[PATCH 4.14 4/4] bpf: Fix truncation handling for mod32 dst reg wrt zero

[Thadeu Lima de Souza Cascardo <cascardo@canonical.com>]

From: Daniel Borkmann <daniel@iogearbox.net>

Commit 9b00f1b78809309163dda2d044d9e94a3c0248a3 upstream.

Recently noticed that when mod32 with a known src reg of 0 is performed,
then the dst register is 32-bit truncated in verifier:

  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  0: (b7) r0 = 0
  1: R0_w=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  1: (b7) r1 = -1
  2: R0_w=inv0 R1_w=inv-1 R10=fp0
  2: (b4) w2 = -1
  3: R0_w=inv0 R1_w=inv-1 R2_w=inv4294967295 R10=fp0
  3: (9c) w1 %= w0
  4: R0_w=inv0 R1_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R2_w=inv4294967295 R10=fp0
  4: (b7) r0 = 1
  5: R0_w=inv1 R1_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R2_w=inv4294967295 R10=fp0
  5: (1d) if r1 == r2 goto pc+1
   R0_w=inv1 R1_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R2_w=inv4294967295 R10=fp0
  6: R0_w=inv1 R1_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R2_w=inv4294967295 R10=fp0
  6: (b7) r0 = 2
  7: R0_w=inv2 R1_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R2_w=inv4294967295 R10=fp0
  7: (95) exit
  7: R0=inv1 R1=inv(id=0,umin_value=4294967295,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R2=inv4294967295 R10=fp0
  7: (95) exit

However, as a runtime result, we get 2 instead of 1, meaning the dst
register does not contain (u32)-1 in this case. The reason is fairly
straight forward given the 0 test leaves the dst register as-is:

  # ./bpftool p d x i 23
   0: (b7) r0 = 0
   1: (b7) r1 = -1
   2: (b4) w2 = -1
   3: (16) if w0 == 0x0 goto pc+1
   4: (9c) w1 %= w0
   5: (b7) r0 = 1
   6: (1d) if r1 == r2 goto pc+1
   7: (b7) r0 = 2
   8: (95) exit

This was originally not an issue given the dst register was marked as
completely unknown (aka 64 bit unknown). However, after 468f6eafa6c4
("bpf: fix 32-bit ALU op verification") the verifier casts the register
output to 32 bit, and hence it becomes 32 bit unknown. Note that for
the case where the src register is unknown, the dst register is marked
64 bit unknown. After the fix, the register is truncated by the runtime
and the test passes:

  # ./bpftool p d x i 23
   0: (b7) r0 = 0
   1: (b7) r1 = -1
   2: (b4) w2 = -1
   3: (16) if w0 == 0x0 goto pc+2
   4: (9c) w1 %= w0
   5: (05) goto pc+1
   6: (bc) w1 = w1
   7: (b7) r0 = 1
   8: (1d) if r1 == r2 goto pc+1
   9: (b7) r0 = 2
  10: (95) exit

Semantics also match with {R,W}x mod{64,32} 0 -> {R,W}x. Invalid div
has always been {R,W}x div{64,32} 0 -> 0. Rewrites are as follows:

  mod32:                            mod64:

  (16) if w0 == 0x0 goto pc+2       (15) if r0 == 0x0 goto pc+1
  (9c) w1 %= w0                     (9f) r1 %= r0
  (05) goto pc+1
  (bc) w1 = w1

Fixes: 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
[Salvatore Bonaccorso: This is an earlier version based on work by
Daniel and John which does not rely on availability of the BPF_JMP32
instruction class. This means it is not even strictly a backport of the
upstream commit mentioned but based on Daniel's and John's work to
address the issue and was finalized by Thadeu Lima de Souza Cascardo.]
Tested-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
 kernel/bpf/verifier.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e7d92a03f8ac..9dccbffde04f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4845,7 +4845,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			bool is64 = BPF_CLASS(insn->code) == BPF_ALU64;
 			struct bpf_insn mask_and_div[] = {
 				BPF_MOV_REG(BPF_CLASS(insn->code), BPF_REG_AX, insn->src_reg),
-				/* Rx div 0 -> 0 */
+				/* [R,W]x div 0 -> 0 */
 				BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, 2),
 				BPF_RAW_REG(*insn, insn->dst_reg, BPF_REG_AX),
 				BPF_JMP_IMM(BPF_JA, 0, 0, 1),
@@ -4853,9 +4853,10 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			};
 			struct bpf_insn mask_and_mod[] = {
 				BPF_MOV_REG(BPF_CLASS(insn->code), BPF_REG_AX, insn->src_reg),
-				/* Rx mod 0 -> Rx */
-				BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, 1),
+				BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, 1 + (is64 ? 0 : 1)),
 				BPF_RAW_REG(*insn, insn->dst_reg, BPF_REG_AX),
+				BPF_JMP_IMM(BPF_JA, 0, 0, 1),
+				BPF_MOV32_REG(insn->dst_reg, insn->dst_reg),
 			};
 			struct bpf_insn *patchlet;
 
@@ -4865,7 +4866,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 				cnt = ARRAY_SIZE(mask_and_div);
 			} else {
 				patchlet = mask_and_mod;
-				cnt = ARRAY_SIZE(mask_and_mod);
+				cnt = ARRAY_SIZE(mask_and_mod) - (is64 ? 2 : 0);
 			}
 
 			new_prog = bpf_patch_insn_data(env, i + delta, patchlet, cnt);
-- 
2.30.2