Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Paul Moore]

On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> passed down the rcu flag to the SELinux AVC, but failed to adjust the
> test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
> Previously, we only returned -ECHILD if generating an audit record with
> LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
> Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
> that dump_common_audit_data() calls d_find_alias() and collects the
> dname from the result if any.
> Other cases that might require similar treatment in the future are
> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
> a path or file is called under RCU-walk.
>
> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> ---
>  security/selinux/avc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> index 74c43ebe34bb..f1fa1072230c 100644
> --- a/security/selinux/avc.c
> +++ b/security/selinux/avc.c
> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
>          * during retry. However this is logically just as if the operation
>          * happened a little later.
>          */
> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
>             (flags & MAY_NOT_BLOCK))
>                 return -ECHILD;

Added the LSM list as I'm beginning to wonder if we should push this
logic down into common_lsm_audit(), this problem around blocking
shouldn't be SELinux specific.

For the LSM folks just joining, the full patchset can be found here:
* https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t

-- 
paul moore
www.paul-moore.com

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Paul Moore]

On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> > passed down the rcu flag to the SELinux AVC, but failed to adjust the
> > test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
> > Previously, we only returned -ECHILD if generating an audit record with
> > LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
> > Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
> > LSM_AUDIT_DATA_INODE only requires this handling due to the fact
> > that dump_common_audit_data() calls d_find_alias() and collects the
> > dname from the result if any.
> > Other cases that might require similar treatment in the future are
> > LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
> > a path or file is called under RCU-walk.
> >
> > Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> > Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> > ---
> >  security/selinux/avc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> > index 74c43ebe34bb..f1fa1072230c 100644
> > --- a/security/selinux/avc.c
> > +++ b/security/selinux/avc.c
> > @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
> >          * during retry. However this is logically just as if the operation
> >          * happened a little later.
> >          */
> > -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
> > +       if ((a->type == LSM_AUDIT_DATA_INODE ||
> > +            a->type == LSM_AUDIT_DATA_DENTRY) &&
> >             (flags & MAY_NOT_BLOCK))
> >                 return -ECHILD;

With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
in dump_common_audit_data() which could block, which is bad, that I
understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
on why that is bad?  It makes a few audit_log*() calls and one call to
d_backing_inode() which is non-blocking and trivial.

What am I missing?

> Added the LSM list as I'm beginning to wonder if we should push this
> logic down into common_lsm_audit(), this problem around blocking
> shouldn't be SELinux specific.
>
> For the LSM folks just joining, the full patchset can be found here:
> * https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t

--
paul moore
www.paul-moore.com

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Stephen Smalley]

On 11/21/19 7:30 PM, Paul Moore wrote:
> On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
>> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
>>> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>> passed down the rcu flag to the SELinux AVC, but failed to adjust the
>>> test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
>>> Previously, we only returned -ECHILD if generating an audit record with
>>> LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
>>> Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
>>> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
>>> that dump_common_audit_data() calls d_find_alias() and collects the
>>> dname from the result if any.
>>> Other cases that might require similar treatment in the future are
>>> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
>>> a path or file is called under RCU-walk.
>>>
>>> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
>>> ---
>>>   security/selinux/avc.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>>> index 74c43ebe34bb..f1fa1072230c 100644
>>> --- a/security/selinux/avc.c
>>> +++ b/security/selinux/avc.c
>>> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
>>>           * during retry. However this is logically just as if the operation
>>>           * happened a little later.
>>>           */
>>> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
>>> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
>>> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
>>>              (flags & MAY_NOT_BLOCK))
>>>                  return -ECHILD;
> 
> With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
> in dump_common_audit_data() which could block, which is bad, that I
> understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
> on why that is bad?  It makes a few audit_log*() calls and one call to
> d_backing_inode() which is non-blocking and trivial.
> 
> What am I missing?

For those who haven't, you may wish to also read the earlier thread here 
that led to this one:
https://lore.kernel.org/selinux/20191119184057.14961-1-will@kernel.org/T/#t

AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY 
case truly block (d_find_alias does not block AFAICT, nor should 
audit_log* as long as we use audit_log_start with GFP_ATOMIC or 
GFP_NOWAIT). My impression from the comment in slow_avc_audit() is that 
the issue is not really about blocking but rather about the inability to 
safely dereference the dentry->d_name during RCU walk, which is 
something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH 
or _FILE, but neither of the latter two are currently used from the two 
hooks that are called during RCU walk, inode_permission and 
inode_follow_link).  Originally _PATH, _DENTRY, and _INODE were all 
under a single _FS type and the original test in slow_avc_audit() was 
against LSM_AUDIT_DATA_FS before the split.

> 
>> Added the LSM list as I'm beginning to wonder if we should push this
>> logic down into common_lsm_audit(), this problem around blocking
>> shouldn't be SELinux specific.

That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to 
common_lsm_audit() just so that it could immediately return if that is 
set and a->type is _INODE or _DENTRY (or _PATH or _FILE).  And the 
individual security module still needs to have its own handling of 
MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security 
module authors from thinking about it.  This is only relevant for 
modules implementing the inode_permission and/or inode_follow_link 
hooks, so it only currently affects SELinux and Smack, and Smack only 
presently implements inode_permission and always returns -ECHILD if 
MAY_NOT_BLOCK (aside from a couple trivial cases), so it will never 
reach common_lsm_audit() in that case.


>>
>> For the LSM folks just joining, the full patchset can be found here:
>> * https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Stephen Smalley]

On 11/22/19 8:37 AM, Stephen Smalley wrote:
> On 11/21/19 7:30 PM, Paul Moore wrote:
>> On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
>>> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> 
>>> wrote:
>>>> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>>> passed down the rcu flag to the SELinux AVC, but failed to adjust the
>>>> test in slow_avc_audit() to also return -ECHILD on 
>>>> LSM_AUDIT_DATA_DENTRY.
>>>> Previously, we only returned -ECHILD if generating an audit record with
>>>> LSM_AUDIT_DATA_INODE since this was only relevant from 
>>>> inode_permission.
>>>> Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
>>>> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
>>>> that dump_common_audit_data() calls d_find_alias() and collects the
>>>> dname from the result if any.
>>>> Other cases that might require similar treatment in the future are
>>>> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
>>>> a path or file is called under RCU-walk.
>>>>
>>>> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
>>>> ---
>>>>   security/selinux/avc.c | 3 ++-
>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>>>> index 74c43ebe34bb..f1fa1072230c 100644
>>>> --- a/security/selinux/avc.c
>>>> +++ b/security/selinux/avc.c
>>>> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state 
>>>> *state,
>>>>           * during retry. However this is logically just as if the 
>>>> operation
>>>>           * happened a little later.
>>>>           */
>>>> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
>>>> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
>>>> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
>>>>              (flags & MAY_NOT_BLOCK))
>>>>                  return -ECHILD;
>>
>> With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
>> in dump_common_audit_data() which could block, which is bad, that I
>> understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
>> on why that is bad?  It makes a few audit_log*() calls and one call to
>> d_backing_inode() which is non-blocking and trivial.
>>
>> What am I missing?
> 
> For those who haven't, you may wish to also read the earlier thread here 
> that led to this one:
> https://lore.kernel.org/selinux/20191119184057.14961-1-will@kernel.org/T/#t
> 
> AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY 
> case truly block (d_find_alias does not block AFAICT, nor should 
> audit_log* as long as we use audit_log_start with GFP_ATOMIC or 
> GFP_NOWAIT). My impression from the comment in slow_avc_audit() is that 
> the issue is not really about blocking but rather about the inability to 
> safely dereference the dentry->d_name during RCU walk, which is 
> something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH 
> or _FILE, but neither of the latter two are currently used from the two 
> hooks that are called during RCU walk, inode_permission and 
> inode_follow_link).  Originally _PATH, _DENTRY, and _INODE were all 
> under a single _FS type and the original test in slow_avc_audit() was 
> against LSM_AUDIT_DATA_FS before the split.
> 
>>
>>> Added the LSM list as I'm beginning to wonder if we should push this
>>> logic down into common_lsm_audit(), this problem around blocking
>>> shouldn't be SELinux specific.
> 
> That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to 
> common_lsm_audit() just so that it could immediately return if that is 
> set and a->type is _INODE or _DENTRY (or _PATH or _FILE).  And the 
> individual security module still needs to have its own handling of 
> MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security 
> module authors from thinking about it.  This is only relevant for 
> modules implementing the inode_permission and/or inode_follow_link 
> hooks, so it only currently affects SELinux and Smack, and Smack only 
> presently implements inode_permission and always returns -ECHILD if 
> MAY_NOT_BLOCK (aside from a couple trivial cases), so it will never 
> reach common_lsm_audit() in that case.

This would also require changing common_lsm_audit() to be able to return 
errors so that it can return -ECHILD and updating all callers to handle 
that.

> 
> 
>>>
>>> For the LSM folks just joining, the full patchset can be found here:
>>> * 
>>> https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t 
>>>

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Paul Moore]

On Fri, Nov 22, 2019 at 8:37 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> On 11/21/19 7:30 PM, Paul Moore wrote:
> > On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
> >> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> >>> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> >>> passed down the rcu flag to the SELinux AVC, but failed to adjust the
> >>> test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
> >>> Previously, we only returned -ECHILD if generating an audit record with
> >>> LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
> >>> Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
> >>> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
> >>> that dump_common_audit_data() calls d_find_alias() and collects the
> >>> dname from the result if any.
> >>> Other cases that might require similar treatment in the future are
> >>> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
> >>> a path or file is called under RCU-walk.
> >>>
> >>> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> >>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> >>> ---
> >>>   security/selinux/avc.c | 3 ++-
> >>>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> >>> index 74c43ebe34bb..f1fa1072230c 100644
> >>> --- a/security/selinux/avc.c
> >>> +++ b/security/selinux/avc.c
> >>> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
> >>>           * during retry. However this is logically just as if the operation
> >>>           * happened a little later.
> >>>           */
> >>> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
> >>> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
> >>> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
> >>>              (flags & MAY_NOT_BLOCK))
> >>>                  return -ECHILD;
> >
> > With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
> > in dump_common_audit_data() which could block, which is bad, that I
> > understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
> > on why that is bad?  It makes a few audit_log*() calls and one call to
> > d_backing_inode() which is non-blocking and trivial.
> >
> > What am I missing?
>
> For those who haven't, you may wish to also read the earlier thread here
> that led to this one:
> https://lore.kernel.org/selinux/20191119184057.14961-1-will@kernel.org/T/#t
>
> AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY
> case truly block (d_find_alias does not block AFAICT, nor should
> audit_log* as long as we use audit_log_start with GFP_ATOMIC or
> GFP_NOWAIT).

Yes, the audit_log*() functions should be safe, if not I would
consider that a bug; I thought d_find_alias() might block, but it's
very likely I'm wrong in that regard.

> My impression from the comment in slow_avc_audit() is that
> the issue is not really about blocking but rather about the inability to
> safely dereference the dentry->d_name during RCU walk, which is
> something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH
> or _FILE, but neither of the latter two are currently used from the two
> hooks that are called during RCU walk, inode_permission and
> inode_follow_link).  Originally _PATH, _DENTRY, and _INODE were all
> under a single _FS type and the original test in slow_avc_audit() was
> against LSM_AUDIT_DATA_FS before the split.

Thanks, I think that is the part I was missing.  I was focused too
much on the VFS stuff that I didn't pay enough attention to
slow_avc_audit().

If that is the case, the comment and code in dentry_cmp() would seem
to indicate that it would be safe to fetch the dentry name string as
long as we use READ_ONCE().  The length field in the qstr might be
off, but the audit_log_untrustedstring() function doesn't use the
qstr's length information.  I suppose if we don't mind the extra
spinlock we could use take_dentry_name_snapshot(); that should be safe
and we are already in the "slow" path.  I didn't check the _PATH or
_FILE cases.

Once again, let me know if I'm missing something.

As an aside, if we somehow can guarantee (e.g. via a name_snapshot)
that qstr length information is valid, we might want to consider
moving from audit_log_unstrustedstring() to
audit_log_n_untrustedstring() to save us a call to strlen().

> >> Added the LSM list as I'm beginning to wonder if we should push this
> >> logic down into common_lsm_audit(), this problem around blocking
> >> shouldn't be SELinux specific.
>
> That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to
> common_lsm_audit() just so that it could immediately return if that is
> set and a->type is _INODE or _DENTRY (or _PATH or _FILE).  And the
> individual security module still needs to have its own handling of
> MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security
> module authors from thinking about it.

Looking at the current SELinux code, all we do is bail out early with
-ECHILD.  If we didn't have that check it looks like the only impact
would be some extra assignments into a struct living on the stack and
a call into common_lsm_audit().  That doesn't seem terrible for a slow
path, especially if it pushes this code into a LSM common function.

> >> For the LSM folks just joining, the full patchset can be found here:
> >> * https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t

-- 
paul moore
www.paul-moore.com

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Stephen Smalley]

On 11/22/19 9:49 AM, Paul Moore wrote:
> On Fri, Nov 22, 2019 at 8:37 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
>> On 11/21/19 7:30 PM, Paul Moore wrote:
>>> On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
>>>> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
>>>>> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>>>> passed down the rcu flag to the SELinux AVC, but failed to adjust the
>>>>> test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
>>>>> Previously, we only returned -ECHILD if generating an audit record with
>>>>> LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
>>>>> Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
>>>>> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
>>>>> that dump_common_audit_data() calls d_find_alias() and collects the
>>>>> dname from the result if any.
>>>>> Other cases that might require similar treatment in the future are
>>>>> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
>>>>> a path or file is called under RCU-walk.
>>>>>
>>>>> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>>>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
>>>>> ---
>>>>>    security/selinux/avc.c | 3 ++-
>>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>>>>> index 74c43ebe34bb..f1fa1072230c 100644
>>>>> --- a/security/selinux/avc.c
>>>>> +++ b/security/selinux/avc.c
>>>>> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
>>>>>            * during retry. However this is logically just as if the operation
>>>>>            * happened a little later.
>>>>>            */
>>>>> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
>>>>> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
>>>>> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
>>>>>               (flags & MAY_NOT_BLOCK))
>>>>>                   return -ECHILD;
>>>
>>> With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
>>> in dump_common_audit_data() which could block, which is bad, that I
>>> understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
>>> on why that is bad?  It makes a few audit_log*() calls and one call to
>>> d_backing_inode() which is non-blocking and trivial.
>>>
>>> What am I missing?
>>
>> For those who haven't, you may wish to also read the earlier thread here
>> that led to this one:
>> https://lore.kernel.org/selinux/20191119184057.14961-1-will@kernel.org/T/#t
>>
>> AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY
>> case truly block (d_find_alias does not block AFAICT, nor should
>> audit_log* as long as we use audit_log_start with GFP_ATOMIC or
>> GFP_NOWAIT).
> 
> Yes, the audit_log*() functions should be safe, if not I would
> consider that a bug; I thought d_find_alias() might block, but it's
> very likely I'm wrong in that regard.

No, it doesn't appear to block.  However, it does take d_lock and 
increment d_lockref.count, which IIUC aren't permitted during rcu-walk.

>> My impression from the comment in slow_avc_audit() is that
>> the issue is not really about blocking but rather about the inability to
>> safely dereference the dentry->d_name during RCU walk, which is
>> something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH
>> or _FILE, but neither of the latter two are currently used from the two
>> hooks that are called during RCU walk, inode_permission and
>> inode_follow_link).  Originally _PATH, _DENTRY, and _INODE were all
>> under a single _FS type and the original test in slow_avc_audit() was
>> against LSM_AUDIT_DATA_FS before the split.
> 
> Thanks, I think that is the part I was missing.  I was focused too
> much on the VFS stuff that I didn't pay enough attention to
> slow_avc_audit().
> 
> If that is the case, the comment and code in dentry_cmp() would seem
> to indicate that it would be safe to fetch the dentry name string as
> long as we use READ_ONCE().  The length field in the qstr might be
> off, but the audit_log_untrustedstring() function doesn't use the
> qstr's length information.  I suppose if we don't mind the extra
> spinlock we could use take_dentry_name_snapshot(); that should be safe
> and we are already in the "slow" path.  I didn't check the _PATH or
> _FILE cases.
> 
> Once again, let me know if I'm missing something.

We can't take any spinlocks on the dentry during rcu-walk IIUC; that 
would defeat the purpose. In looking for a parallel with filesystem 
implementations, I noted that fs/namei.c:get_link() doesn't even pass 
the dentry to the filesystem get_link() method in the rcu-walk case, 
only doing so under ref-walk.  So they won't permit the filesystem 
implementations to ever dereference the dentry for get_link() under 
rcu-walk.  Not sure why it gets passed to security_inode_follow_link() 
then, or if it is ever safe for a security module to dereference its fields.

I was hoping to get fsdevel folks to comment since I feel like we're 
guessing about exactly what guarantees we have in this area.

> 
> As an aside, if we somehow can guarantee (e.g. via a name_snapshot)
> that qstr length information is valid, we might want to consider
> moving from audit_log_unstrustedstring() to
> audit_log_n_untrustedstring() to save us a call to strlen().
> 
>>>> Added the LSM list as I'm beginning to wonder if we should push this
>>>> logic down into common_lsm_audit(), this problem around blocking
>>>> shouldn't be SELinux specific.
>>
>> That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to
>> common_lsm_audit() just so that it could immediately return if that is
>> set and a->type is _INODE or _DENTRY (or _PATH or _FILE).  And the
>> individual security module still needs to have its own handling of
>> MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security
>> module authors from thinking about it.
> 
> Looking at the current SELinux code, all we do is bail out early with
> -ECHILD.  If we didn't have that check it looks like the only impact
> would be some extra assignments into a struct living on the stack and
> a call into common_lsm_audit().  That doesn't seem terrible for a slow
> path, especially if it pushes this code into a LSM common function.

Not terrible, just not sure if it ends up being a worthwhile change.  If 
the LSM maintainers would like it that way, I can do that.

> 
>>>> For the LSM folks just joining, the full patchset can be found here:
>>>> * https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t
> 

Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Stephen Smalley]

On 11/22/19 10:09 AM, Stephen Smalley wrote:
> On 11/22/19 9:49 AM, Paul Moore wrote:
>> On Fri, Nov 22, 2019 at 8:37 AM Stephen Smalley <sds@tycho.nsa.gov> 
>> wrote:
>>> On 11/21/19 7:30 PM, Paul Moore wrote:
>>>> On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
>>>>> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> 
>>>>> wrote:
>>>>>> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk 
>>>>>> aware")
>>>>>> passed down the rcu flag to the SELinux AVC, but failed to adjust the
>>>>>> test in slow_avc_audit() to also return -ECHILD on 
>>>>>> LSM_AUDIT_DATA_DENTRY.
>>>>>> Previously, we only returned -ECHILD if generating an audit record 
>>>>>> with
>>>>>> LSM_AUDIT_DATA_INODE since this was only relevant from 
>>>>>> inode_permission.
>>>>>> Return -ECHILD on either LSM_AUDIT_DATA_INODE or 
>>>>>> LSM_AUDIT_DATA_DENTRY.
>>>>>> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
>>>>>> that dump_common_audit_data() calls d_find_alias() and collects the
>>>>>> dname from the result if any.
>>>>>> Other cases that might require similar treatment in the future are
>>>>>> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
>>>>>> a path or file is called under RCU-walk.
>>>>>>
>>>>>> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk 
>>>>>> aware")
>>>>>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
>>>>>> ---
>>>>>>    security/selinux/avc.c | 3 ++-
>>>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>>>>>> index 74c43ebe34bb..f1fa1072230c 100644
>>>>>> --- a/security/selinux/avc.c
>>>>>> +++ b/security/selinux/avc.c
>>>>>> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct 
>>>>>> selinux_state *state,
>>>>>>            * during retry. However this is logically just as if 
>>>>>> the operation
>>>>>>            * happened a little later.
>>>>>>            */
>>>>>> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
>>>>>> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
>>>>>> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
>>>>>>               (flags & MAY_NOT_BLOCK))
>>>>>>                   return -ECHILD;
>>>>
>>>> With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
>>>> in dump_common_audit_data() which could block, which is bad, that I
>>>> understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
>>>> on why that is bad?  It makes a few audit_log*() calls and one call to
>>>> d_backing_inode() which is non-blocking and trivial.
>>>>
>>>> What am I missing?
>>>
>>> For those who haven't, you may wish to also read the earlier thread here
>>> that led to this one:
>>> https://lore.kernel.org/selinux/20191119184057.14961-1-will@kernel.org/T/#t 
>>>
>>>
>>> AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY
>>> case truly block (d_find_alias does not block AFAICT, nor should
>>> audit_log* as long as we use audit_log_start with GFP_ATOMIC or
>>> GFP_NOWAIT).
>>
>> Yes, the audit_log*() functions should be safe, if not I would
>> consider that a bug; I thought d_find_alias() might block, but it's
>> very likely I'm wrong in that regard.
> 
> No, it doesn't appear to block.  However, it does take d_lock and 
> increment d_lockref.count, which IIUC aren't permitted during rcu-walk.
> 
>>> My impression from the comment in slow_avc_audit() is that
>>> the issue is not really about blocking but rather about the inability to
>>> safely dereference the dentry->d_name during RCU walk, which is
>>> something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH
>>> or _FILE, but neither of the latter two are currently used from the two
>>> hooks that are called during RCU walk, inode_permission and
>>> inode_follow_link).  Originally _PATH, _DENTRY, and _INODE were all
>>> under a single _FS type and the original test in slow_avc_audit() was
>>> against LSM_AUDIT_DATA_FS before the split.
>>
>> Thanks, I think that is the part I was missing.  I was focused too
>> much on the VFS stuff that I didn't pay enough attention to
>> slow_avc_audit().
>>
>> If that is the case, the comment and code in dentry_cmp() would seem
>> to indicate that it would be safe to fetch the dentry name string as
>> long as we use READ_ONCE().  The length field in the qstr might be
>> off, but the audit_log_untrustedstring() function doesn't use the
>> qstr's length information.  I suppose if we don't mind the extra
>> spinlock we could use take_dentry_name_snapshot(); that should be safe
>> and we are already in the "slow" path.  I didn't check the _PATH or
>> _FILE cases.
>>
>> Once again, let me know if I'm missing something.
> 
> We can't take any spinlocks on the dentry during rcu-walk IIUC; that 
> would defeat the purpose. In looking for a parallel with filesystem 
> implementations, I noted that fs/namei.c:get_link() doesn't even pass 
> the dentry to the filesystem get_link() method in the rcu-walk case, 
> only doing so under ref-walk.  So they won't permit the filesystem 
> implementations to ever dereference the dentry for get_link() under 
> rcu-walk.  Not sure why it gets passed to security_inode_follow_link() 
> then, or if it is ever safe for a security module to dereference its 
> fields.
> 
> I was hoping to get fsdevel folks to comment since I feel like we're 
> guessing about exactly what guarantees we have in this area.
> 
>>
>> As an aside, if we somehow can guarantee (e.g. via a name_snapshot)
>> that qstr length information is valid, we might want to consider
>> moving from audit_log_unstrustedstring() to
>> audit_log_n_untrustedstring() to save us a call to strlen().
>>
>>>>> Added the LSM list as I'm beginning to wonder if we should push this
>>>>> logic down into common_lsm_audit(), this problem around blocking
>>>>> shouldn't be SELinux specific.
>>>
>>> That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to
>>> common_lsm_audit() just so that it could immediately return if that is
>>> set and a->type is _INODE or _DENTRY (or _PATH or _FILE).  And the
>>> individual security module still needs to have its own handling of
>>> MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security
>>> module authors from thinking about it.
>>
>> Looking at the current SELinux code, all we do is bail out early with
>> -ECHILD.  If we didn't have that check it looks like the only impact
>> would be some extra assignments into a struct living on the stack and
>> a call into common_lsm_audit().  That doesn't seem terrible for a slow
>> path, especially if it pushes this code into a LSM common function.
> 
> Not terrible, just not sure if it ends up being a worthwhile change.  If 
> the LSM maintainers would like it that way, I can do that.

I think this rendered moot by viro's suggestion, since we are taking the 
handling of MAY_NOT_BLOCK up even earlier in the processing and the 
flags don't need to be passed down to slow_avc_audit() anymore.  Sure, 
we could still pass them down and defer the handling to 
common_lsm_audit(), but that's just extra wasted work before we bail 
out, and we are no longer even testing the a->type field with the new 
logic so there is no longer anything related to the lsm_audit 
implementation.

> 
>>
>>>>> For the LSM folks just joining, the full patchset can be found here:
>>>>> * 
>>>>> https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t 
>>>>>
>>
>