Re: [PATCH bpf] riscv, bpf: Fix inconsistent JIT image generation

From: Daniel Borkmann
Date: Tue Jul 11 2023 - 16:04:17 EST

On 7/11/23 7:50 PM, Palmer Dabbelt wrote:
On Mon, 10 Jul 2023 00:41:31 PDT (-0700), bjorn@xxxxxxxxxx wrote:
From: Björn Töpel <bjorn@xxxxxxxxxxxx>

In order to generate the prologue and epilogue, the BPF JIT needs to
know which registers that are clobbered. Therefore, the during
pre-final passes, the prologue is generated after the body of the
program body-prologue-epilogue. Then, in the final pass, a proper
prologue-body-epilogue JITted image is generated.

This scheme has worked most of the time. However, for some large
programs with many jumps, e.g. the test_kmod.sh BPF selftest with
hardening enabled (blinding constants), this has shown to be
incorrect. For the final pass, when the proper prologue-body-epilogue
is generated, the image has not converged. This will lead to that the
final image will have incorrect jump offsets. The following is an
excerpt from an incorrect image:

| ...
| 3b8: 00c50663 beq a0,a2,3c4 <.text+0x3c4>
| 3bc: 0020e317 auipc t1,0x20e
| 3c0: 49630067 jalr zero,1174(t1) # 20e852 <.text+0x20e852>
| ...
| 20e84c: 8796 c.mv a5,t0
| 20e84e: 6422 c.ldsp s0,8(sp) # Epilogue start
| 20e850: 6141 c.addi16sp sp,16
| 20e852: 853e c.mv a0,a5 # Incorrect jump target
| 20e854: 8082 c.jr ra

The image has shrunk, and the epilogue offset is incorrect in the
final pass.

Correct the problem by always generating proper prologue-body-epilogue
outputs, which means that the first pass will only generate the body
to track what registers that are touched.

Fixes: 2353ecc6f91f ("bpf, riscv: add BPF JIT for RV64G")
Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
---
arch/riscv/net/bpf_jit.h | 6 +++---
arch/riscv/net/bpf_jit_core.c | 19 +++++++++++++------
2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
index bf9802a63061..2717f5490428 100644
--- a/arch/riscv/net/bpf_jit.h
+++ b/arch/riscv/net/bpf_jit.h
@@ -69,7 +69,7 @@ struct rv_jit_context {
struct bpf_prog *prog;
u16 *insns; /* RV insns */
int ninsns;
- int body_len;
+ int prologue_len;
int epilogue_offset;
int *offset; /* BPF to RV */
int nexentries;
@@ -216,8 +216,8 @@ static inline int rv_offset(int insn, int off, struct rv_jit_context *ctx)
int from, to;

off++; /* BPF branch is from PC+1, RV is from PC */
- from = (insn > 0) ? ctx->offset[insn - 1] : 0;
- to = (insn + off > 0) ? ctx->offset[insn + off - 1] : 0;
+ from = (insn > 0) ? ctx->offset[insn - 1] : ctx->prologue_len;
+ to = (insn + off > 0) ? ctx->offset[insn + off - 1] : ctx->prologue_len;
return ninsns_rvoff(to - from);
}

diff --git a/arch/riscv/net/bpf_jit_core.c b/arch/riscv/net/bpf_jit_core.c
index 737baf8715da..7a26a3e1c73c 100644
--- a/arch/riscv/net/bpf_jit_core.c
+++ b/arch/riscv/net/bpf_jit_core.c
@@ -44,7 +44,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
unsigned int prog_size = 0, extable_size = 0;
bool tmp_blinded = false, extra_pass = false;
struct bpf_prog *tmp, *orig_prog = prog;
- int pass = 0, prev_ninsns = 0, prologue_len, i;
+ int pass = 0, prev_ninsns = 0, i;
struct rv_jit_data *jit_data;
struct rv_jit_context *ctx;

@@ -83,6 +83,12 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
prog = orig_prog;
goto out_offset;
}
+
+ if (build_body(ctx, extra_pass, NULL)) {
+ prog = orig_prog;
+ goto out_offset;
+ }
+
for (i = 0; i < prog->len; i++) {
prev_ninsns += 32;
ctx->offset[i] = prev_ninsns;
@@ -91,12 +97,15 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
for (i = 0; i < NR_JIT_ITERATIONS; i++) {
pass++;
ctx->ninsns = 0;
+
+ bpf_jit_build_prologue(ctx);
+ ctx->prologue_len = ctx->ninsns;
+
if (build_body(ctx, extra_pass, ctx->offset)) {
prog = orig_prog;
goto out_offset;
}
- ctx->body_len = ctx->ninsns;
- bpf_jit_build_prologue(ctx);
+
ctx->epilogue_offset = ctx->ninsns;
bpf_jit_build_epilogue(ctx);

@@ -162,10 +171,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)

if (!prog->is_func || extra_pass) {
bpf_jit_binary_lock_ro(jit_data->header);
- prologue_len = ctx->epilogue_offset - ctx->body_len;
for (i = 0; i < prog->len; i++)
- ctx->offset[i] = ninsns_rvoff(prologue_len +
- ctx->offset[i]);
+ ctx->offset[i] = ninsns_rvoff(ctx->offset[i]);
bpf_prog_fill_jited_linfo(prog, ctx->offset);
out_offset:
kfree(ctx->offset);

base-commit: 496720b7cfb6574a8f6f4d434f23e3d1e6cfaeb9

Acked-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Reviewed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>

I'm assuming this is aimed at the BPF tree, but LMK if you guys want me
to pick it up -- I've already got something for this week, so it's easy
on my end. I'm dropping it from my queue and patchwork for now, though.

Sounds good, we applied it to bpf already.

Thanks,
Daniel