random: do not use jump labels before they are initialized

Stephen reported that a static key warning splat appears during early
boot on systems that credit randomness from device trees that contain an
"rng-seed" property, because because setup_machine_fdt() is called
before jump_label_init() during setup_arch():

 static_key_enable_cpuslocked(): static key '0xffffffe51c6fcfc0' used before call to jump_label_init()
 WARNING: CPU: 0 PID: 0 at kernel/jump_label.c:166 static_key_enable_cpuslocked+0xb0/0xb8
 Modules linked in:
 CPU: 0 PID: 0 Comm: swapper Not tainted 5.18.0+ #224 44b43e377bfc84bc99bb5ab885ff694984ee09ff
 pstate: 600001c9 (nZCv dAIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : static_key_enable_cpuslocked+0xb0/0xb8
 lr : static_key_enable_cpuslocked+0xb0/0xb8
 sp : ffffffe51c393cf0
 x29: ffffffe51c393cf0 x28: 000000008185054c x27: 00000000f1042f10
 x26: 0000000000000000 x25: 00000000f10302b2 x24: 0000002513200000
 x23: 0000002513200000 x22: ffffffe51c1c9000 x21: fffffffdfdc00000
 x20: ffffffe51c2f0831 x19: ffffffe51c6fcfc0 x18: 00000000ffff1020
 x17: 00000000e1e2ac90 x16: 00000000000000e0 x15: ffffffe51b710708
 x14: 0000000000000066 x13: 0000000000000018 x12: 0000000000000000
 x11: 0000000000000000 x10: 00000000ffffffff x9 : 0000000000000000
 x8 : 0000000000000000 x7 : 61632065726f6665 x6 : 6220646573752027
 x5 : ffffffe51c641d25 x4 : ffffffe51c13142c x3 : ffff0a00ffffff05
 x2 : 40000000ffffe003 x1 : 00000000000001c0 x0 : 0000000000000065
 Call trace:
  static_key_enable_cpuslocked+0xb0/0xb8
  static_key_enable+0x2c/0x40
  crng_set_ready+0x24/0x30
  execute_in_process_context+0x80/0x90
  _credit_init_bits+0x100/0x154
  add_bootloader_randomness+0x64/0x78
  early_init_dt_scan_chosen+0x140/0x184
  early_init_dt_scan_nodes+0x28/0x4c
  early_init_dt_scan+0x40/0x44
  setup_machine_fdt+0x7c/0x120
  setup_arch+0x74/0x1d8
  start_kernel+0x84/0x44c
  __primary_switched+0xc0/0xc8
 ---[ end trace 0000000000000000 ]---
 random: crng init done
 Machine model: Google Lazor (rev1 - 2) with LTE

A trivial fix went in to address this on arm64, 73e2d827 ("arm64:
Initialize jump labels before setup_machine_fdt()"). I wrote patches as
well for arm32 and risc-v. But still patches are needed on xtensa,
powerpc, arc, and mips. So that's 7 platforms where things aren't quite
right. This sort of points to larger issues that might need a larger
solution.

Instead, this commit just defers setting the static branch until later
in the boot process. random_init() is called after jump_label_init() has
been called, and so is always a safe place from which to adjust the
static branch.

Fixes: f5bda35f ("random: use static branch for crng_ready()")
Reported-by: Stephen Boyd <swboyd@chromium.org>
Reported-by: Phil Elwell <phil@raspberrypi.com>
Tested-by: Phil Elwell <phil@raspberrypi.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -650,6 +650,7 @@ static void __cold _credit_init_bits(size_t bits)
 
 	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
 		crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
+		if (static_key_initialized)
 			execute_in_process_context(crng_set_ready, &set_ready);
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
@@ -798,6 +799,14 @@ int __init random_init(const char *command_line)
 	_mix_pool_bytes(command_line, strlen(command_line));
 	add_latent_entropy();
 
+	/*
+	 * If we were initialized by the bootloader before jump labels are
+	 * initialized, then we should enable the static branch here, where
+	 * it's guaranteed that jump labels have been initialized.
+	 */
+	if (!static_branch_likely(&crng_is_ready) && crng_init >= CRNG_READY)
+		crng_set_ready(NULL);
+
 	if (crng_ready())
 		crng_reseed();
 	else if (trust_cpu)