[PATCH net-next v2 5/5] selinux: bpf: Add addtional check for bpf object file receive
Chenbo Feng
fengc at google.com
Tue Oct 10 17:48:35 UTC 2017
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.
> 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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