[PATCH] bpf: add diagnostics for rejected memory and map accesses

[Suchit Karunakaran <suchitkarunakaran@gmail.com>]

Emit structured verifier diagnostics when BPF programs are rejected for
unsupported or prohibited memory operations. Cover sign-extending arena
loads without JIT support, read/write access restrictions on maps, writes
through read-only pointers, direct packet writes, and modifying helpers
used with BPF_F_RDONLY_PROG.

Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.com>
---
 kernel/bpf/fixups.c   |  5 +++++
 kernel/bpf/verifier.c | 16 ++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index 2add8001c3ec..b8baf77d933e 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -9,6 +9,7 @@
 #include <linux/sort.h>
 #include <linux/perf_event.h>
 #include <net/xdp.h>
+#include "diagnostics.h"
 #include "disasm.h"
 
 #define verbose(env, fmt, args...) bpf_verifier_log_write(env, fmt, ##args)
@@ -958,6 +959,10 @@ int bpf_convert_ctx_accesses(struct bpf_verifier_env *env)
 			if (BPF_MODE(insn->code) == BPF_MEMSX) {
 				if (!bpf_jit_supports_insn(insn, true)) {
 					verbose(env, "sign extending loads from arena are not supported yet\n");
+					bpf_diag_policy(
+						env, i + delta, "sign-extending arena load",
+						"the current JIT does not support this load instruction for arena memory",
+						"Use a kernel and architecture with JIT support for this arena load.");
 					return -EOPNOTSUPP;
 				}
 				insn->code = BPF_CLASS(insn->code) | BPF_PROBE_MEM32SX | BPF_SIZE(insn->code);
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3a676fc70f4b..ebfa53930276 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4365,12 +4365,18 @@ static int check_map_access_type(struct bpf_verifier_env *env, struct bpf_reg_st
 	if (type == BPF_WRITE && !(cap & BPF_MAP_CAN_WRITE)) {
 		verbose(env, "write into map forbidden, value_size=%d off=%lld size=%d\n",
 			map->value_size, reg_smin(reg) + off, size);
+		bpf_diag_policy(env, env->insn_idx, "map value write",
+				"this map does not permit BPF programs to write its values",
+				"Remove the write, or use a map that allows writes from BPF programs.");
 		return -EACCES;
 	}
 
 	if (type == BPF_READ && !(cap & BPF_MAP_CAN_READ)) {
 		verbose(env, "read from map forbidden, value_size=%d off=%lld size=%d\n",
 			map->value_size, reg_smin(reg) + off, size);
+		bpf_diag_policy(env, env->insn_idx, "map value read",
+				 "this map does not permit BPF programs to read its values",
+				 "Remove the read, or use a map that allows reads from BPF programs.");
 		return -EACCES;
 	}
 
@@ -6527,6 +6533,9 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 		if (t == BPF_WRITE && rdonly_mem) {
 			verbose(env, "%s cannot write into %s\n",
 				reg_arg_name(env, argno), reg_type_str(env, reg->type));
+			bpf_diag_policy(env, insn_idx, "write through a read-only memory pointer",
+					"this pointer permits reads only",
+					"Use a writable destination, or copy the data into a writable buffer before modifying it.");
 			return -EACCES;
 		}
 
@@ -6613,6 +6622,9 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 	} else if (reg_is_pkt_pointer(reg)) {
 		if (t == BPF_WRITE && !may_access_direct_pkt_data(env, NULL, t)) {
 			verbose(env, "cannot write into packet\n");
+			bpf_diag_policy(env, insn_idx, "direct packet write",
+					"direct packet writes are disabled for this program type",
+					"Remove the direct write, or perform the modification in a program type and hook that support packet writes.");
 			return -EACCES;
 		}
 		if (t == BPF_WRITE && value_regno >= 0 &&
@@ -11188,6 +11200,10 @@ record_func_map(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
 	     func_id == BPF_FUNC_map_push_elem ||
 	     func_id == BPF_FUNC_map_pop_elem)) {
 		verbose(env, "write into map forbidden\n");
+		bpf_diag_policy(env, insn_idx,
+				bpf_diag_fmt(env, "call to %s", func_id_name(func_id)),
+				"this helper modifies the map, but BPF_F_RDONLY_PROG forbids modifications from BPF programs",
+				"Remove the modifying helper call, or use a map created without BPF_F_RDONLY_PROG if updates from BPF programs are intended.");
 		return -EACCES;
 	}
 
-- 
2.55.0