netdev - [PATCH bpf-next v3 3/9] bpf/verifier: refine retval R0 state for bpf_get

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Message-ID: <20180420221842.742330-4-yhs@fb.com>
Date:   Fri, 20 Apr 2018 15:18:36 -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 3/9] bpf/verifier: refine retval R0 state for bpf_get_stack helper

The special property of return values for helpers bpf_get_stack
and bpf_probe_read_str are captured in verifier.
Both helpers return a negative error code or
a length, which is equal to or smaller than the buffer
size argument. This additional information in the
verifier can avoid the condition such as "retval > bufsize"
in the bpf program. For example, for the code blow,
    usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
    if (usize < 0 || usize > max_len)
        return 0;
The verifier may have the following errors:
    52: (85) call bpf_get_stack#65
     R0=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R1_w=ctx(id=0,off=0,imm=0)
     R2_w=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R3_w=inv800 R4_w=inv256
     R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
     R9_w=inv800 R10=fp0,call_-1
    53: (bf) r8 = r0
    54: (bf) r1 = r8
    55: (67) r1 <<= 32
    56: (bf) r2 = r1
    57: (77) r2 >>= 32
    58: (25) if r2 > 0x31f goto pc+33
     R0=inv(id=0) R1=inv(id=0,smax_value=9223372032559808512,
                         umax_value=18446744069414584320,
                         var_off=(0x0; 0xffffffff00000000))
     R2=inv(id=0,umax_value=799,var_off=(0x0; 0x3ff))
     R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
     R8=inv(id=0) R9=inv800 R10=fp0,call_-1
    59: (1f) r9 -= r8
    60: (c7) r1 s>>= 32
    61: (bf) r2 = r7
    62: (0f) r2 += r1
    math between map_value pointer and register with unbounded
    min value is not allowed
The failure is due to llvm compiler optimization where register "r2",
which is a copy of "r1", is tested for condition while later on "r1"
is used for map_ptr operation. The verifier is not able to track such
inst sequence effectively.

Without the "usize > max_len" condition, there is no llvm optimization
and the below generated code passed verifier:
    52: (85) call bpf_get_stack#65
     R0=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R1_w=ctx(id=0,off=0,imm=0)
     R2_w=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R3_w=inv800 R4_w=inv256
     R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
     R9_w=inv800 R10=fp0,call_-1
    53: (b7) r1 = 0
    54: (bf) r8 = r0
    55: (67) r8 <<= 32
    56: (c7) r8 s>>= 32
    57: (6d) if r1 s> r8 goto pc+24
     R0=inv(id=0,umax_value=800) R1=inv0 R6=ctx(id=0,off=0,imm=0)
     R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0)
     R8=inv(id=0,umax_value=800,var_off=(0x0; 0x3ff)) R9=inv800
     R10=fp0,call_-1
    58: (bf) r2 = r7
    59: (0f) r2 += r8
    60: (1f) r9 -= r8
    61: (bf) r1 = r6

Signed-off-by: Yonghong Song <yhs@...com>
---
 kernel/bpf/verifier.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index aba9425..3c8bb92 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -164,6 +164,8 @@ struct bpf_call_arg_meta {
 	bool pkt_access;
 	int regno;
 	int access_size;
+	s64 msize_smax_value;
+	u64 msize_umax_value;
 };
 
 static DEFINE_MUTEX(bpf_verifier_lock);
@@ -2027,6 +2029,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
 		err = check_helper_mem_access(env, regno - 1,
 					      reg->umax_value,
 					      zero_size_allowed, meta);
+
+		if (!err && !!meta) {
+			/* remember the mem_size which may be used later
+			 * to refine return values.
+			 */
+			meta->msize_smax_value = reg->smax_value;
+			meta->msize_umax_value = reg->umax_value;
+		}
 	}
 
 	return err;
@@ -2333,6 +2343,21 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 	return 0;
 }
 
+static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type,
+				   int func_id,
+				   struct bpf_call_arg_meta *meta)
+{
+	struct bpf_reg_state *ret_reg = &regs[BPF_REG_0];
+
+	if (ret_type != RET_INTEGER ||
+	    (func_id != BPF_FUNC_get_stack &&
+	     func_id != BPF_FUNC_probe_read_str))
+		return;
+
+	ret_reg->smax_value = meta->msize_smax_value;
+	ret_reg->umax_value = meta->msize_umax_value;
+}
+
 static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn_idx)
 {
 	const struct bpf_func_proto *fn = NULL;
@@ -2456,6 +2481,8 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 		return -EINVAL;
 	}
 
+	do_refine_retval_range(regs, fn->ret_type, func_id, &meta);
+
 	err = check_map_func_compatibility(env, meta.map_ptr, func_id);
 	if (err)
 		return err;
-- 
2.9.5