
> built-in only), but affect the 3.18 API at near EOL. AFAIK no one wants
> that?

I'll just do this.  3.18 is EOL, this is only being done for a
distro-specific tree (i.e. AOSP).

> 3) Do nothing more. Make this a distro concern only. Leave this posted as a
> back-port for the record, but never merged, for those that are _interested_
> and declare 3.18 stable as noisy for sepolicy and overlayfs under some usage
> patterns with few user space mitigation unless they explicitly take this
> back-port into their tree (eg: android common kernel) if used built-in. This
> way, in 3.18.y at least the module and built-in version behave the _same_ in
> stable.

I'll just add the export to the patch and check this into AOSP, thanks!

greg k-h