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Message-ID: <CAEjxPJ7aXgOCP4+1Lbfe2b5fjB9Mu1n2h2juDY1RjPgP10PUxQ@mail.gmail.com>
Date: Thu, 20 Feb 2025 15:33:16 -0500
From: Stephen Smalley <stephen.smalley.work@...il.com>
To: Casey Schaufler <casey@...aufler-ca.com>
Cc: Paul Moore <paul@...l-moore.com>, linux-security-module@...r.kernel.org,
jmorris@...ei.org, serge@...lyn.com, keescook@...omium.org,
john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp,
linux-kernel@...r.kernel.org, selinux@...r.kernel.org, mic@...ikod.net,
ceph-devel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: [PATCH v3 4/5] LSM: lsm_context in security_dentry_init_security
On Thu, Feb 20, 2025 at 3:31 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
>
> On 2/20/2025 11:37 AM, Stephen Smalley wrote:
> > On Thu, Feb 20, 2025 at 2:33 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
> >> On 2/20/2025 10:16 AM, Stephen Smalley wrote:
> >>> On Thu, Feb 20, 2025 at 1:02 PM Stephen Smalley
> >>> <stephen.smalley.work@...il.com> wrote:
> >>>> On Thu, Feb 20, 2025 at 12:54 PM Paul Moore <paul@...l-moore.com> wrote:
> >>>>> On Thu, Feb 20, 2025 at 12:40 PM Paul Moore <paul@...l-moore.com> wrote:
> >>>>>> On Thu, Feb 20, 2025 at 11:43 AM Stephen Smalley
> >>>>>> <stephen.smalley.work@...il.com> wrote:
> >>>>>>> On Wed, Oct 23, 2024 at 5:23 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
> >>>>>>>> Replace the (secctx,seclen) pointer pair with a single lsm_context
> >>>>>>>> pointer to allow return of the LSM identifier along with the context
> >>>>>>>> and context length. This allows security_release_secctx() to know how
> >>>>>>>> to release the context. Callers have been modified to use or save the
> >>>>>>>> returned data from the new structure.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
> >>>>>>>> Cc: ceph-devel@...r.kernel.org
> >>>>>>>> Cc: linux-nfs@...r.kernel.org
> >>>>>>>> ---
> >>>>>>>> fs/ceph/super.h | 3 +--
> >>>>>>>> fs/ceph/xattr.c | 16 ++++++----------
> >>>>>>>> fs/fuse/dir.c | 35 ++++++++++++++++++-----------------
> >>>>>>>> fs/nfs/nfs4proc.c | 20 ++++++++++++--------
> >>>>>>>> include/linux/lsm_hook_defs.h | 2 +-
> >>>>>>>> include/linux/security.h | 26 +++-----------------------
> >>>>>>>> security/security.c | 9 ++++-----
> >>>>>>>> security/selinux/hooks.c | 9 +++++----
> >>>>>>>> 8 files changed, 50 insertions(+), 70 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> >>>>>>>> index 76776d716744..0b116ef3a752 100644
> >>>>>>>> --- a/fs/nfs/nfs4proc.c
> >>>>>>>> +++ b/fs/nfs/nfs4proc.c
> >>>>>>>> @@ -114,6 +114,7 @@ static inline struct nfs4_label *
> >>>>>>>> nfs4_label_init_security(struct inode *dir, struct dentry *dentry,
> >>>>>>>> struct iattr *sattr, struct nfs4_label *label)
> >>>>>>>> {
> >>>>>>>> + struct lsm_context shim;
> >>>>>>>> int err;
> >>>>>>>>
> >>>>>>>> if (label == NULL)
> >>>>>>>> @@ -128,21 +129,24 @@ nfs4_label_init_security(struct inode *dir, struct dentry *dentry,
> >>>>>>>> label->label = NULL;
> >>>>>>>>
> >>>>>>>> err = security_dentry_init_security(dentry, sattr->ia_mode,
> >>>>>>>> - &dentry->d_name, NULL,
> >>>>>>>> - (void **)&label->label, &label->len);
> >>>>>>>> - if (err == 0)
> >>>>>>>> - return label;
> >>>>>>>> + &dentry->d_name, NULL, &shim);
> >>>>>>>> + if (err)
> >>>>>>>> + return NULL;
> >>>>>>>>
> >>>>>>>> - return NULL;
> >>>>>>>> + label->label = shim.context;
> >>>>>>>> + label->len = shim.len;
> >>>>>>>> + return label;
> >>>>>>>> }
> >>>>>>>> static inline void
> >>>>>>>> nfs4_label_release_security(struct nfs4_label *label)
> >>>>>>>> {
> >>>>>>>> - struct lsm_context scaff; /* scaffolding */
> >>>>>>>> + struct lsm_context shim;
> >>>>>>>>
> >>>>>>>> if (label) {
> >>>>>>>> - lsmcontext_init(&scaff, label->label, label->len, 0);
> >>>>>>>> - security_release_secctx(&scaff);
> >>>>>>>> + shim.context = label->label;
> >>>>>>>> + shim.len = label->len;
> >>>>>>>> + shim.id = LSM_ID_UNDEF;
> >>>>>>> Is there a patch that follows this one to fix this? Otherwise, setting
> >>>>>>> this to UNDEF causes SELinux to NOT free the context, which produces a
> >>>>>>> memory leak for every NFS inode security context. Reported by kmemleak
> >>>>>>> when running the selinux-testsuite NFS tests.
> >>>>>> I don't recall seeing anything related to this, but patches are
> >>>>>> definitely welcome.
> >>>>> Looking at this quickly, this is an interesting problem as I don't
> >>>>> believe we have enough context in nfs4_label_release_security() to
> >>>>> correctly set the shim.id value. If there is a positive, it is that
> >>>>> lsm_context is really still just a string wrapped up with some
> >>>>> metadata, e.g. length/ID, so we kfree()'ing shim.context is going to
> >>>>> be okay-ish, at least for the foreseeable future.
> >>>>>
> >>>>> I can think of two ways to fix this, but I'd love to hear other ideas too.
> >>>>>
> >>>>> 1. Handle the LSM_ID_UNDEF case directly in security_release_secctx()
> >>>>> and skip any individual LSM processing.
> >>>>>
> >>>>> 2. Define a new LSM_ID_ANY value and update all of the LSMs to also
> >>>>> process the ANY case as well as their own.
> >>>>>
> >>>>> I'm not finding either option very exciting, but option #2 looks
> >>>>> particularly ugly, so I think I'd prefer to see someone draft a patch
> >>>>> for option #1 assuming nothing better is presented.
> >>>> We could perhaps add a u32 lsmid to struct nfs4_label, save it from
> >>>> the shim.id obtained in nfs4_label_init_security(), and use it in
> >>>> nfs4_label_release_security(). Not sure why that wasn't done in the
> >>>> first place.
> >>> Something like this (not tested yet). If this looks sane, will submit
> >>> separately.
> >>>
> >>> commit b530104f50e8 ("lsm: lsm_context in security_dentry_init_security")
> >>> did not preserve the lsm id for subsequent release calls, which results
> >>> in a memory leak. Fix it by saving the lsm id in the nfs4_label and
> >>> providing it on the subsequent release call.
> >>>
> >>> Fixes: b530104f50e8 ("lsm: lsm_context in security_dentry_init_security")
> >>> Signed-off-by: Stephen Smalley <stephen.smalley.work@...il.com>
> >> I'm not a fan of adding secids into other subsystems, especially in cases
> >> where they've tried to avoid them in the past.
> >>
> >> The better solution, which I'm tracking down the patch for now, is for
> >> the individual LSMs to always do their release, and for security_release_secctx()
> >> to check the lsm_id and call the appropriate LSM specific hook. Until there
> >> are multiple LSMs with contexts, LSM_ID_UNDEF is as good as a match.
> >>
> >> Please don't use this patch.
> > It doesn't add a secid; it just saves the LSM id obtained from
> > lsm_context populated by the security_dentry_init_security() hook call
> > and passes it back in the lsm_context to the security_release_secctx()
> > call.
>
> Right. Sorry. If you're going to do that, the nfs_label struct should
> just include a lsm_context instead. But that hit opposition when proposed
> initially.
>
> The practical solution has to acknowledge that at this stage there can only
> be one LSM providing contexts, and each LSM can release the context if the
> LSM is matches the LSM or is LSM_ID_UNDEF. That will change before SELinux,
> AppArmor and Smack can co-exist, but that's not yet available. For now the
> check
>
> if (cp->id == LSM_ID_SELINUX)
>
> can either be removed or changed to
>
> if (cp->id == LSM_ID_SELINUX || cp->id == LSM_ID_UNDEF)
>
> In a system that respects LSM_FLAG_LEGACY_MAJOR the id isn't relevant
> with the context using LSMs all being thus identified.
Shrug. My patch seemed cleaner, but I don't really care as long as it
is fixed, preferably before 6.14 goes final.
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