[Non-DoD Source] Re: [PATCH net-next v2 5/5] selinux: bpf: Add addtional check for bpf object file receive
Stephen Smalley
sds at tycho.nsa.gov
Tue Oct 10 19:23:28 UTC 2017
On Tue, 2017-10-10 at 10:48 -0700, Chenbo Feng wrote:
> On Tue, Oct 10, 2017 at 7:24 AM, Stephen Smalley <sds at tycho.nsa.gov>
> wrote:
> > On Mon, 2017-10-09 at 15:20 -0700, Chenbo Feng wrote:
> > > From: Chenbo Feng <fengc at google.com>
> > >
> > > Introduce a bpf object related check when sending and receiving
> > > files
> > > through unix domain socket as well as binder. It checks if the
> > > receiving
> > > process have privilege to read/write the bpf map or use the bpf
> > > program.
> > > This check is necessary because the bpf maps and programs are
> > > using a
> > > anonymous inode as their shared inode so the normal way of
> > > checking
> > > the
> > > files and sockets when passing between processes cannot work
> > > properly
> > > on
> > > eBPF object. This check only works when the BPF_SYSCALL is
> > > configured.
> > > The information stored inside the file security struct is the
> > > same as
> > > the information in bpf object security struct.
> > >
> > > Signed-off-by: Chenbo Feng <fengc at google.com>
> > > ---
> > > include/linux/bpf.h | 3 +++
> > > include/linux/lsm_hooks.h | 17 +++++++++++++
> > > include/linux/security.h | 9 +++++++
> > > kernel/bpf/syscall.c | 4 ++--
> > > security/security.c | 8 +++++++
> > > security/selinux/hooks.c | 61
> > > +++++++++++++++++++++++++++++++++++++++++++++++
> > > 6 files changed, 100 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index 225740688ab7..81d6c01b8825 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -285,6 +285,9 @@ int bpf_prog_array_copy_to_user(struct
> > > bpf_prog_array __rcu *progs,
> > > #ifdef CONFIG_BPF_SYSCALL
> > > DECLARE_PER_CPU(int, bpf_prog_active);
> > >
> > > +extern const struct file_operations bpf_map_fops;
> > > +extern const struct file_operations bpf_prog_fops;
> > > +
> > > #define BPF_PROG_TYPE(_id, _ops) \
> > > extern const struct bpf_verifier_ops _ops;
> > > #define BPF_MAP_TYPE(_id, _ops) \
> > > diff --git a/include/linux/lsm_hooks.h
> > > b/include/linux/lsm_hooks.h
> > > index 7161d8e7ee79..517dea60b87b 100644
> > > --- a/include/linux/lsm_hooks.h
> > > +++ b/include/linux/lsm_hooks.h
> > > @@ -1385,6 +1385,19 @@
> > > * @bpf_prog_free_security:
> > > * Clean up the security information stored inside bpf prog.
> > > *
> > > + * @bpf_map_file:
> > > + * When creating a bpf map fd, set up the file security
> > > information with
> > > + * the bpf security information stored in the map struct. So
> > > when the map
> > > + * fd is passed between processes, the security module can
> > > directly read
> > > + * the security information from file security struct rather
> > > than the bpf
> > > + * security struct.
> > > + *
> > > + * @bpf_prog_file:
> > > + * When creating a bpf prog fd, set up the file security
> > > information with
> > > + * the bpf security information stored in the prog struct. So
> > > when the prog
> > > + * fd is passed between processes, the security module can
> > > directly read
> > > + * the security information from file security struct rather
> > > than the bpf
> > > + * security struct.
> > > */
> > > union security_list_options {
> > > int (*binder_set_context_mgr)(struct task_struct *mgr);
> > > @@ -1726,6 +1739,8 @@ union security_list_options {
> > > void (*bpf_map_free_security)(struct bpf_map *map);
> > > int (*bpf_prog_alloc_security)(struct bpf_prog_aux *aux);
> > > void (*bpf_prog_free_security)(struct bpf_prog_aux *aux);
> > > + void (*bpf_map_file)(struct bpf_map *map, struct file
> > > *file);
> > > + void (*bpf_prog_file)(struct bpf_prog_aux *aux, struct file
> > > *file);
> > > #endif /* CONFIG_BPF_SYSCALL */
> > > };
> > >
> > > @@ -1954,6 +1969,8 @@ struct security_hook_heads {
> > > struct list_head bpf_map_free_security;
> > > struct list_head bpf_prog_alloc_security;
> > > struct list_head bpf_prog_free_security;
> > > + struct list_head bpf_map_file;
> > > + struct list_head bpf_prog_file;
> > > #endif /* CONFIG_BPF_SYSCALL */
> > > } __randomize_layout;
> > >
> > > diff --git a/include/linux/security.h b/include/linux/security.h
> > > index 18800b0911e5..57573b794e2d 100644
> > > --- a/include/linux/security.h
> > > +++ b/include/linux/security.h
> > > @@ -1740,6 +1740,8 @@ extern int security_bpf_map_alloc(struct
> > > bpf_map *map);
> > > extern void security_bpf_map_free(struct bpf_map *map);
> > > extern int security_bpf_prog_alloc(struct bpf_prog_aux *aux);
> > > extern void security_bpf_prog_free(struct bpf_prog_aux *aux);
> > > +extern void security_bpf_map_file(struct bpf_map *map, struct
> > > file
> > > *file);
> > > +extern void security_bpf_prog_file(struct bpf_prog_aux *aux,
> > > struct
> > > file *file);
> > > #else
> > > static inline int security_bpf(int cmd, union bpf_attr *attr,
> > > unsigned int size)
> > > @@ -1772,6 +1774,13 @@ static inline int
> > > security_bpf_prog_alloc(struct bpf_prog_aux *aux)
> > >
> > > static inline void security_bpf_prog_free(struct bpf_prog_aux
> > > *aux)
> > > { }
> > > +
> > > +static inline void security_bpf_map_file(struct bpf_map *map,
> > > struct
> > > file *file)
> > > +{ }
> > > +
> > > +static inline void security_bpf_prog_file(struct bpf_prog_aux
> > > *aux,
> > > + struct file *file)
> > > +{ }
> > > #endif /* CONFIG_SECURITY */
> > > #endif /* CONFIG_BPF_SYSCALL */
> > >
> > > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > > index 1cf31ddd7616..b144181d3f3a 100644
> > > --- a/kernel/bpf/syscall.c
> > > +++ b/kernel/bpf/syscall.c
> > > @@ -313,7 +313,7 @@ static ssize_t bpf_dummy_write(struct file
> > > *filp,
> > > const char __user *buf,
> > > return -EINVAL;
> > > }
> > >
> > > -static const struct file_operations bpf_map_fops = {
> > > +const struct file_operations bpf_map_fops = {
> > > #ifdef CONFIG_PROC_FS
> > > .show_fdinfo = bpf_map_show_fdinfo,
> > > #endif
> > > @@ -964,7 +964,7 @@ static void bpf_prog_show_fdinfo(struct
> > > seq_file
> > > *m, struct file *filp)
> > > }
> > > #endif
> > >
> > > -static const struct file_operations bpf_prog_fops = {
> > > +const struct file_operations bpf_prog_fops = {
> > > #ifdef CONFIG_PROC_FS
> > > .show_fdinfo = bpf_prog_show_fdinfo,
> > > #endif
> > > diff --git a/security/security.c b/security/security.c
> > > index 1cd8526cb0b7..dacf649b8cfa 100644
> > > --- a/security/security.c
> > > +++ b/security/security.c
> > > @@ -1734,4 +1734,12 @@ void security_bpf_prog_free(struct
> > > bpf_prog_aux *aux)
> > > {
> > > call_void_hook(bpf_prog_free_security, aux);
> > > }
> > > +void security_bpf_map_file(struct bpf_map *map, struct file
> > > *file)
> > > +{
> > > + call_void_hook(bpf_map_file, map, file);
> > > +}
> > > +void security_bpf_prog_file(struct bpf_prog_aux *aux, struct
> > > file
> > > *file)
> > > +{
> > > + call_void_hook(bpf_prog_file, aux, file);
> > > +}
> > > #endif /* CONFIG_BPF_SYSCALL */
> > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > index 41aba4e3d57c..fea88655e0ee 100644
> > > --- a/security/selinux/hooks.c
> > > +++ b/security/selinux/hooks.c
> > > @@ -1815,6 +1815,10 @@ static inline int file_path_has_perm(const
> > > struct cred *cred,
> > > return inode_has_perm(cred, file_inode(file), av, &ad);
> > > }
> > >
> > > +#ifdef CONFIG_BPF_SYSCALL
> > > +static int bpf_file_check(struct file *file, u32 sid);
> > > +#endif
> > > +
> > > /* Check whether a task can use an open file descriptor to
> > > access an inode in a given way. Check access to the
> > > descriptor itself, and then use dentry_has_perm to
> > > @@ -1845,6 +1849,12 @@ static int file_has_perm(const struct cred
> > > *cred,
> > > goto out;
> > > }
> > >
> > > +#ifdef CONFIG_BPF_SYSCALL
> > > + rc = bpf_file_check(file, cred_sid(cred));
> > > + if (rc)
> > > + goto out;
> > > +#endif
> > > +
> > > /* av is zero if only checking access to the descriptor. */
> > > rc = 0;
> > > if (av)
> > > @@ -2165,6 +2175,12 @@ static int
> > > selinux_binder_transfer_file(struct
> > > task_struct *from,
> > > return rc;
> > > }
> > >
> > > +#ifdef CONFIG_BPF_SYSCALL
> > > + rc = bpf_file_check(file, sid);
> > > + if (rc)
> > > + return rc;
> > > +#endif
> > > +
> > > if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> > > return 0;
> > >
> > > @@ -6288,6 +6304,33 @@ static u32 bpf_map_fmode_to_av(fmode_t
> > > fmode)
> > > return av;
> > > }
> > >
> > > +/* This function will check the file pass through unix socket or
> > > binder to see
> > > + * if it is a bpf related object. And apply correspinding checks
> > > on
> > > the bpf
> > > + * object based on the type. The bpf maps and programs, not like
> > > other files and
> > > + * socket, are using a shared anonymous inode inside the kernel
> > > as
> > > their inode.
> > > + * So checking that inode cannot identify if the process have
> > > privilege to
> > > + * access the bpf object and that's why we have to add this
> > > additional check in
> > > + * selinux_file_receive and selinux_binder_transfer_files.
> > > + */
> > > +static int bpf_file_check(struct file *file, u32 sid)
> > > +{
> > > + struct file_security_struct *fsec = file->f_security;
> > > + int ret;
> > > +
> > > + if (file->f_op == &bpf_map_fops) {
> > > + ret = avc_has_perm(sid, fsec->sid,
> > > SECCLASS_BPF_MAP,
> > > + bpf_map_fmode_to_av(file-
> > > > f_mode), NULL);
> > >
> > > + if (ret)
> > > + return ret;
> > > + } else if (file->f_op == &bpf_prog_fops) {
> > > + ret = avc_has_perm(sid, fsec->sid,
> > > SECCLASS_BPF_PROG,
> > > + BPF_PROG__USE, NULL);
> > > + if (ret)
> > > + return ret;
> > > + }
> > > + return 0;
> > > +}
> > > +
> > > static int selinux_bpf_map(struct bpf_map *map, fmode_t fmode)
> > > {
> > > u32 sid = current_sid();
> > > @@ -6351,6 +6394,22 @@ static void selinux_bpf_prog_free(struct
> > > bpf_prog_aux *aux)
> > > aux->security = NULL;
> > > kfree(bpfsec);
> > > }
> > > +
> > > +static void selinux_bpf_map_file(struct bpf_map *map, struct
> > > file
> > > *file)
> > > +{
> > > + struct bpf_security_struct *bpfsec = map->security;
> > > + struct file_security_struct *fsec = file->f_security;
> > > +
> > > + fsec->sid = bpfsec->sid;
> > > +}
> > > +
> > > +static void selinux_bpf_prog_file(struct bpf_prog_aux *aux,
> > > struct
> > > file *file)
> > > +{
> > > + struct bpf_security_struct *bpfsec = aux->security;
> > > + struct file_security_struct *fsec = file->f_security;
> > > +
> > > + fsec->sid = bpfsec->sid;
> >
> > I could be wrong, but isn't it the case that fsec->sid already will
> > equal bpfsec->sid, because they are both created by the same thread
> > during the same system call, and they each inherit the SID of the
> > current task?
> >
>
> This is true when bpf object is created by the same process that
> obtains the fd. But there are other ways of getting a bpf object fd
> from the kernel such as bpf_obj_get and bpf_get_obj_fd_by_id. These
> action will ask the kernel to allocate a new file for the bpf object
> and the file sid would be the process ask for fd while the bpfsec-
> >sid
> is the sid when bpf object get created. These two could be different.
Oh, in that case you shouldn't change the fsec->sid; you'll need to use
the bpfsec->sid in your checks instead. But you can still do what I
described below.
> > What I expected you to do was to add and set a flags field in the
> > file_security_struct to indicate that this is a bpf map or prog,
> > and
> > then test for that in your bpf_file_check() function instead of
> > having
> > to export and test the fops structures.
> >
> >
> > > +}
> > > #endif
> > >
> > > static struct security_hook_list selinux_hooks[]
> > > __lsm_ro_after_init
> > > = {
> > > @@ -6581,6 +6640,8 @@ static struct security_hook_list
> > > selinux_hooks[] __lsm_ro_after_init = {
> > > LSM_HOOK_INIT(bpf_prog_alloc_security,
> > > selinux_bpf_prog_alloc),
> > > LSM_HOOK_INIT(bpf_map_free_security, selinux_bpf_map_free),
> > > LSM_HOOK_INIT(bpf_prog_free_security,
> > > selinux_bpf_prog_free),
> > > + LSM_HOOK_INIT(bpf_map_file, selinux_bpf_map_file),
> > > + LSM_HOOK_INIT(bpf_prog_file, selinux_bpf_prog_file),
> > > #endif
> > > };
> > >
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