bpf: Add strict alignment flag for BPF_PROG_LOAD.

Add a new field, "prog_flags", and an initial flag value
BPF_F_STRICT_ALIGNMENT.

When set, the verifier will enforce strict pointer alignment
regardless of the setting of CONFIG_EFFICIENT_UNALIGNED_ACCESS.

The verifier, in this mode, will also use a fixed value of "2" in
place of NET_IP_ALIGN.

This facilitates test cases that will exercise and validate this part
of the verifier even when run on architectures where alignment doesn't
matter.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>

6 files changed, 40 insertions, 9 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 7c6a51924afc..d5093b52b485 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -90,6 +90,7 @@ struct bpf_verifier_env {
         struct bpf_prog *prog;          /* eBPF program being verified */
         struct bpf_verifier_stack_elem *head; /* stack of verifier states to be processed */
         int stack_size;                 /* number of states to be processed */
+        bool strict_alignment;          /* perform strict pointer alignment checks */
         struct bpf_verifier_state cur_state; /* current verifier state */
         struct bpf_verifier_state_list **explored_states; /* search pruning optimization */
         const struct bpf_ext_analyzer_ops *analyzer_ops; /* external analyzer ops */
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 945a1f5f63c5..94dfa9def355 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -132,6 +132,13 @@ enum bpf_attach_type {
  */
 #define BPF_F_ALLOW_OVERRIDE    (1U << 0)
 
+/* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
+ * verifier will perform strict alignment checking as if the kernel
+ * has been built with CONFIG_EFFICIENT_UNALIGNED_ACCESS not set,
+ * and NET_IP_ALIGN defined to 2.
+ */
+#define BPF_F_STRICT_ALIGNMENT  (1U << 0)
+
 #define BPF_PSEUDO_MAP_FD       1
 
 /* flags for BPF_MAP_UPDATE_ELEM command */
@@ -177,6 +184,7 @@ union bpf_attr {
                 __u32           log_size;       /* size of user buffer */
                 __aligned_u64   log_buf;        /* user supplied buffer */
                 __u32           kern_version;   /* checked when prog_type=kprobe */
+                __u32           prog_flags;
         };
 
         struct { /* anonymous struct used by BPF_OBJ_* commands */
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index fd2411fd6914..265a0d854e33 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -783,7 +783,7 @@ struct bpf_prog *bpf_prog_get_type(u32 ufd, enum bpf_prog_type type)
 EXPORT_SYMBOL_GPL(bpf_prog_get_type);
 
 /* last field in 'union bpf_attr' used by this command */
-#define BPF_PROG_LOAD_LAST_FIELD kern_version
+#define BPF_PROG_LOAD_LAST_FIELD prog_flags
 
 static int bpf_prog_load(union bpf_attr *attr)
 {
@@ -796,6 +796,9 @@ static int bpf_prog_load(union bpf_attr *attr)
         if (CHECK_ATTR(BPF_PROG_LOAD))
                 return -EINVAL;
 
+        if (attr->prog_flags & ~BPF_F_STRICT_ALIGNMENT)
+                return -EINVAL;
+
         /* copy eBPF program license from user space */
         if (strncpy_from_user(license, u64_to_user_ptr(attr->license),
                               sizeof(license) - 1) < 0)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ff2bfe1d656a..e74fb1b87855 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -791,6 +791,7 @@ static bool is_pointer_value(struct bpf_verifier_env *env, int regno)
 static int check_pkt_ptr_alignment(const struct bpf_reg_state *reg,
                                    int off, int size, bool strict)
 {
+        int ip_align;
         int reg_off;
 
         /* Byte size accesses are always allowed. */
@@ -807,10 +808,14 @@ static int check_pkt_ptr_alignment(const struct bpf_reg_state *reg,
                 reg_off += reg->aux_off;
         }
 
-        /* skb->data is NET_IP_ALIGN-ed */
-        if ((NET_IP_ALIGN + reg_off + off) % size != 0) {
+        /* skb->data is NET_IP_ALIGN-ed, but for strict alignment checking
+         * we force this to 2 which is universally what architectures use
+         * when they don't set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS.
+         */
+        ip_align = strict ? 2 : NET_IP_ALIGN;
+        if ((ip_align + reg_off + off) % size != 0) {
                 verbose("misaligned packet access off %d+%d+%d size %d\n",
-                        NET_IP_ALIGN, reg_off, off, size);
+                        ip_align, reg_off, off, size);
                 return -EACCES;
         }
 
@@ -828,10 +833,11 @@ static int check_val_ptr_alignment(const struct bpf_reg_state *reg,
         return 0;
 }
 
-static int check_ptr_alignment(const struct bpf_reg_state *reg,
+static int check_ptr_alignment(struct bpf_verifier_env *env,
+                               const struct bpf_reg_state *reg,
                                int off, int size)
 {
-        bool strict = false;
+        bool strict = env->strict_alignment;
 
         if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
                 strict = true;
@@ -873,7 +879,7 @@ static int check_mem_access(struct bpf_verifier_env *env, u32 regno, int off,
         if (size < 0)
                 return size;
 
-        err = check_ptr_alignment(reg, off, size);
+        err = check_ptr_alignment(env, reg, off, size);
         if (err)
                 return err;
 
@@ -3568,6 +3574,10 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
         } else {
                 log_level = 0;
         }
+        if (attr->prog_flags & BPF_F_STRICT_ALIGNMENT)
+                env->strict_alignment = true;
+        else
+                env->strict_alignment = false;
 
         ret = replace_map_fd_with_map_ptr(env);
         if (ret < 0)
@@ -3673,6 +3683,7 @@ int bpf_analyzer(struct bpf_prog *prog, const struct bpf_ext_analyzer_ops *ops,
         mutex_lock(&bpf_verifier_lock);
 
         log_level = 0;
+        env->strict_alignment = false;
 
         env->explored_states = kcalloc(env->prog->len,
                                        sizeof(struct bpf_verifier_state_list *),
diff --git a/tools/build/feature/test-bpf.c b/tools/build/feature/test-bpf.c
index ebc6dceddb58..7598361ef1f1 100644
--- a/tools/build/feature/test-bpf.c
+++ b/tools/build/feature/test-bpf.c
@@ -29,6 +29,7 @@ int main(void)
         attr.log_size = 0;
         attr.log_level = 0;
         attr.kern_version = 0;
+        attr.prog_flags = 0;
 
         /*
          * Test existence of __NR_bpf and BPF_PROG_LOAD.
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index e553529929f6..94dfa9def355 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -132,6 +132,13 @@ enum bpf_attach_type {
  */
 #define BPF_F_ALLOW_OVERRIDE    (1U << 0)
 
+/* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
+ * verifier will perform strict alignment checking as if the kernel
+ * has been built with CONFIG_EFFICIENT_UNALIGNED_ACCESS not set,
+ * and NET_IP_ALIGN defined to 2.
+ */
+#define BPF_F_STRICT_ALIGNMENT  (1U << 0)
+
 #define BPF_PSEUDO_MAP_FD       1
 
 /* flags for BPF_MAP_UPDATE_ELEM command */
@@ -177,6 +184,7 @@ union bpf_attr {
                 __u32           log_size;       /* size of user buffer */
                 __aligned_u64   log_buf;        /* user supplied buffer */
                 __u32           kern_version;   /* checked when prog_type=kprobe */
+                __u32           prog_flags;
         };
 
         struct { /* anonymous struct used by BPF_OBJ_* commands */
@@ -481,8 +489,7 @@ union bpf_attr {
  * u32 bpf_get_socket_uid(skb)