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

[Ard Biesheuvel]

Hi Jason,

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. ]
>

What more direct fix did you have in mind here?

> 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>
> ---
>  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);

Can we just drop this entirely, and rely on the hunk below to set the
static key? What justifies having two code paths that set the static
key in different ways on different architectures?

The use of the static key in general seems like a reasonable idea,
even though it is not clear what we actually gain by it (it omits a
single load from memory, right?)

So I'd argue that the impact of deferring the static key assignment is
so limited that there is really no reason for doing it this early, as
this clearly has unanticipated side effects that are difficult to
diagnose in some cases (i.e., boot crashes before the early console
comes up)

>                 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
>