[PATCH 5.10 45/54] bpf: Fix 32 bit src register truncation on div/mod

Greg Kroah-Hartman
2021-02-11T15:37:08Z

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.

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>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/verifier.c | 28 +++++++++++++---------------
1 file changed, 13 insertions(+), 15 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10866,30 +10866,28 @@ static int fixup_bpf_calls(struct bpf_ve
insn->code == (BPF_ALU | BPF_MOD | BPF_X) ||
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),
+ bool isdiv = BPF_OP(insn->code) == BPF_DIV;
+ struct bpf_insn *patchlet;
+ struct bpf_insn chk_and_div[] = {
/* Rx div 0 -> 0 */
- BPF_JMP_IMM(BPF_JNE, insn->src_reg, 0, 2),
+ BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
+ BPF_JNE | BPF_K, insn->src_reg,
+ 0, 2, 0),
BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
*insn,
};
- struct bpf_insn mask_and_mod[] = {
- BPF_MOV32_REG(insn->src_reg, insn->src_reg),
+ struct bpf_insn chk_and_mod[] = {
/* Rx mod 0 -> Rx */
- BPF_JMP_IMM(BPF_JEQ, insn->src_reg, 0, 1),
+ BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
+ BPF_JEQ | BPF_K, insn->src_reg,
+ 0, 1, 0),
*insn,
};
- 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);
- } else {
- patchlet = mask_and_mod + (is64 ? 1 : 0);
- cnt = ARRAY_SIZE(mask_and_mod) - (is64 ? 1 : 0);
- }
+ patchlet = isdiv ? chk_and_div : chk_and_mod;
+ cnt = isdiv ? ARRAY_SIZE(chk_and_div) :
+ ARRAY_SIZE(chk_and_mod);

new_prog = bpf_patch_insn_data(env, i + delta, patchlet, cnt);
if (!new_prog)