From 9c3095ad40f9f52be636826878319e5f626db1a0 Mon Sep 17 00:00:00 2001 From: Puranjay Mohan Date: Mon, 13 May 2024 10:02:48 +0000 Subject: powerpc/bpf: enforce full ordering for ATOMIC operations with BPF_FETCH commit b1e7cee96127468c2483cf10c2899c9b5cf79bf8 upstream. The Linux Kernel Memory Model [1][2] requires RMW operations that have a return value to be fully ordered. BPF atomic operations with BPF_FETCH (including BPF_XCHG and BPF_CMPXCHG) return a value back so they need to be JITed to fully ordered operations. POWERPC currently emits relaxed operations for these. We can show this by running the following litmus-test: PPC SB+atomic_add+fetch { 0:r0=x; (* dst reg assuming offset is 0 *) 0:r1=2; (* src reg *) 0:r2=1; 0:r4=y; (* P0 writes to this, P1 reads this *) 0:r5=z; (* P1 writes to this, P0 reads this *) 0:r6=0; 1:r2=1; 1:r4=y; 1:r5=z; } P0 | P1 ; stw r2, 0(r4) | stw r2,0(r5) ; | ; loop:lwarx r3, r6, r0 | ; mr r8, r3 | ; add r3, r3, r1 | sync ; stwcx. r3, r6, r0 | ; bne loop | ; mr r1, r8 | ; | ; lwa r7, 0(r5) | lwa r7,0(r4) ; ~exists(0:r7=0 /\ 1:r7=0) Witnesses Positive: 9 Negative: 3 Condition ~exists (0:r7=0 /\ 1:r7=0) Observation SB+atomic_add+fetch Sometimes 3 9 This test shows that the older store in P0 is reordered with a newer load to a different address. Although there is a RMW operation with fetch between them. Adding a sync before and after RMW fixes the issue: Witnesses Positive: 9 Negative: 0 Condition ~exists (0:r7=0 /\ 1:r7=0) Observation SB+atomic_add+fetch Never 0 9 [1] https://www.kernel.org/doc/Documentation/memory-barriers.txt [2] https://www.kernel.org/doc/Documentation/atomic_t.txt Fixes: aea7ef8a82c0 ("powerpc/bpf/32: add support for BPF_ATOMIC bitwise operations") Fixes: 2d9206b22743 ("powerpc/bpf/32: Add instructions for atomic_[cmp]xchg") Fixes: dbe6e2456fb0 ("powerpc/bpf/64: add support for atomic fetch operations") Fixes: 1e82dfaa7819 ("powerpc/bpf/64: Add instructions for atomic_[cmp]xchg") Cc: stable@vger.kernel.org # v6.0+ Signed-off-by: Puranjay Mohan Reviewed-by: Christophe Leroy Reviewed-by: Naveen N Rao Acked-by: Paul E. McKenney Signed-off-by: Michael Ellerman Link: https://msgid.link/20240513100248.110535-1-puranjay@kernel.org Signed-off-by: Greg Kroah-Hartman --- arch/powerpc/net/bpf_jit_comp32.c | 12 ++++++++++++ arch/powerpc/net/bpf_jit_comp64.c | 12 ++++++++++++ 2 files changed, 24 insertions(+) diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c index 7f91ea064c08..06f886850a93 100644 --- a/arch/powerpc/net/bpf_jit_comp32.c +++ b/arch/powerpc/net/bpf_jit_comp32.c @@ -851,6 +851,15 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context * /* Get offset into TMP_REG */ EMIT(PPC_RAW_LI(tmp_reg, off)); + /* + * Enforce full ordering for operations with BPF_FETCH by emitting a 'sync' + * before and after the operation. + * + * This is a requirement in the Linux Kernel Memory Model. + * See __cmpxchg_u32() in asm/cmpxchg.h as an example. + */ + if ((imm & BPF_FETCH) && IS_ENABLED(CONFIG_SMP)) + EMIT(PPC_RAW_SYNC()); tmp_idx = ctx->idx * 4; /* load value from memory into r0 */ EMIT(PPC_RAW_LWARX(_R0, tmp_reg, dst_reg, 0)); @@ -904,6 +913,9 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context * /* For the BPF_FETCH variant, get old data into src_reg */ if (imm & BPF_FETCH) { + /* Emit 'sync' to enforce full ordering */ + if (IS_ENABLED(CONFIG_SMP)) + EMIT(PPC_RAW_SYNC()); EMIT(PPC_RAW_MR(ret_reg, ax_reg)); if (!fp->aux->verifier_zext) EMIT(PPC_RAW_LI(ret_reg - 1, 0)); /* higher 32-bit */ diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c index 0f8048f6dad6..2239ce5e8501 100644 --- a/arch/powerpc/net/bpf_jit_comp64.c +++ b/arch/powerpc/net/bpf_jit_comp64.c @@ -803,6 +803,15 @@ emit_clear: /* Get offset into TMP_REG_1 */ EMIT(PPC_RAW_LI(tmp1_reg, off)); + /* + * Enforce full ordering for operations with BPF_FETCH by emitting a 'sync' + * before and after the operation. + * + * This is a requirement in the Linux Kernel Memory Model. + * See __cmpxchg_u64() in asm/cmpxchg.h as an example. + */ + if ((imm & BPF_FETCH) && IS_ENABLED(CONFIG_SMP)) + EMIT(PPC_RAW_SYNC()); tmp_idx = ctx->idx * 4; /* load value from memory into TMP_REG_2 */ if (size == BPF_DW) @@ -865,6 +874,9 @@ emit_clear: PPC_BCC_SHORT(COND_NE, tmp_idx); if (imm & BPF_FETCH) { + /* Emit 'sync' to enforce full ordering */ + if (IS_ENABLED(CONFIG_SMP)) + EMIT(PPC_RAW_SYNC()); EMIT(PPC_RAW_MR(ret_reg, _R0)); /* * Skip unnecessary zero-extension for 32-bit cmpxchg. -- cgit v1.2.3