bpf: Prevent memory disambiguation attack

BugLink: https://bugs.launchpad.net/bugs/1811080

commit af86ca4e upstream.

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 <ast@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 4.4:
 - Add verifier_env parameter to check_stack_write()
 - Look up stack slot_types with state->stack_slot_type[] rather than
   state->stack[].slot_type[]
 - Drop bpf_verifier_env argument to verbose()
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/bpf/verifier.c

@@ -192,6 +192,7 @@ struct bpf_insn_aux_data {
 		enum bpf_reg_type ptr_type;	/* pointer type for load/store insns */
 		unsigned long map_state;	/* pointer/poison value for maps */
 	};
+	int sanitize_stack_off; /* stack slot to be cleared */
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
@@ -585,8 +586,9 @@ static bool is_spillable_regtype(enum bpf_reg_type type)
 /* check_stack_read/write functions track spill/fill of registers,
  * stack boundary and alignment are checked in check_mem_access()
  */
-static int check_stack_write(struct verifier_state *state, int off, int size,
-			     int value_regno)
+static int check_stack_write(struct verifier_env *env,
+			     struct verifier_state *state, int off,
+			     int size, int value_regno, int insn_idx)
 {
 	int i, spi = (MAX_BPF_STACK + off) / BPF_REG_SIZE;
 	/* caller checked that off % size == 0 and -MAX_BPF_STACK <= off < 0,
@@ -605,8 +607,32 @@ static int check_stack_write(struct verifier_state *state, int off, int size,
 		/* save register state */
 		state->spilled_regs[spi] = state->regs[value_regno];
 
-		for (i = 0; i < BPF_REG_SIZE; i++)
+		for (i = 0; i < BPF_REG_SIZE; i++) {
+			if (state->stack_slot_type[MAX_BPF_STACK + off + 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("insn %d cannot access two stack slots fp%d and fp%d",
+						insn_idx, *poff, soff);
+					return -EINVAL;
+				}
+				*poff = soff;
+			}
 			state->stack_slot_type[MAX_BPF_STACK + off + i] = STACK_SPILL;
+		}
 	} else {
 		/* regular write of data into stack */
 		state->spilled_regs[spi] = (struct reg_state) {};
@@ -762,7 +788,8 @@ static int check_mem_access(struct verifier_env *env, int insn_idx, u32 regno, i
 				verbose("attempt to corrupt spilled pointer on stack\n");
 				return -EACCES;
 			}
-			err = check_stack_write(state, off, size, value_regno);
+			err = check_stack_write(env, state, off, size,
+						value_regno, insn_idx);
 		} else {
 			err = check_stack_read(state, off, size, value_regno);
 		}
@@ -2260,6 +2287,34 @@ static int convert_ctx_accesses(struct verifier_env *env)
 		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;