bpf: Prevent memory disambiguation attack

Detect code patterns where malicious 'speculative store bypass' can be used
and sanitize such patterns.

39: (bf) r3 = r10
40: (07) r3 += -216
41: (79) r8 = *(u64 *)(r7 +0) // slow read
42: (7a) *(u64 *)(r10 -72) = 0 // verifier inserts this instruction
43: (7b) *(u64 *)(r8 +0) = r3 // this store becomes slow due to r8
44: (79) r1 = *(u64 *)(r6 +0) // cpu speculatively executes this load
45: (71) r2 = *(u8 *)(r1 +0) // speculatively arbitrary 'load byte'
// is now sanitized

Above code after x86 JIT becomes:
e5: mov %rbp,%rdx
e8: add $0xffffffffffffff28,%rdx
ef: mov 0x0(%r13),%r14
f3: movq $0x0,-0x48(%rbp)
fb: mov %rdx,0x0(%r14)
ff: mov 0x0(%rbx),%rdi
103: movzbq 0x0(%rdi),%rsi

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

include/linux/bpf_verifier.h

···
 		s32 call_imm;			/* saved imm field of call insn */
 	};
 	int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
+	int sanitize_stack_off; /* stack slot to be cleared */
 	bool seen; /* this insn was processed by the verifier */
 };
 

kernel/bpf/verifier.c

···
  */
 static int check_stack_write(struct bpf_verifier_env *env,
 			     struct bpf_func_state *state, /* func where register points to */
-			     int off, int size, int value_regno)
+			     int off, int size, int value_regno, int insn_idx)
 {
 	struct bpf_func_state *cur; /* state of the current function */
 	int i, slot = -off - 1, spi = slot / BPF_REG_SIZE, err;
···
 		state->stack[spi].spilled_ptr = cur->regs[value_regno];
 		state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
 
-		for (i = 0; i < BPF_REG_SIZE; i++)
+		for (i = 0; i < BPF_REG_SIZE; i++) {
+			if (state->stack[spi].slot_type[i] == STACK_MISC &&
+			    !env->allow_ptr_leaks) {
+				int *poff = &env->insn_aux_data[insn_idx].sanitize_stack_off;
+				int soff = (-spi - 1) * BPF_REG_SIZE;
+
+				/* detected reuse of integer stack slot with a pointer
+				 * which means either llvm is reusing stack slot or
+				 * an attacker is trying to exploit CVE-2018-3639
+				 * (speculative store bypass)
+				 * Have to sanitize that slot with preemptive
+				 * store of zero.
+				 */
+				if (*poff && *poff != soff) {
+					/* disallow programs where single insn stores
+					 * into two different stack slots, since verifier
+					 * cannot sanitize them
+					 */
+					verbose(env,
+						"insn %d cannot access two stack slots fp%d and fp%d",
+						insn_idx, *poff, soff);
+					return -EINVAL;
+				}
+				*poff = soff;
+			}
 			state->stack[spi].slot_type[i] = STACK_SPILL;
+		}
 	} else {
 		u8 type = STACK_MISC;
 
···
 
 		if (t == BPF_WRITE)
 			err = check_stack_write(env, state, off, size,
-						value_regno);
+						value_regno, insn_idx);
 		else
 			err = check_stack_read(env, state, off, size,
 					       value_regno);
···
 			type = BPF_WRITE;
 		else
 			continue;
+
+		if (type == BPF_WRITE &&
+		    env->insn_aux_data[i + delta].sanitize_stack_off) {
+			struct bpf_insn patch[] = {
+				/* Sanitize suspicious stack slot with zero.
+				 * There are no memory dependencies for this store,
+				 * since it's only using frame pointer and immediate
+				 * constant of zero
+				 */
+				BPF_ST_MEM(BPF_DW, BPF_REG_FP,
+					   env->insn_aux_data[i + delta].sanitize_stack_off,
+					   0),
+				/* the original STX instruction will immediately
+				 * overwrite the same stack slot with appropriate value
+				 */
+				*insn,
+			};
+
+			cnt = ARRAY_SIZE(patch);
+			new_prog = bpf_patch_insn_data(env, i + delta, patch, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta    += cnt - 1;
+			env->prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
+			continue;
+		}
 
 		if (env->insn_aux_data[i + delta].ptr_type != PTR_TO_CTX)
 			continue;