Re: [kernel-hardening] Re: [RFC][PATCH] nfsd: add +1 to reference counting scheme for struct nfsd4_session

On Mon, Feb 13, 2017 at 06:46:19AM -0500, David Windsor wrote:

On Mon, Feb 13, 2017 at 5:54 AM, Hans Liljestrand [off-list ref] wrote:

quoted

On Sat, Feb 11, 2017 at 01:42:53AM -0500, David Windsor wrote:

quoted

<snip>

quoted

Signed-off-by: David Windsor <redacted>
---
 fs/nfsd/nfs4state.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index a0dee8a..b0f3010 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c

@@ -196,7 +196,7 @@ static void nfsd4_put_session_locked(struct

nfsd4_session *ses)

        lockdep_assert_held(&nn->client_lock);

-       if (atomic_dec_and_test(&ses->se_ref) && is_session_dead(ses))
+       if (!atomic_add_unless(&ses->se_ref, -1, 1) &&
is_session_des(ses))

This should read:
if (!atomic_add_unless(&ses->se_ref, -1, 1) && is_session_dead(ses))

quoted

                free_session(ses);

Hi,
I'm not sure if I have this correctly; But both before and after the patch
free_session gets called when se_ref count was 1, shouldn't this have
changed with the +1 scheme?

Also, since the !atomic_add_unless doesn't actually decrement when at 1,
doesn't this leave the se_ref as 1 when it's destroyed? The function seems
to always be locked, so perhaps this doesn't matter, but still seems a bit
risky.

Yes; I forgot the additional call to atomic_dec_and_test() before
free_session().  Thanks!

I'll resubmit this after seeing how the rest of this discussion goes.
We may end up abandoning this refcounting case.

I could live with it.

My knee jerk reaction is like Jeff's--it just seems more natural to me
for reference count 0 to mean "not in use, OK to free" in cases like
this--but maybe I just need to get used to the idea.

It'd be interesting to see what the final result looks like after
conversion to refcount_t.

--b.
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