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Date:   Tue, 10 Oct 2023 11:14:00 +0200
From:   =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mic@digikod.net>
To:     Paul Moore <paul@paul-moore.com>
Cc:     Casey Schaufler <casey@schaufler-ca.com>,
        linux-security-module@vger.kernel.org, jmorris@namei.org,
        serge@hallyn.com, keescook@chromium.org,
        john.johansen@canonical.com, penguin-kernel@i-love.sakura.ne.jp,
        stephen.smalley.work@gmail.com, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org
Subject: Re: [PATCH v15 04/11] LSM: syscalls for current process attributes
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On Mon, Oct 09, 2023 at 12:04:31PM -0400, Paul Moore wrote:
> On Mon, Oct 9, 2023 at 11:37 AM Mickaël Salaün <mic@digikod.net> wrote:
> > On Thu, Oct 05, 2023 at 09:04:34PM -0400, Paul Moore wrote:
> > > On Tue, Oct 3, 2023 at 10:09 AM Mickaël Salaün <mic@digikod.net> wrote:
> > > > On Tue, Sep 12, 2023 at 01:56:49PM -0700, Casey Schaufler wrote:
> > > > > Create a system call lsm_get_self_attr() to provide the security
> > > > > module maintained attributes of the current process.
> > > > > Create a system call lsm_set_self_attr() to set a security
> > > > > module maintained attribute of the current process.
> > > > > Historically these attributes have been exposed to user space via
> > > > > entries in procfs under /proc/self/attr.
> > > > >
> > > > > The attribute value is provided in a lsm_ctx structure. The structure
> > > > > identifies the size of the attribute, and the attribute value. The format
> > > > > of the attribute value is defined by the security module. A flags field
> > > > > is included for LSM specific information. It is currently unused and must
> > > > > be 0. The total size of the data, including the lsm_ctx structure and any
> > > > > padding, is maintained as well.
> > > > >
> > > > > struct lsm_ctx {
> > > > >         __u64 id;
> > > > >         __u64 flags;
> > > > >         __u64 len;
> > > > >         __u64 ctx_len;
> > > > >         __u8 ctx[];
> > > > > };
> > > > >
> > > > > Two new LSM hooks are used to interface with the LSMs.
> > > > > security_getselfattr() collects the lsm_ctx values from the
> > > > > LSMs that support the hook, accounting for space requirements.
> > > > > security_setselfattr() identifies which LSM the attribute is
> > > > > intended for and passes it along.
> > > > >
> > > > > Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> > > > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > > > Reviewed-by: Serge Hallyn <serge@hallyn.com>
> > > > > Reviewed-by: John Johansen <john.johansen@canonical.com>
> > > > > ---
> > > > >  Documentation/userspace-api/lsm.rst |  70 +++++++++++++
> > > > >  include/linux/lsm_hook_defs.h       |   4 +
> > > > >  include/linux/lsm_hooks.h           |   1 +
> > > > >  include/linux/security.h            |  19 ++++
> > > > >  include/linux/syscalls.h            |   5 +
> > > > >  include/uapi/linux/lsm.h            |  36 +++++++
> > > > >  kernel/sys_ni.c                     |   2 +
> > > > >  security/Makefile                   |   1 +
> > > > >  security/lsm_syscalls.c             |  57 +++++++++++
> > > > >  security/security.c                 | 152 ++++++++++++++++++++++++++++
> > > > >  10 files changed, 347 insertions(+)
> > > > >  create mode 100644 Documentation/userspace-api/lsm.rst
> > > > >  create mode 100644 security/lsm_syscalls.c
> > >
> > > ...
> > >
> > > > > diff --git a/security/security.c b/security/security.c
> > > > > index a3489c04b783..0d179750d964 100644
> > > > > --- a/security/security.c
> > > > > +++ b/security/security.c
> > > > > @@ -3837,6 +3837,158 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
> > > > >  }
> > > > >  EXPORT_SYMBOL(security_d_instantiate);
> > > > >
> > > > > +/*
> > > > > + * Please keep this in sync with it's counterpart in security/lsm_syscalls.c
> > > > > + */
> > > > > +
> > > > > +/**
> > > > > + * security_getselfattr - Read an LSM attribute of the current process.
> > > > > + * @attr: which attribute to return
> > > > > + * @uctx: the user-space destination for the information, or NULL
> > > > > + * @size: pointer to the size of space available to receive the data
> > > > > + * @flags: special handling options. LSM_FLAG_SINGLE indicates that only
> > > > > + * attributes associated with the LSM identified in the passed @ctx be
> > > > > + * reported.
> > > > > + *
> > > > > + * A NULL value for @uctx can be used to get both the number of attributes
> > > > > + * and the size of the data.
> > > > > + *
> > > > > + * Returns the number of attributes found on success, negative value
> > > > > + * on error. @size is reset to the total size of the data.
> > > > > + * If @size is insufficient to contain the data -E2BIG is returned.
> > > > > + */
> > > > > +int security_getselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
> > > > > +                      size_t __user *size, u32 flags)
> > > > > +{
> > > > > +     struct security_hook_list *hp;
> > > > > +     struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
> > > > > +     u8 __user *base = (u8 __user *)uctx;
> > > > > +     size_t total = 0;
> > > > > +     size_t entrysize;
> > > > > +     size_t left;
> > > > > +     bool toobig = false;
> > > > > +     bool single = false;
> > > > > +     int count = 0;
> > > > > +     int rc;
> > > > > +
> > > > > +     if (attr == LSM_ATTR_UNDEF)
> > > > > +             return -EINVAL;
> > > > > +     if (size == NULL)
> > > > > +             return -EINVAL;
> > > > > +     if (get_user(left, size))
> > > > > +             return -EFAULT;
> > > > > +
> > > > > +     if (flags) {
> > > > > +             /*
> > > > > +              * Only flag supported is LSM_FLAG_SINGLE
> > > > > +              */
> > > > > +             if (flags != LSM_FLAG_SINGLE)
> > > > > +                     return -EINVAL;
> > > > > +             if (uctx && copy_from_user(&lctx, uctx, sizeof(lctx)))
> > > >
> > > > I'm not sure if we should return -EINVAL or -EFAULT when uctx == NULL.
> > > > Because uctx is optional (when LSM_FLAG_SINGLE is not set), I guess
> > > > -EINVAL is OK.
> > >
> > > That's a good point, we should probably the error codes here: if uctx
> > > is NULL in the LSM_FLAG_SINGLE case we should return -EINVAL, if the
> > > copy_from_user() fails we should return -EFAULT.
> > >
> > > > > +                     return -EFAULT;
> > > > > +             /*
> > > > > +              * If the LSM ID isn't specified it is an error.
> > > > > +              */
> > > > > +             if (lctx.id == LSM_ID_UNDEF)
> > > > > +                     return -EINVAL;
> > > > > +             single = true;
> > > > > +     }
> > > > > +
> > > > > +     /*
> > > > > +      * In the usual case gather all the data from the LSMs.
> > > > > +      * In the single case only get the data from the LSM specified.
> > > > > +      */
> > > > > +     hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
> > > > > +             if (single && lctx.id != hp->lsmid->id)
> > > > > +                     continue;
> > > > > +             entrysize = left;
> > > > > +             if (base)
> > > > > +                     uctx = (struct lsm_ctx __user *)(base + total);
> > > > > +             rc = hp->hook.getselfattr(attr, uctx, &entrysize, flags);
> > > > > +             if (rc == -EOPNOTSUPP) {
> > > > > +                     rc = 0;
> > > > > +                     continue;
> > > > > +             }
> > > > > +             if (rc == -E2BIG) {
> > > > > +                     toobig = true;
> > > > > +                     left = 0;
> > > > > +             } else if (rc < 0)
> > > > > +                     return rc;
> > > > > +             else
> > > > > +                     left -= entrysize;
> > > > > +
> > > > > +             total += entrysize;
> > > > > +             count += rc;
> > > >
> > > > There is a bug if rc == -E2BIG
> > >
> > > Can you elaborate a bit more on this? Nothing is jumping out at me as
> > > obviously broken... are you talking about @count becoming garbage due
> > > to @rc being equal to -E2BIG?  If that is the case it should be okay
> > > since we explicitly return -E2BIG, not @count, if @toobig is true.
> >
> > Indeed, in this case count will not be returned thanks to toobig. I'd
> > suggest to "continue" if rc == -E2BIG (like for -EOPNOTSUPP) to avoid an
> > inconsistent count value, which could bite us one day.
> 
> Okay, how about we reset @rc to zero in the -E2BIG case?  We don't
> want to bypass the lower part of the loop in this case as we still
> need to update @total.
> 
>   if (rc == -E2BIG) {
>     rc = 0;
>     left = 0;
>     toobig = true;
>   }

Indeed, this is better.

To avoid useless calls to getselfattr hooks, we could also patch the
last part of the loop:
-		if (single)
+		if (single || left <= 0)
 			break;

> 
> -- 
> paul-moore.com