Vulnerability: The do_check function in kernel/bpf/verifier.c in the Linux kernel before 4.11.1 does not make the allow_ptr_leaks value available for restricting the output of the print_bpf_insn function, which allows local users to obtain sensitive address information via crafted bpf system calls.
Vulnerability ID: CVE-2017-9150
CWE: CWE-200
Severity score: 2.9
Project: https://github.com/torvalds/linux
Published: 2017-05-22
File: kernel/bpf/verifier.c
Commit message: bpf: don't let ldimm64 leak map addresses on unprivileged

The patch fixes two things at once:

1) It checks the env->allow_ptr_leaks and only prints the map address to
   the log if we have the privileges to do so, otherwise it just dumps 0
   as we would when kptr_restrict is enabled on %pK. Given the latter is
   off by default and not every distro sets it, I don't want to rely on
   this, hence the 0 by default for unprivileged.

2) Printing of ldimm64 in the verifier log is currently broken in that
   we don't print the full immediate, but only the 32 bit part of the
   first insn part for ldimm64. Thus, fix this up as well; it's okay to
   access, since we verified all ldimm64 earlier already (including just
   constants) through replace_map_fd_with_map_ptr().

Fixes: 1be7f75d1668 ("bpf: enable non-root eBPF programs")
Fixes: cbd357008604 ("bpf: verifier (add ability to receive verification log)")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Commit author: Daniel Borkmann

Vulnerable version (before fix):
 static const char *const bpf_jmp_string[16] = { 	[BPF_EXIT >> 4] = "exit", };  static void print_bpf_insn(struct bpf_insn *insn) { 	u8 class = BPF_CLASS(insn->code);  static void print_bpf_insn(struct bpf_insn *insn) 				insn->code, 				bpf_ldst_string[BPF_SIZE(insn->code) >> 3], 				insn->src_reg, insn->imm); 		} else if (BPF_MODE(insn->code) == BPF_IMM) { 			verbose("(%02x) r%d = 0x%x\n", 				insn->code, insn->dst_reg, insn->imm); 		} else { 			verbose("BUG_ld_%02x\n", insn->code); 			return; static int do_check(struct bpf_verifier_env *env)  		if (log_level) { 			verbose("%d: ", insn_idx); 			print_bpf_insn(insn); 		}  		err = ext_analyzer_insn_hook(env, insn_idx, prev_insn_idx);

Fixed version (after fix):
 static const char *const bpf_jmp_string[16] = { 	[BPF_EXIT >> 4] = "exit", };  static void print_bpf_insn(const struct bpf_verifier_env *env, 			   const struct bpf_insn *insn) { 	u8 class = BPF_CLASS(insn->code);  static void print_bpf_insn(struct bpf_insn *insn) 				insn->code, 				bpf_ldst_string[BPF_SIZE(insn->code) >> 3], 				insn->src_reg, insn->imm); 		} else if (BPF_MODE(insn->code) == BPF_IMM && 			   BPF_SIZE(insn->code) == BPF_DW) { 			/* At this point, we already made sure that the second 			 * part of the ldimm64 insn is accessible. 			 */ 			u64 imm = ((u64)(insn + 1)->imm << 32) | (u32)insn->imm; 			bool map_ptr = insn->src_reg == BPF_PSEUDO_MAP_FD;  			if (map_ptr && !env->allow_ptr_leaks) 				imm = 0;  			verbose("(%02x) r%d = 0x%llx\n", insn->code, 				insn->dst_reg, (unsigned long long)imm); 		} else { 			verbose("BUG_ld_%02x\n", insn->code); 			return; static int do_check(struct bpf_verifier_env *env)  		if (log_level) { 			verbose("%d: ", insn_idx); 			print_bpf_insn(env, insn); 		}  		err = ext_analyzer_insn_hook(env, insn_idx, prev_insn_idx);

Patch:
patch: @@ -298,7 +298,8 @@ static const char *const bpf_jmp_string[16] = {
 	[BPF_EXIT >> 4] = "exit",
 };
 
-static void print_bpf_insn(struct bpf_insn *insn)
+static void print_bpf_insn(const struct bpf_verifier_env *env,
+			   const struct bpf_insn *insn)
 {
 	u8 class = BPF_CLASS(insn->code);
 
@@ -362,9 +363,19 @@ static void print_bpf_insn(struct bpf_insn *insn)
 				insn->code,
 				bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
 				insn->src_reg, insn->imm);
-		} else if (BPF_MODE(insn->code) == BPF_IMM) {
-			verbose("(%02x) r%d = 0x%x\n",
-				insn->code, insn->dst_reg, insn->imm);
+		} else if (BPF_MODE(insn->code) == BPF_IMM &&
+			   BPF_SIZE(insn->code) == BPF_DW) {
+			/* At this point, we already made sure that the second
+			 * part of the ldimm64 insn is accessible.
+			 */
+			u64 imm = ((u64)(insn + 1)->imm << 32) | (u32)insn->imm;
+			bool map_ptr = insn->src_reg == BPF_PSEUDO_MAP_FD;
+
+			if (map_ptr && !env->allow_ptr_leaks)
+				imm = 0;
+
+			verbose("(%02x) r%d = 0x%llx\n", insn->code,
+				insn->dst_reg, (unsigned long long)imm);
 		} else {
 			verbose("BUG_ld_%02x\n", insn->code);
 			return;
@@ -2853,7 +2864,7 @@ static int do_check(struct bpf_verifier_env *env)
 
 		if (log_level) {
 			verbose("%d: ", insn_idx);
-			print_bpf_insn(insn);
+			print_bpf_insn(env, insn);
 		}
 
 		err = ext_analyzer_insn_hook(env, insn_idx, prev_insn_idx);
path: kernel/bpf/verifier.c