[PATCH v3 20/39] x86/ibt,bpf: Add ENDBR instructions to prologue and trampoline

Peter Zijlstra
2022-03-03T11:33:01Z

With IBT enabled builds we need ENDBR instructions at indirect jump
target sites, since we start execution of the JIT'ed code through an
indirect jump, the very first instruction needs to be ENDBR.

Similarly, since eBPF tail-calls use indirect branches, their landing
site needs to be an ENDBR too.

The trampolines need similar adjustment.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/net/bpf_jit_comp.c | 16 ++++++++++++++--
kernel/bpf/trampoline.c | 20 ++++----------------
2 files changed, 18 insertions(+), 18 deletions(-)

--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -46,6 +46,12 @@ static u8 *emit_code(u8 *ptr, u32 bytes,
#define EMIT4_off32(b1, b2, b3, b4, off) \
do { EMIT4(b1, b2, b3, b4); EMIT(off, 4); } while (0)

+#ifdef CONFIG_X86_KERNEL_IBT
+#define EMIT_ENDBR() EMIT(gen_endbr(), 4)
+#else
+#define EMIT_ENDBR()
+#endif
+
static bool is_imm8(int value)
{
return value <= 127 && value >= -128;
@@ -241,7 +247,7 @@ struct jit_context {
/* Number of bytes emit_patch() needs to generate instructions */
#define X86_PATCH_SIZE 5
/* Number of bytes that will be skipped on tailcall */
-#define X86_TAIL_CALL_OFFSET 11
+#define X86_TAIL_CALL_OFFSET (11 + ENDBR_INSN_SIZE)

static void push_callee_regs(u8 **pprog, bool *callee_regs_used)
{
@@ -286,6 +292,7 @@ static void emit_prologue(u8 **pprog, u3
/* BPF trampoline can be made to work without these nops,
* but let's waste 5 bytes for now and optimize later
*/
+ EMIT_ENDBR();
memcpy(prog, x86_nops[5], X86_PATCH_SIZE);
prog += X86_PATCH_SIZE;
if (!ebpf_from_cbpf) {
@@ -296,6 +303,10 @@ static void emit_prologue(u8 **pprog, u3
}
EMIT1(0x55); /* push rbp */
EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
+
+ /* X86_TAIL_CALL_OFFSET is here */
+ EMIT_ENDBR();
+
/* sub rsp, rounded_stack_depth */
if (stack_depth)
EMIT3_off32(0x48, 0x81, 0xEC, round_up(stack_depth, 8));
@@ -2028,10 +2039,11 @@ int arch_prepare_bpf_trampoline(struct b
/* skip patched call instruction and point orig_call to actual
* body of the kernel function.
*/
- orig_call += X86_PATCH_SIZE;
+ orig_call += X86_PATCH_SIZE + ENDBR_INSN_SIZE;

prog = image;

+ EMIT_ENDBR();
EMIT1(0x55); /* push rbp */
EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -117,18 +117,6 @@ static void bpf_trampoline_module_put(st
tr->mod = NULL;
}

-static int is_ftrace_location(void *ip)
-{
- long addr;
-
- addr = ftrace_location((long)ip);
- if (!addr)
- return 0;
- if (WARN_ON_ONCE(addr != (long)ip))
- return -EFAULT;
- return 1;
-}
-
static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
{
void *ip = tr->func.addr;
@@ -160,12 +148,12 @@ static int modify_fentry(struct bpf_tram
static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
{
void *ip = tr->func.addr;
+ unsigned long faddr;
int ret;

- ret = is_ftrace_location(ip);
- if (ret < 0)
- return ret;
- tr->func.ftrace_managed = ret;
+ faddr = ftrace_location((unsigned long)ip);
+ if (faddr)
+ tr->func.ftrace_managed = true;

if (bpf_trampoline_module_get(tr))
return -ENOENT;