netdev - [PATCH bpf-next v3 4/9] bpf/verifier: improve register value range tracking with ARSH

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Message-ID: <20180420221842.742330-5-yhs@fb.com>
Date:   Fri, 20 Apr 2018 15:18:37 -0700
From:   Yonghong Song <yhs@...com>
To:     <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>
CC:     <kernel-team@...com>
Subject: [PATCH bpf-next v3 4/9] 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.

Signed-off-by: Yonghong Song <yhs@...com>
---
 kernel/bpf/verifier.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3c8bb92..01c215d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2975,6 +2975,32 @@ 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;
+		}
+		if (dst_reg->smin_value < 0)
+			dst_reg->smin_value >>= umin_val;
+		else
+			dst_reg->smin_value >>= umax_val;
+		if (dst_reg->smax_value < 0)
+			dst_reg->smax_value >>= umax_val;
+		else
+			dst_reg->smax_value >>= umin_val;
+		if (src_known)
+			dst_reg->var_off = tnum_rshift(dst_reg->var_off,
+						       umin_val);
+		else
+			dst_reg->var_off = tnum_rshift(tnum_unknown, umin_val);
+		dst_reg->umin_value >>= umax_val;
+		dst_reg->umax_value >>= umin_val;
+		/* We may learn something more from the var_off */
+		__update_reg_bounds(dst_reg);
+		break;
 	default:
 		mark_reg_unknown(env, regs, insn->dst_reg);
 		break;
-- 
2.9.5