[PATCH 4.14 v2 3/4] bpf: Fix 32 bit src register truncation on div/mod

Edward Liaw
2023-02-22T19:30:11Z

From: Daniel Borkmann <daniel@iogearbox.net>

Commit e88b2c6e5a4d9ce30d75391e4d950da74bb2bd90 upstream.

While reviewing a different fix, John and I noticed an oddity in one of the
BPF program dumps that stood out, for example:

# bpftool p d x i 13
0: (b7) r0 = 808464450
1: (b4) w4 = 808464432
2: (bc) w0 = w0
3: (15) if r0 == 0x0 goto pc+1
4: (9c) w4 %= w0
[...]

In line 2 we noticed that the mov32 would 32 bit truncate the original src
register for the div/mod operation. While for the two operations the dst
register is typically marked unknown e.g. from adjust_scalar_min_max_vals()
the src register is not, and thus verifier keeps tracking original bounds,
simplified:

0: R1=ctx(id=0,off=0,imm=0) R10=fp0
0: (b7) r0 = -1
1: R0_w=invP-1 R1=ctx(id=0,off=0,imm=0) R10=fp0
1: (b7) r1 = -1
2: R0_w=invP-1 R1_w=invP-1 R10=fp0
2: (3c) w0 /= w1
3: R0_w=invP(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R1_w=invP-1 R10=fp0
3: (77) r1 >>= 32
4: R0_w=invP(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R1_w=invP4294967295 R10=fp0
4: (bf) r0 = r1
5: R0_w=invP4294967295 R1_w=invP4294967295 R10=fp0
5: (95) exit
processed 6 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

Runtime result of r0 at exit is 0 instead of expected -1. Remove the
verifier mov32 src rewrite in div/mod and replace it with a jmp32 test
instead. After the fix, we result in the following code generation when
having dividend r1 and divisor r6:

div, 64 bit: div, 32 bit:

0: (b7) r6 = 8 0: (b7) r6 = 8
1: (b7) r1 = 8 1: (b7) r1 = 8
2: (55) if r6 != 0x0 goto pc+2 2: (56) if w6 != 0x0 goto pc+2
3: (ac) w1 ^= w1 3: (ac) w1 ^= w1
4: (05) goto pc+1 4: (05) goto pc+1
5: (3f) r1 /= r6 5: (3c) w1 /= w6
6: (b7) r0 = 0 6: (b7) r0 = 0
7: (95) exit 7: (95) exit

mod, 64 bit: mod, 32 bit:

0: (b7) r6 = 8 0: (b7) r6 = 8
1: (b7) r1 = 8 1: (b7) r1 = 8
2: (15) if r6 == 0x0 goto pc+1 2: (16) if w6 == 0x0 goto pc+1
3: (9f) r1 %= r6 3: (9c) w1 %= w6
4: (b7) r0 = 0 4: (b7) r0 = 0
5: (95) exit 5: (95) exit

x86 in particular can throw a 'divide error' exception for div
instruction not only for divisor being zero, but also for the case
when the quotient is too large for the designated register. For the
edx:eax and rdx:rax dividend pair it is not an issue in x86 BPF JIT
since we always zero edx (rdx). Hence really the only protection
needed is against divisor being zero.

[Salvatore Bonaccorso: This is an earlier version of the patch provided
by Daniel Borkmann 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.]

Fixes: 68fda450a7df ("bpf: fix 32-bit divide by zero")
Co-developed-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
include/linux/filter.h | 24 ++++++++++++++++++++++++
kernel/bpf/verifier.c | 22 +++++++++++-----------
2 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 7c0e616362f0..55a639609070 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -67,6 +67,14 @@ struct bpf_prog_aux;

/* ALU ops on registers, bpf_add|sub|...: dst_reg += src_reg */

+#define BPF_ALU_REG(CLASS, OP, DST, SRC) \
+ ((struct bpf_insn) { \
+ .code = CLASS | BPF_OP(OP) | BPF_X, \
+ .dst_reg = DST, \
+ .src_reg = SRC, \
+ .off = 0, \
+ .imm = 0 })
+
#define BPF_ALU64_REG(OP, DST, SRC) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_OP(OP) | BPF_X, \
@@ -113,6 +121,14 @@ struct bpf_prog_aux;

/* Short form of mov, dst_reg = src_reg */

+#define BPF_MOV_REG(CLASS, DST, SRC) \
+ ((struct bpf_insn) { \
+ .code = CLASS | BPF_MOV | BPF_X, \
+ .dst_reg = DST, \
+ .src_reg = SRC, \
+ .off = 0, \
+ .imm = 0 })
+
#define BPF_MOV64_REG(DST, SRC) \
((struct bpf_insn) { \
.code = BPF_ALU64 | BPF_MOV | BPF_X, \
@@ -147,6 +163,14 @@ struct bpf_prog_aux;
.off = 0, \
.imm = IMM })

+#define BPF_RAW_REG(insn, DST, SRC) \
+ ((struct bpf_insn) { \
+ .code = (insn).code, \
+ .dst_reg = DST, \
+ .src_reg = SRC, \
+ .off = (insn).off, \
+ .imm = (insn).imm })
+
/* BPF_LD_IMM64 macro encodes single 'load 64-bit immediate' insn */
#define BPF_LD_IMM64(DST, IMM) \
BPF_LD_IMM64_RAW(DST, 0, IMM)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 836791403129..9f04d413df92 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4845,28 +4845,28 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
insn->code == (BPF_ALU | BPF_DIV | BPF_X)) {
bool is64 = BPF_CLASS(insn->code) == BPF_ALU64;
struct bpf_insn mask_and_div[] = {
- BPF_MOV32_REG(insn->src_reg, insn->src_reg),
+ BPF_MOV_REG(BPF_CLASS(insn->code), BPF_REG_AX, insn->src_reg),
/* Rx div 0 -> 0 */
- BPF_JMP_IMM(BPF_JNE, insn->src_reg, 0, 2),
- BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg),
+ 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),
- *insn,
+ BPF_ALU_REG(BPF_CLASS(insn->code), BPF_XOR, insn->dst_reg, insn->dst_reg),
};
struct bpf_insn mask_and_mod[] = {
- BPF_MOV32_REG(insn->src_reg, insn->src_reg),
+ BPF_MOV_REG(BPF_CLASS(insn->code), BPF_REG_AX, insn->src_reg),
/* Rx mod 0 -> Rx */
- BPF_JMP_IMM(BPF_JEQ, insn->src_reg, 0, 1),
- *insn,
+ BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, 1),
+ BPF_RAW_REG(*insn, insn->dst_reg, BPF_REG_AX),
};
struct bpf_insn *patchlet;

if (insn->code == (BPF_ALU64 | BPF_DIV | BPF_X) ||
insn->code == (BPF_ALU | BPF_DIV | BPF_X)) {
- patchlet = mask_and_div + (is64 ? 1 : 0);
- cnt = ARRAY_SIZE(mask_and_div) - (is64 ? 1 : 0);
+ patchlet = mask_and_div;
+ cnt = ARRAY_SIZE(mask_and_div);
} else {
- patchlet = mask_and_mod + (is64 ? 1 : 0);
- cnt = ARRAY_SIZE(mask_and_mod) - (is64 ? 1 : 0);
+ patchlet = mask_and_mod;
+ cnt = ARRAY_SIZE(mask_and_mod);
}

new_prog = bpf_patch_insn_data(env, i + delta, patchlet, cnt);
--
2.39.2.637.g21b0678d19-goog