Commit e688c3db authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Daniel Borkmann
Browse files

bpf: Fix register equivalence tracking. 



The 64-bit JEQ/JNE handling in reg_set_min_max() was clearing reg->id in either
true or false branch. In the case 'if (reg->id)' check was done on the other
branch the counter part register would have reg->id == 0 when called into
find_equal_scalars(). In such case the helper would incorrectly identify other
registers with id == 0 as equivalent and propagate the state incorrectly.
Fix it by preserving ID across reg_set_min_max().

In other words any kind of comparison operator on the scalar register
should preserve its ID to recognize:

r1 = r2
if (r1 == 20) {
  #1 here both r1 and r2 == 20
} else if (r2 < 20) {
  #2 here both r1 and r2 < 20
}

The patch is addressing #1 case. The #2 was working correctly already.

Fixes: 75748837 ("bpf: Propagate scalar ranges through register assignments.")
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Tested-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201014175608.1416-1-alexei.starovoitov@gmail.com
parent ccdf7fae

kernel/bpf/verifier.c

+25 −13
Original line number Diff line number Diff line
@@ -1010,14 +1010,9 @@ static const int caller_saved[CALLER_SAVED_REGS] = { 
static void __mark_reg_not_init(const struct bpf_verifier_env *env,

				struct bpf_reg_state *reg);



/* Mark the unknown part of a register (variable offset or scalar value) as

 * known to have the value @imm.

 */

static void __mark_reg_known(struct bpf_reg_state *reg, u64 imm)

/* This helper doesn't clear reg->id */

static void ___mark_reg_known(struct bpf_reg_state *reg, u64 imm)

{

	/* Clear id, off, and union(map_ptr, range) */

	memset(((u8 *)reg) + sizeof(reg->type), 0,

	       offsetof(struct bpf_reg_state, var_off) - sizeof(reg->type));

	reg->var_off = tnum_const(imm);

	reg->smin_value = (s64)imm;

	reg->smax_value = (s64)imm;
@@ -1030,6 +1025,17 @@ static void __mark_reg_known(struct bpf_reg_state *reg, u64 imm) 
	reg->u32_max_value = (u32)imm;

}



/* Mark the unknown part of a register (variable offset or scalar value) as

 * known to have the value @imm.

 */

static void __mark_reg_known(struct bpf_reg_state *reg, u64 imm)

{

	/* Clear id, off, and union(map_ptr, range) */

	memset(((u8 *)reg) + sizeof(reg->type), 0,

	       offsetof(struct bpf_reg_state, var_off) - sizeof(reg->type));

	___mark_reg_known(reg, imm);

}



static void __mark_reg32_known(struct bpf_reg_state *reg, u64 imm)

{

	reg->var_off = tnum_const_subreg(reg->var_off, imm);
@@ -7001,14 +7007,18 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg, 
		struct bpf_reg_state *reg =

			opcode == BPF_JEQ ? true_reg : false_reg;



		/* For BPF_JEQ, if this is false we know nothing Jon Snow, but

		 * if it is true we know the value for sure. Likewise for

		 * BPF_JNE.

		/* JEQ/JNE comparison doesn't change the register equivalence.

		 * r1 = r2;

		 * if (r1 == 42) goto label;

		 * ...

		 * label: // here both r1 and r2 are known to be 42.

		 *

		 * Hence when marking register as known preserve it's ID.

		 */

		if (is_jmp32)

			__mark_reg32_known(reg, val32);

		else

			__mark_reg_known(reg, val);

			___mark_reg_known(reg, val);

		break;

	}

	case BPF_JSET:
@@ -7551,7 +7561,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
				reg_combine_min_max(&other_branch_regs[insn->src_reg],

						    &other_branch_regs[insn->dst_reg],

						    src_reg, dst_reg, opcode);

			if (src_reg->id) {

			if (src_reg->id &&

			    !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {

				find_equal_scalars(this_branch, src_reg);

				find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);

			}
@@ -7563,7 +7574,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
					opcode, is_jmp32);

	}



	if (dst_reg->type == SCALAR_VALUE && dst_reg->id) {

	if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&

	    !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {

		find_equal_scalars(this_branch, dst_reg);

		find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]);

	}

tools/testing/selftests/bpf/verifier/regalloc.c

+26 −0
Original line number Diff line number Diff line
@@ -241,3 +241,29 @@ 
	.result = ACCEPT,

	.prog_type = BPF_PROG_TYPE_TRACEPOINT,

},

{

	"regalloc, spill, JEQ",

	.insns = {

	BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),

	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),

	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),

	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),

	BPF_LD_MAP_FD(BPF_REG_1, 0),

	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),

	BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_0, -8), /* spill r0 */

	BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 0),

	/* The verifier will walk the rest twice with r0 == 0 and r0 == map_value */

	BPF_EMIT_CALL(BPF_FUNC_get_prandom_u32),

	BPF_MOV64_REG(BPF_REG_2, BPF_REG_0),

	BPF_JMP_IMM(BPF_JEQ, BPF_REG_2, 20, 0),

	/* The verifier will walk the rest two more times with r0 == 20 and r0 == unknown */

	BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_10, -8), /* fill r3 with map_value */

	BPF_JMP_IMM(BPF_JEQ, BPF_REG_3, 0, 1), /* skip ldx if map_value == NULL */

	/* Buggy verifier will think that r3 == 20 here */

	BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_3, 0), /* read from map_value */

	BPF_EXIT_INSN(),

	},

	.fixup_map_hash_48b = { 4 },

	.result = ACCEPT,

	.prog_type = BPF_PROG_TYPE_TRACEPOINT,

},

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