[PATCH bpf] bpf, x64: implement retpoline for tail call

[Daniel Borkmann <daniel@iogearbox.net>]

Implement a retpoline [0] for the BPF tail call JIT'ing that converts
the indirect jump via jmp %rax that is used to make the long jump into
another JITed BPF image. Since this is subject to speculative execution,
we need to control the transient instruction sequence here as well
when CONFIG_RETPOLINE is set, and direct it into a pause + lfence loop.
The latter aligns also with what gcc / clang emits (e.g. [1]).

JIT dump after patch:

  # bpftool p d x i 1
   0: (18) r2 = map[id:1]
   2: (b7) r3 = 0
   3: (85) call bpf_tail_call#12
   4: (b7) r0 = 2
   5: (95) exit

With CONFIG_RETPOLINE:

  # bpftool p d j i 1
  [...]
  33:	cmp    %edx,0x24(%rsi)
  36:	jbe    0x0000000000000072  |*
  38:	mov    0x24(%rbp),%eax
  3e:	cmp    $0x20,%eax
  41:	ja     0x0000000000000072  |
  43:	add    $0x1,%eax
  46:	mov    %eax,0x24(%rbp)
  4c:	mov    0x90(%rsi,%rdx,8),%rax
  54:	test   %rax,%rax
  57:	je     0x0000000000000072  |
  59:	mov    0x28(%rax),%rax
  5d:	add    $0x25,%rax
  61:	callq  0x000000000000006d  |+
  66:	pause                      |
  68:	lfence                     |
  6b:	jmp    0x0000000000000066  |
  6d:	mov    %rax,(%rsp)         |
  71:	retq                       |
  72:	mov    $0x2,%eax
  [...]

  * relative fall-through jumps in error case
  + retpoline for indirect jump

Without CONFIG_RETPOLINE:

  # bpftool p d j i 1
  [...]
  33:	cmp    %edx,0x24(%rsi)
  36:	jbe    0x0000000000000063  |*
  38:	mov    0x24(%rbp),%eax
  3e:	cmp    $0x20,%eax
  41:	ja     0x0000000000000063  |
  43:	add    $0x1,%eax
  46:	mov    %eax,0x24(%rbp)
  4c:	mov    0x90(%rsi,%rdx,8),%rax
  54:	test   %rax,%rax
  57:	je     0x0000000000000063  |
  59:	mov    0x28(%rax),%rax
  5d:	add    $0x25,%rax
  61:	jmpq   *%rax               |-
  63:	mov    $0x2,%eax
  [...]

  * relative fall-through jumps in error case
  - plain indirect jump as before

  [0] https://support.google.com/faqs/answer/7625886
  [1] https://github.com/gcc-mirror/gcc/commit/a31e654fa107be968b802786d747e962c2fcdb2b

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 arch/x86/net/bpf_jit_comp.c | 52 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 42 insertions(+), 10 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 4923d92..7e8d562 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -261,6 +261,35 @@ static void emit_prologue(u8 **pprog, u32 stack_depth)
 	*pprog = prog;
 }
 
+#ifdef CONFIG_RETPOLINE
+# define RETPOLINE_SIZE	17
+# define OFFSET_ADJ	RETPOLINE_SIZE
+
+/* Instead of plain jmp %rax, we emit a retpoline to control
+ * speculative execution for the indirect branch.
+ */
+static void emit_retpoline_rax_trampoline(u8 **pprog)
+{
+	u8 *prog = *pprog;
+	int cnt = 0;
+
+	EMIT1_off32(0xE8, 7);	 /* callq <set_up_target> */
+	/* capture_spec: */
+	EMIT2(0xF3, 0x90);	 /* pause */
+	EMIT3(0x0F, 0xAE, 0xE8); /* lfence */
+	EMIT2(0xEB, 0xF9);	 /* jmp <capture_spec> */
+	/* set_up_target: */
+	EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */
+	EMIT1(0xC3);		 /* retq */
+
+	BUILD_BUG_ON(cnt != RETPOLINE_SIZE);
+	*pprog = prog;
+}
+#else
+/* Plain jmp %rax version used. */
+# define OFFSET_ADJ	2
+#endif
+
 /* generate the following code:
  * ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) ...
  *   if (index >= array->map.max_entries)
@@ -290,7 +319,7 @@ static void emit_bpf_tail_call(u8 **pprog)
 	EMIT2(0x89, 0xD2);                        /* mov edx, edx */
 	EMIT3(0x39, 0x56,                         /* cmp dword ptr [rsi + 16], edx */
 	      offsetof(struct bpf_array, map.max_entries));
-#define OFFSET1 43 /* number of bytes to jump */
+#define OFFSET1 (41 + OFFSET_ADJ) /* number of bytes to jump */
 	EMIT2(X86_JBE, OFFSET1);                  /* jbe out */
 	label1 = cnt;
 
@@ -299,7 +328,7 @@ static void emit_bpf_tail_call(u8 **pprog)
 	 */
 	EMIT2_off32(0x8B, 0x85, 36);              /* mov eax, dword ptr [rbp + 36] */
 	EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT);     /* cmp eax, MAX_TAIL_CALL_CNT */
-#define OFFSET2 32
+#define OFFSET2 (30 + OFFSET_ADJ)
 	EMIT2(X86_JA, OFFSET2);                   /* ja out */
 	label2 = cnt;
 	EMIT3(0x83, 0xC0, 0x01);                  /* add eax, 1 */
@@ -313,7 +342,7 @@ static void emit_bpf_tail_call(u8 **pprog)
 	 *   goto out;
 	 */
 	EMIT3(0x48, 0x85, 0xC0);		  /* test rax,rax */
-#define OFFSET3 10
+#define OFFSET3 (8 + OFFSET_ADJ)
 	EMIT2(X86_JE, OFFSET3);                   /* je out */
 	label3 = cnt;
 
@@ -326,16 +355,19 @@ static void emit_bpf_tail_call(u8 **pprog)
 	 * rdi == ctx (1st arg)
 	 * rax == prog->bpf_func + prologue_size
 	 */
-	EMIT2(0xFF, 0xE0);                        /* jmp rax */
-
-	/* out: */
-	BUILD_BUG_ON(cnt - label1 != OFFSET1);
-	BUILD_BUG_ON(cnt - label2 != OFFSET2);
-	BUILD_BUG_ON(cnt - label3 != OFFSET3);
+	BUILD_BUG_ON(cnt - label1 != OFFSET1 - OFFSET_ADJ);
+	BUILD_BUG_ON(cnt - label2 != OFFSET2 - OFFSET_ADJ);
+	BUILD_BUG_ON(cnt - label3 != OFFSET3 - OFFSET_ADJ);
+#ifdef CONFIG_RETPOLINE
 	*pprog = prog;
+	emit_retpoline_rax_trampoline(pprog);
+#else
+	EMIT2(0xFF, 0xE0);	/* jmp rax */
+	*pprog = prog;
+#endif
+	/* out: */
 }
 
-
 static void emit_load_skb_data_hlen(u8 **pprog)
 {
 	u8 *prog = *pprog;
-- 
2.9.5