[PATCH 5.10 060/148] bpf,x86: Respect X86_FEATURE_RETPOLINE*

Greg Kroah-Hartman
2022-07-23T10:05:14Z

From: Peter Zijlstra <peterz@infradead.org>

commit 87c87ecd00c54ecd677798cb49ef27329e0fab41 upstream.

Current BPF codegen doesn't respect X86_FEATURE_RETPOLINE* flags and
unconditionally emits a thunk call, this is sub-optimal and doesn't
match the regular, compiler generated, code.

Update the i386 JIT to emit code equal to what the compiler emits for
the regular kernel text (IOW. a plain THUNK call).

Update the x86_64 JIT to emit code similar to the result of compiler
and kernel rewrites as according to X86_FEATURE_RETPOLINE* flags.
Inlining RETPOLINE_AMD (lfence; jmp *%reg) and !RETPOLINE (jmp *%reg),
while doing a THUNK call for RETPOLINE.

This removes the hard-coded retpoline thunks and shrinks the generated
code. Leaving a single retpoline thunk definition in the kernel.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.614772675@infradead.org
[cascardo: RETPOLINE_AMD was renamed to RETPOLINE_LFENCE]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
[bwh: Backported to 5.10: add the necessary cnt variable to
emit_indirect_jump()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/nospec-branch.h | 59 -----------------------------------
arch/x86/net/bpf_jit_comp.c | 49 +++++++++++++----------------
arch/x86/net/bpf_jit_comp32.c | 22 +++++++++++--
3 files changed, 42 insertions(+), 88 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -318,63 +318,4 @@ static inline void mds_idle_clear_cpu_bu

#endif /* __ASSEMBLY__ */

-/*
- * Below is used in the eBPF JIT compiler and emits the byte sequence
- * for the following assembly:
- *
- * With retpolines configured:
- *
- * callq do_rop
- * spec_trap:
- * pause
- * lfence
- * jmp spec_trap
- * do_rop:
- * mov %rcx,(%rsp) for x86_64
- * mov %edx,(%esp) for x86_32
- * retq
- *
- * Without retpolines configured:
- *
- * jmp *%rcx for x86_64
- * jmp *%edx for x86_32
- */
-#ifdef CONFIG_RETPOLINE
-# ifdef CONFIG_X86_64
-# define RETPOLINE_RCX_BPF_JIT_SIZE 17
-# define RETPOLINE_RCX_BPF_JIT() \
-do { \
- EMIT1_off32(0xE8, 7); /* callq do_rop */ \
- /* spec_trap: */ \
- EMIT2(0xF3, 0x90); /* pause */ \
- EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
- EMIT2(0xEB, 0xF9); /* jmp spec_trap */ \
- /* do_rop: */ \
- EMIT4(0x48, 0x89, 0x0C, 0x24); /* mov %rcx,(%rsp) */ \
- EMIT1(0xC3); /* retq */ \
-} while (0)
-# else /* !CONFIG_X86_64 */
-# define RETPOLINE_EDX_BPF_JIT() \
-do { \
- EMIT1_off32(0xE8, 7); /* call do_rop */ \
- /* spec_trap: */ \
- EMIT2(0xF3, 0x90); /* pause */ \
- EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
- EMIT2(0xEB, 0xF9); /* jmp spec_trap */ \
- /* do_rop: */ \
- EMIT3(0x89, 0x14, 0x24); /* mov %edx,(%esp) */ \
- EMIT1(0xC3); /* ret */ \
-} while (0)
-# endif
-#else /* !CONFIG_RETPOLINE */
-# ifdef CONFIG_X86_64
-# define RETPOLINE_RCX_BPF_JIT_SIZE 2
-# define RETPOLINE_RCX_BPF_JIT() \
- EMIT2(0xFF, 0xE1); /* jmp *%rcx */
-# else /* !CONFIG_X86_64 */
-# define RETPOLINE_EDX_BPF_JIT() \
- EMIT2(0xFF, 0xE2) /* jmp *%edx */
-# endif
-#endif
-
#endif /* _ASM_X86_NOSPEC_BRANCH_H_ */
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -379,6 +379,26 @@ int bpf_arch_text_poke(void *ip, enum bp
return __bpf_arch_text_poke(ip, t, old_addr, new_addr, true);
}

+#define EMIT_LFENCE() EMIT3(0x0F, 0xAE, 0xE8)
+
+static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
+{
+ u8 *prog = *pprog;
+ int cnt = 0;
+
+#ifdef CONFIG_RETPOLINE
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) {
+ EMIT_LFENCE();
+ EMIT2(0xFF, 0xE0 + reg);
+ } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
+ emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
+ } else
+#endif
+ EMIT2(0xFF, 0xE0 + reg);
+
+ *pprog = prog;
+}
+
/*
* Generate the following code:
*
@@ -460,7 +480,7 @@ static void emit_bpf_tail_call_indirect(
* rdi == ctx (1st arg)
* rcx == prog->bpf_func + X86_TAIL_CALL_OFFSET
*/
- RETPOLINE_RCX_BPF_JIT();
+ emit_indirect_jump(&prog, 1 /* rcx */, ip + (prog - start));

/* out: */
ctx->tail_call_indirect_label = prog - start;
@@ -1099,8 +1119,7 @@ static int do_jit(struct bpf_prog *bpf_p
/* speculation barrier */
case BPF_ST | BPF_NOSPEC:
if (boot_cpu_has(X86_FEATURE_XMM2))
- /* Emit 'lfence' */
- EMIT3(0x0F, 0xAE, 0xE8);
+ EMIT_LFENCE();
break;

/* ST: *(u8*)(dst_reg + off) = imm */
@@ -1878,26 +1897,6 @@ cleanup:
return ret;
}

-static int emit_fallback_jump(u8 **pprog)
-{
- u8 *prog = *pprog;
- int err = 0;
-
-#ifdef CONFIG_RETPOLINE
- /* Note that this assumes the the compiler uses external
- * thunks for indirect calls. Both clang and GCC use the same
- * naming convention for external thunks.
- */
- err = emit_jump(&prog, __x86_indirect_thunk_rdx, prog);
-#else
- int cnt = 0;
-
- EMIT2(0xFF, 0xE2); /* jmp rdx */
-#endif
- *pprog = prog;
- return err;
-}
-
static int emit_bpf_dispatcher(u8 **pprog, int a, int b, s64 *progs)
{
u8 *jg_reloc, *prog = *pprog;
@@ -1919,9 +1918,7 @@ static int emit_bpf_dispatcher(u8 **ppro
if (err)
return err;

- err = emit_fallback_jump(&prog); /* jmp thunk/indirect */
- if (err)
- return err;
+ emit_indirect_jump(&prog, 2 /* rdx */, prog);

*pprog = prog;
return 0;
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -15,6 +15,7 @@
#include <asm/cacheflush.h>
#include <asm/set_memory.h>
#include <asm/nospec-branch.h>
+#include <asm/asm-prototypes.h>
#include <linux/bpf.h>

/*
@@ -1267,6 +1268,21 @@ static void emit_epilogue(u8 **pprog, u3
*pprog = prog;
}

+static int emit_jmp_edx(u8 **pprog, u8 *ip)
+{
+ u8 *prog = *pprog;
+ int cnt = 0;
+
+#ifdef CONFIG_RETPOLINE
+ EMIT1_off32(0xE9, (u8 *)__x86_indirect_thunk_edx - (ip + 5));
+#else
+ EMIT2(0xFF, 0xE2);
+#endif
+ *pprog = prog;
+
+ return cnt;
+}
+
/*
* Generate the following code:
* ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) ...
@@ -1280,7 +1296,7 @@ static void emit_epilogue(u8 **pprog, u3
* goto *(prog->bpf_func + prologue_size);
* out:
*/
-static void emit_bpf_tail_call(u8 **pprog)
+static void emit_bpf_tail_call(u8 **pprog, u8 *ip)
{
u8 *prog = *pprog;
int cnt = 0;
@@ -1362,7 +1378,7 @@ static void emit_bpf_tail_call(u8 **ppro
* eax == ctx (1st arg)
* edx == prog->bpf_func + prologue_size
*/
- RETPOLINE_EDX_BPF_JIT();
+ cnt += emit_jmp_edx(&prog, ip + cnt);

if (jmp_label1 == -1)
jmp_label1 = cnt;
@@ -1929,7 +1945,7 @@ static int do_jit(struct bpf_prog *bpf_p
break;
}
case BPF_JMP | BPF_TAIL_CALL:
- emit_bpf_tail_call(&prog);
+ emit_bpf_tail_call(&prog, image + addrs[i - 1]);
break;

/* cond jump */