[PATCH] random: do not use jump labels before they are initialized

[Ard Biesheuvel]

On Tue, 7 Jun 2022 at 12:04, Jason A. Donenfeld <Jason at zx2c4.com> wrote:
>
> [ I would like to pursue fixing this more directly first before actually
>   merging this, but I thought I'd send this to the list now anyway as a
>   the "backup" plan. If I can't figure out how to make headway on the
>   main plan in the next few days, it'll be easy to just do this. ]
>
> 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, 73e2d827a501 ("arm64:
> Initialize jump labels before setup_machine_fdt()"). But it appears that
> fixing it on other platforms might not be so trivial. Instead, defer the
> setting of the static branch until later in the boot process.
>
> Fixes: f5bda35fba61 ("random: use static branch for crng_ready()")
> Reported-by: Stephen Boyd <swboyd at chromium.org>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Russell King <linux at armlinux.org.uk>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Phil Elwell <phil at raspberrypi.com>
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>

Reviewed-by: Ard Biesheuvel <ardb at kernel.org>

> ---
>  drivers/char/random.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 4862d4d3ec49..f9a020ec08b9 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -650,7 +650,8 @@ 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. */
> -               execute_in_process_context(crng_set_ready, &set_ready);
> +               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);
>                 pr_notice("crng init done\n");
> @@ -779,6 +780,14 @@ int __init random_init(const char *command_line)
>         unsigned int i, arch_bytes;
>         unsigned long 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 defined(LATENT_ENTROPY_PLUGIN)
>         static const u8 compiletime_seed[BLAKE2S_BLOCK_SIZE] __initconst __latent_entropy;
>         _mix_pool_bytes(compiletime_seed, sizeof(compiletime_seed));
> --
> 2.35.1
>