[PATCH bpf] arm: bpf: Fix bugs with ALU64 {RSH, ARSH} BPF_K shift by 0

[PATCH bpf] arm: bpf: Fix bugs with ALU64 {RSH, ARSH} BPF_K shift by 0

[Luke Nelson]

The current arm BPF JIT does not correctly compile RSH or ARSH when the
immediate shift amount is 0. This causes the "rsh64 by 0 imm" and "arsh64
by 0 imm" BPF selftests to hang the kernel by reaching an instruction
the verifier determines to be unreachable.

The root cause is in how immediate right shifts are encoded on arm.
For LSR and ASR (logical and arithmetic right shift), a bit-pattern
of 00000 in the immediate encodes a shift amount of 32. When the BPF
immediate is 0, the generated code shifts by 32 instead of the expected
behavior (a no-op).

This patch fixes the bugs by adding an additional check if the BPF
immediate is 0. After the change, the above mentioned BPF selftests pass.

Fixes: 39c13c204bb11 ("arm: eBPF JIT compiler")
Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
---
 arch/arm/net/bpf_jit_32.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index cc29869d12a3..d124f78e20ac 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -929,7 +929,11 @@ static inline void emit_a32_rsh_i64(const s8 dst[],
 	rd = arm_bpf_get_reg64(dst, tmp, ctx);
 
 	/* Do LSR operation */
-	if (val < 32) {
+	if (val == 0) {
+		/* An immediate value of 0 encodes a shift amount of 32
+		 * for LSR. To shift by 0, don't do anything.
+		 */
+	} else if (val < 32) {
 		emit(ARM_MOV_SI(tmp2[1], rd[1], SRTYPE_LSR, val), ctx);
 		emit(ARM_ORR_SI(rd[1], tmp2[1], rd[0], SRTYPE_ASL, 32 - val), ctx);
 		emit(ARM_MOV_SI(rd[0], rd[0], SRTYPE_LSR, val), ctx);
@@ -955,7 +959,11 @@ static inline void emit_a32_arsh_i64(const s8 dst[],
 	rd = arm_bpf_get_reg64(dst, tmp, ctx);
 
 	/* Do ARSH operation */
-	if (val < 32) {
+	if (val == 0) {
+		/* An immediate value of 0 encodes a shift amount of 32
+		 * for ASR. To shift by 0, don't do anything.
+		 */
+	} else if (val < 32) {
 		emit(ARM_MOV_SI(tmp2[1], rd[1], SRTYPE_LSR, val), ctx);
 		emit(ARM_ORR_SI(rd[1], tmp2[1], rd[0], SRTYPE_ASL, 32 - val), ctx);
 		emit(ARM_MOV_SI(rd[0], rd[0], SRTYPE_ASR, val), ctx);
-- 
2.17.1

Re: [PATCH bpf] arm: bpf: Fix bugs with ALU64 {RSH, ARSH} BPF_K shift by 0

[Daniel Borkmann]

On 4/8/20 8:12 PM, Luke Nelson wrote:
> The current arm BPF JIT does not correctly compile RSH or ARSH when the
> immediate shift amount is 0. This causes the "rsh64 by 0 imm" and "arsh64
> by 0 imm" BPF selftests to hang the kernel by reaching an instruction
> the verifier determines to be unreachable.
> 
> The root cause is in how immediate right shifts are encoded on arm.
> For LSR and ASR (logical and arithmetic right shift), a bit-pattern
> of 00000 in the immediate encodes a shift amount of 32. When the BPF
> immediate is 0, the generated code shifts by 32 instead of the expected
> behavior (a no-op).
> 
> This patch fixes the bugs by adding an additional check if the BPF
> immediate is 0. After the change, the above mentioned BPF selftests pass.
> 
> Fixes: 39c13c204bb11 ("arm: eBPF JIT compiler")
> Co-developed-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>

Yikes, thanks for fixing, applied. Looks like noone was running BPF kselftests
on arm32 for quite a while. :(