If add_bootloader_randomness() or add_hwgenerator_randomness() is
called for the first time during early boot, crng_init equals 0. Then,
crng_fast_load() gets called -- which is safe to do even if the input
pool is not yet properly set up.
If the added entropy suffices to increase crng_init to 1, future calls
to add_bootloader_randomness() or add_hwgenerator_randomness() used to
progress to credit_entropy_bits(). However, if the input pool is not yet
properly set up, the cmpxchg call within that function can lead to an
infinite recursion. This is not only a hypothetical problem, as qemu
on arm64 may provide bootloader entropy via EFI and via devicetree.
As crng_global_init_time is set to != 0 once the input pool is properly
set up, check (also) for this condition to determine which branch to take.
Calls to crng_fast_load() do not modify the input pool; therefore, the
entropy_count for the input pool must not be modified at that early
stage.
Reported-by: Ivan T. Ivanov <iivanov@xxxxxxx>
Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness")
Tested-by: Ivan T. Ivanov <iivanov@xxxxxxx>
Signed-off-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
---
v2->v3: onle one unlikely (Ard Biesheuvel)
v1->v2: fix commit message; unmerge Reported-and-tested-by-tag (Ard Biesheuvel)
drivers/char/random.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 605969ed0f96..18fe804c1bf8 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1763,8 +1763,8 @@ static void __init init_std_data(struct entropy_store *r)
}
/*
- * Note that setup_arch() may call add_device_randomness()
- * long before we get here. This allows seeding of the pools
+ * add_device_randomness() or add_bootloader_randomness() may be
+ * called long before we get here. This allows seeding of the pools
* with some platform dependent data very early in the boot
* process. But it limits our options here. We must use
* statically allocated structures that already have all
@@ -2274,7 +2274,12 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
{
struct entropy_store *poolp = &input_pool;
- if (unlikely(crng_init == 0)) {
+ /* We cannot do much with the input pool until it is set up in
+ * rand_initalize(); therefore just mix into the crng state.
+ * As this does not affect the input pool, we cannot credit
+ * entropy for this.
+ */
+ if (unlikely(crng_init == 0 || crng_global_init_time == 0)) {
crng_fast_load(buffer, count);
return;
}
