Commit 9cbe1f5a authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov
Browse files

bpf/verifier: improve register value range tracking with ARSH 



When helpers like bpf_get_stack returns an int value
and later on used for arithmetic computation, the LSH and ARSH
operations are often required to get proper sign extension into
64-bit. For example, without this patch:
    54: R0=inv(id=0,umax_value=800)
    54: (bf) r8 = r0
    55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
    55: (67) r8 <<= 32
    56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
    56: (c7) r8 s>>= 32
    57: R8=inv(id=0)
With this patch:
    54: R0=inv(id=0,umax_value=800)
    54: (bf) r8 = r0
    55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
    55: (67) r8 <<= 32
    56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
    56: (c7) r8 s>>= 32
    57: R8=inv(id=0, umax_value=800,var_off=(0x0; 0x3ff))
With better range of "R8", later on when "R8" is added to other register,
e.g., a map pointer or scalar-value register, the better register
range can be derived and verifier failure may be avoided.

In our later example,
    ......
    usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
    if (usize < 0)
        return 0;
    ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
    ......
Without improving ARSH value range tracking, the register representing
"max_len - usize" will have smin_value equal to S64_MIN and will be
rejected by verifier.

Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent afbe1a5b

include/linux/tnum.h

+3 −1
Original line number Diff line number Diff line
@@ -23,8 +23,10 @@ struct tnum tnum_range(u64 min, u64 max); 
/* Arithmetic and logical ops */

/* Shift a tnum left (by a fixed shift) */

struct tnum tnum_lshift(struct tnum a, u8 shift);

/* Shift a tnum right (by a fixed shift) */

/* Shift (rsh) a tnum right (by a fixed shift) */

struct tnum tnum_rshift(struct tnum a, u8 shift);

/* Shift (arsh) a tnum right (by a fixed min_shift) */

struct tnum tnum_arshift(struct tnum a, u8 min_shift);

/* Add two tnums, return @a + @b */

struct tnum tnum_add(struct tnum a, struct tnum b);

/* Subtract two tnums, return @a - @b */

kernel/bpf/tnum.c

+10 −0
Original line number Diff line number Diff line
@@ -43,6 +43,16 @@ struct tnum tnum_rshift(struct tnum a, u8 shift) 
	return TNUM(a.value >> shift, a.mask >> shift);

}



struct tnum tnum_arshift(struct tnum a, u8 min_shift)

{

	/* if a.value is negative, arithmetic shifting by minimum shift

	 * will have larger negative offset compared to more shifting.

	 * If a.value is nonnegative, arithmetic shifting by minimum shift

	 * will have larger positive offset compare to more shifting.

	 */

	return TNUM((s64)a.value >> min_shift, (s64)a.mask >> min_shift);

}



struct tnum tnum_add(struct tnum a, struct tnum b)

{

	u64 sm, sv, sigma, chi, mu;

kernel/bpf/verifier.c

+23 −0
Original line number Diff line number Diff line
@@ -2974,6 +2974,29 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, 
		/* We may learn something more from the var_off */

		__update_reg_bounds(dst_reg);

		break;

	case BPF_ARSH:

		if (umax_val >= insn_bitness) {

			/* Shifts greater than 31 or 63 are undefined.

			 * This includes shifts by a negative number.

			 */

			mark_reg_unknown(env, regs, insn->dst_reg);

			break;

		}



		/* Upon reaching here, src_known is true and

		 * umax_val is equal to umin_val.

		 */

		dst_reg->smin_value >>= umin_val;

		dst_reg->smax_value >>= umin_val;

		dst_reg->var_off = tnum_arshift(dst_reg->var_off, umin_val);



		/* blow away the dst_reg umin_value/umax_value and rely on

		 * dst_reg var_off to refine the result.

		 */

		dst_reg->umin_value = 0;

		dst_reg->umax_value = U64_MAX;

		__update_reg_bounds(dst_reg);

		break;

	default:

		mark_reg_unknown(env, regs, insn->dst_reg);

		break;

CRA Git | Maintained and supported by SUSTech CRA and CCSE