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Subject[RFC v3 21/22] bpf,landlock: Add optional skb pointer in the Landlock context
Date
This is a proof of concept to expose optional values that could depend
of the process access rights.

There is two dedicated flags: LANDLOCK_FLAG_ACCESS_SKB_READ and
LANDLOCK_FLAG_ACCESS_SKB_WRITE. Each of them can be activated to access
eBPF functions manipulating a skb in a read or write way.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Sargun Dhillon <sargun@sargun.me>
---
include/linux/bpf.h | 2 ++
include/uapi/linux/bpf.h | 7 ++++++-
kernel/bpf/verifier.c | 6 ++++++
security/landlock/lsm.c | 26 ++++++++++++++++++++++++++
4 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f7325c17f720..218973777612 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -88,6 +88,7 @@ enum bpf_arg_type {

ARG_PTR_TO_STRUCT_FILE, /* pointer to struct file */
ARG_CONST_PTR_TO_LANDLOCK_HANDLE_FS, /* pointer to Landlock FS handle */
+ ARG_PTR_TO_STRUCT_SKB, /* pointer to struct skb */
};

/* type of values returned from helper functions */
@@ -150,6 +151,7 @@ enum bpf_reg_type {
/* Landlock */
PTR_TO_STRUCT_FILE,
CONST_PTR_TO_LANDLOCK_HANDLE_FS,
+ PTR_TO_STRUCT_SKB,
};

struct bpf_prog;
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 8cfc2de2ab76..7d9e56952ed9 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -586,7 +586,9 @@ enum landlock_hook_id {
/* context of function access flags */
#define LANDLOCK_FLAG_ACCESS_UPDATE (1 << 0)
#define LANDLOCK_FLAG_ACCESS_DEBUG (1 << 1)
-#define _LANDLOCK_FLAG_ACCESS_MASK ((1ULL << 2) - 1)
+#define LANDLOCK_FLAG_ACCESS_SKB_READ (1 << 2)
+#define LANDLOCK_FLAG_ACCESS_SKB_WRITE (1 << 3)
+#define _LANDLOCK_FLAG_ACCESS_MASK ((1ULL << 4) - 1)

/* Handle check flags */
#define LANDLOCK_FLAG_FS_DENTRY (1 << 0)
@@ -619,12 +621,15 @@ struct landlock_handle {
* @args: LSM hook arguments, see include/linux/lsm_hooks.h for there
* description and the LANDLOCK_HOOK* definitions from
* security/landlock/lsm.c for their types.
+ * @opt_skb: optional skb pointer, accessible with the
+ * LANDLOCK_FLAG_ACCESS_SKB_* flags for network-related hooks.
*/
struct landlock_data {
__u32 hook; /* enum landlock_hook_id */
__u16 origin; /* LANDLOCK_FLAG_ORIGIN_* */
__u16 cookie; /* seccomp RET_LANDLOCK */
__u64 args[6];
+ __u64 opt_skb;
};

#endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8d7b18574f5a..a95154c1a60f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -247,6 +247,7 @@ static const char * const reg_type_str[] = {
[PTR_TO_PACKET_END] = "pkt_end",
[PTR_TO_STRUCT_FILE] = "struct_file",
[CONST_PTR_TO_LANDLOCK_HANDLE_FS] = "landlock_handle_fs",
+ [PTR_TO_STRUCT_SKB] = "struct_skb",
};

static void print_verifier_state(struct verifier_state *state)
@@ -559,6 +560,7 @@ static bool is_spillable_regtype(enum bpf_reg_type type)
case CONST_PTR_TO_MAP:
case PTR_TO_STRUCT_FILE:
case CONST_PTR_TO_LANDLOCK_HANDLE_FS:
+ case PTR_TO_STRUCT_SKB:
return true;
default:
return false;
@@ -984,6 +986,10 @@ static int check_func_arg(struct verifier_env *env, u32 regno,
expected_type = CONST_PTR_TO_LANDLOCK_HANDLE_FS;
if (type != expected_type)
goto err_type;
+ } else if (arg_type == ARG_PTR_TO_STRUCT_SKB) {
+ expected_type = PTR_TO_STRUCT_SKB;
+ if (type != expected_type)
+ goto err_type;
} else if (arg_type == ARG_PTR_TO_STACK ||
arg_type == ARG_PTR_TO_RAW_STACK) {
expected_type = PTR_TO_STACK;
diff --git a/security/landlock/lsm.c b/security/landlock/lsm.c
index 56c45abe979c..8b0e6f0eb6b7 100644
--- a/security/landlock/lsm.c
+++ b/security/landlock/lsm.c
@@ -281,6 +281,7 @@ static bool __is_valid_access(int off, int size, enum bpf_access_type type,
break;
case offsetof(struct landlock_data, args[0]) ...
offsetof(struct landlock_data, args[5]):
+ case offsetof(struct landlock_data, opt_skb):
expected_size = sizeof(__u64);
break;
default:
@@ -299,6 +300,13 @@ static bool __is_valid_access(int off, int size, enum bpf_access_type type,
if (*reg_type == NOT_INIT)
return false;
break;
+ case offsetof(struct landlock_data, opt_skb):
+ if (!(prog_subtype->landlock_hook.access &
+ (LANDLOCK_FLAG_ACCESS_SKB_READ |
+ LANDLOCK_FLAG_ACCESS_SKB_WRITE)))
+ return false;
+ *reg_type = PTR_TO_STRUCT_SKB;
+ break;
}

return true;
@@ -401,6 +409,24 @@ static inline bool bpf_landlock_is_valid_subtype(
if (prog_subtype->landlock_hook.access & LANDLOCK_FLAG_ACCESS_DEBUG &&
!capable(CAP_SYS_ADMIN))
return false;
+ /*
+ * Capability checks must be enforced for every landlocked process.
+ * To support user namespaces/capabilities, we must then check the
+ * namespaces of a task before putting it in a landlocked cgroup.
+ * This could be implemented in the future.
+ */
+ if (prog_subtype->landlock_hook.access & LANDLOCK_FLAG_ACCESS_SKB_READ &&
+ !capable(CAP_NET_ADMIN))
+ return false;
+ /*
+ * It is interesting to differentiate read and write access to be able
+ * to securely delegate some work to unprivileged (and potentially
+ * compromised/untrusted) processes. This different type of access can
+ * be checked for function calls or context accesses.
+ */
+ if (prog_subtype->landlock_hook.access & LANDLOCK_FLAG_ACCESS_SKB_WRITE &&
+ !capable(CAP_NET_ADMIN))
+ return false;

return true;
}
--
2.9.3
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