[PATCH bpf-next v2 1/2] bpf: Improve verifier u32 scalar equality checking

From: Yonghong Song <yhs@fb.com>
To: <bpf@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>, <kernel-team@fb.com>,
	Martin KaFai Lau <martin.lau@kernel.org>
Subject: [PATCH bpf-next v2 1/2] bpf: Improve verifier u32 scalar equality checking
Date: Mon, 17 Apr 2023 15:21:34 -0700	[thread overview]
Message-ID: <20230417222134.359714-1-yhs@fb.com> (raw)

In [1], I tried to remove bpf-specific codes to prevent certain
llvm optimizations, and add llvm TTI (target transform info) hooks
to prevent those optimizations. During this process, I found
if I enable llvm SimplifyCFG:shouldFoldTwoEntryPHINode
transformation, I will hit the following verification failure with selftests:

  ...
  8: (18) r1 = 0xffffc900001b2230       ; R1_w=map_value(off=560,ks=4,vs=564,imm=0)
  10: (61) r1 = *(u32 *)(r1 +0)         ; R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
  ; if (skb->tstamp == EGRESS_ENDHOST_MAGIC)
  11: (79) r2 = *(u64 *)(r6 +152)       ; R2_w=scalar() R6=ctx(off=0,imm=0)
  ; if (skb->tstamp == EGRESS_ENDHOST_MAGIC)
  12: (55) if r2 != 0xb9fbeef goto pc+10        ; R2_w=195018479
  13: (bc) w2 = w1                      ; R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
  ; if (test < __NR_TESTS)
  14: (a6) if w1 < 0x9 goto pc+1 16: R0=2 R1_w=scalar(umax=8,var_off=(0x0; 0xf)) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R6=ctx(off=0,imm=0) R10=fp0
  ;
  16: (27) r2 *= 28                     ; R2_w=scalar(umax=120259084260,var_off=(0x0; 0x1ffffffffc),s32_max=2147483644,u32_max=-4)
  17: (18) r3 = 0xffffc900001b2118      ; R3_w=map_value(off=280,ks=4,vs=564,imm=0)
  19: (0f) r3 += r2                     ; R2_w=scalar(umax=120259084260,var_off=(0x0; 0x1ffffffffc),s32_max=2147483644,u32_max=-4) R3_w=map_value(off=280,ks=4,vs=564,umax=120259084260,var_off=(0x0; 0x1ffffffffc),s32_max=2147483644,u32_max=-4)
  20: (61) r2 = *(u32 *)(r3 +0)
  R3 unbounded memory access, make sure to bounds check any such access
  processed 97 insns (limit 1000000) max_states_per_insn 1 total_states 10 peak_states 10 mark_read 6
  -- END PROG LOAD LOG --
  libbpf: prog 'ingress_fwdns_prio100': failed to load: -13
  libbpf: failed to load object 'test_tc_dtime'
  libbpf: failed to load BPF skeleton 'test_tc_dtime': -13
  ...

At insn 14, with condition 'w1 < 9', register r1 is changed from an arbitrary
u32 value to `scalar(umax=8,var_off=(0x0; 0xf))`. Register r2, however, remains
as an arbitrary u32 value. Current verifier won't claim r1/r2 equality if
the previous mov is alu32 ('w2 = w1').

If r1 upper 32bit value is not 0, we indeed cannot clamin r1/r2 equality
after 'w2 = w1'. But in this particular case, we know r1 upper 32bit value
is 0, so it is safe to claim r1/r2 equality. This patch exactly did this.
For a 32bit subreg mov, if the src register upper 32bit is 0,
it is okay to claim equality between src and dst registers.

With this patch, the above verification sequence becomes

  ...
  8: (18) r1 = 0xffffc9000048e230       ; R1_w=map_value(off=560,ks=4,vs=564,imm=0)
  10: (61) r1 = *(u32 *)(r1 +0)         ; R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
  ; if (skb->tstamp == EGRESS_ENDHOST_MAGIC)
  11: (79) r2 = *(u64 *)(r6 +152)       ; R2_w=scalar() R6=ctx(off=0,imm=0)
  ; if (skb->tstamp == EGRESS_ENDHOST_MAGIC)
  12: (55) if r2 != 0xb9fbeef goto pc+10        ; R2_w=195018479
  13: (bc) w2 = w1                      ; R1_w=scalar(id=6,umax=4294967295,var_off=(0x0; 0xffffffff)) R2_w=scalar(id=6,umax=4294967295,var_off=(0x0; 0xffffffff))
  ; if (test < __NR_TESTS)
  14: (a6) if w1 < 0x9 goto pc+1        ; R1_w=scalar(id=6,umin=9,umax=4294967295,var_off=(0x0; 0xffffffff))
  ...
  from 14 to 16: R0=2 R1_w=scalar(id=6,umax=8,var_off=(0x0; 0xf)) R2_w=scalar(id=6,umax=8,var_off=(0x0; 0xf)) R6=ctx(off=0,imm=0) R10=fp0
  16: (27) r2 *= 28                     ; R2_w=scalar(umax=224,var_off=(0x0; 0xfc))
  17: (18) r3 = 0xffffc9000048e118      ; R3_w=map_value(off=280,ks=4,vs=564,imm=0)
  19: (0f) r3 += r2
  20: (61) r2 = *(u32 *)(r3 +0)         ; R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R3_w=map_value(off=280,ks=4,vs=564,umax=224,var_off=(0x0; 0xfc),s32_max=252,u32_max=252)
  ...

and eventually the bpf program can be verified successfully.

  [1] https://reviews.llvm.org/D147968

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 kernel/bpf/verifier.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5dae11ee01c3..1e05355facdc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12421,12 +12421,17 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 						insn->src_reg);
 					return -EACCES;
 				} else if (src_reg->type == SCALAR_VALUE) {
+					bool is_src_reg_u32 = src_reg->umax_value <= U32_MAX;
+
+					if (is_src_reg_u32 && !src_reg->id)
+						src_reg->id = ++env->id_gen;
 					copy_register_state(dst_reg, src_reg);
-					/* Make sure ID is cleared otherwise
+					/* Make sure ID is cleared if src_reg is not in u32 range otherwise
 					 * dst_reg min/max could be incorrectly
 					 * propagated into src_reg by find_equal_scalars()
 					 */
-					dst_reg->id = 0;
+					if (!is_src_reg_u32)
+						dst_reg->id = 0;
 					dst_reg->live |= REG_LIVE_WRITTEN;
 					dst_reg->subreg_def = env->insn_idx + 1;
 				} else {
-- 
2.34.1

