[PATCH] LoongArch: Fix calling smp_processor_id() in preemptible code

[Xi Ruoyao]

Fix the warning recently exploited by commit 4ab17e762b34 ("LoongArch:
BPF: Use BPF prog pack allocator"):

    BUG: using smp_processor_id() in preemptible [00000000] code: systemd/1
    caller is larch_insn_text_copy+0x40/0xf0

Simply changing it to raw_smp_processor_id() is not enough: if preempt
and CPU hotplug happens after raw_smp_processor_id() but before
stop_machine(), the CPU where raw_smp_processor_id() has run may be
offline when stop_machine() and no CPU will run copy_to_kernel_nofault()
in text_copy_cb().  Thus guard the larch_insn_text_copy() calls with
cpus_read_lock() and change stop_machine() to stop_machine_cpuslocked()
to prevent this.

Fixes: 9fbd18cf4c69 ("LoongArch: BPF: Add dynamic code modification support")
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
---

P.S. I've considered moving the locks inside larch_insn_text_copy() but
doing so seems not an easy hack.  In bpf_arch_text_poke() obviously the
memcpy() call must be guarded by text_mutex, so we have to leave the
acquire of text_mutex out of larch_insn_text_copy.  But in the entire
kernel the acquire of mutexes is always after cpus_read_lock(), so we
cannot put cpus_read_lock() into larch_insn_text_copy() while leaving
the text_mutex acquire out (or we risk a deadlock due to inconsistent
lock acquire order).  So I'd just submit this as-is to fix the bug and
leave the posssible refactor for others.

 arch/loongarch/kernel/inst.c | 4 ++--
 arch/loongarch/net/bpf_jit.c | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/loongarch/kernel/inst.c b/arch/loongarch/kernel/inst.c
index bf037f0c6b26..db236e2b34a7 100644
--- a/arch/loongarch/kernel/inst.c
+++ b/arch/loongarch/kernel/inst.c
@@ -263,14 +263,14 @@ int larch_insn_text_copy(void *dst, void *src, size_t len)
 		.dst = dst,
 		.src = src,
 		.len = len,
-		.cpu = smp_processor_id(),
+		.cpu = raw_smp_processor_id(),
 	};
 
 	start = round_down((size_t)dst, PAGE_SIZE);
 	end   = round_up((size_t)dst + len, PAGE_SIZE);
 
 	set_memory_rw(start, (end - start) / PAGE_SIZE);
-	ret = stop_machine(text_copy_cb, &copy, cpu_online_mask);
+	ret = stop_machine_cpuslocked(text_copy_cb, &copy, cpu_online_mask);
 	set_memory_rox(start, (end - start) / PAGE_SIZE);
 
 	return ret;
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 3bd89f55960d..e8e0ad34928c 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -1379,9 +1379,11 @@ void *bpf_arch_text_copy(void *dst, void *src, size_t len)
 {
 	int ret;
 
+	cpus_read_lock();
 	mutex_lock(&text_mutex);
 	ret = larch_insn_text_copy(dst, src, len);
 	mutex_unlock(&text_mutex);
+	cpus_read_unlock();
 
 	return ret ? ERR_PTR(-EINVAL) : dst;
 }
@@ -1429,10 +1431,12 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t,
 	if (ret)
 		return ret;
 
+	cpus_read_lock();
 	mutex_lock(&text_mutex);
 	if (memcmp(ip, new_insns, LOONGARCH_LONG_JUMP_NBYTES))
 		ret = larch_insn_text_copy(ip, new_insns, LOONGARCH_LONG_JUMP_NBYTES);
 	mutex_unlock(&text_mutex);
+	cpus_read_unlock();
 
 	return ret;
 }
@@ -1450,10 +1454,12 @@ int bpf_arch_text_invalidate(void *dst, size_t len)
 	for (i = 0; i < (len / sizeof(u32)); i++)
 		inst[i] = INSN_BREAK;
 
+	cpus_read_lock();
 	mutex_lock(&text_mutex);
 	if (larch_insn_text_copy(dst, inst, len))
 		ret = -EINVAL;
 	mutex_unlock(&text_mutex);
+	cpus_read_unlock();
 
 	kvfree(inst);
 
-- 
2.53.0