[PATCH 5.4 222/320] random: do not throw away excess input to crng_fast_load

Greg Kroah-Hartman
2022-01-24T22:58:53Z

From: Jason A. Donenfeld <Jason@zx2c4.com>

[ Upstream commit 73c7733f122e8d0107f88655a12011f68f69e74b ]

When crng_fast_load() is called by add_hwgenerator_randomness(), we
currently will advance to crng_init==1 once we've acquired 64 bytes, and
then throw away the rest of the buffer. Usually, that is not a problem:
When add_hwgenerator_randomness() gets called via EFI or DT during
setup_arch(), there won't be any IRQ randomness. Therefore, the 64 bytes
passed by EFI exactly matches what is needed to advance to crng_init==1.
Usually, DT seems to pass 64 bytes as well -- with one notable exception
being kexec, which hands over 128 bytes of entropy to the kexec'd kernel.
In that case, we'll advance to crng_init==1 once 64 of those bytes are
consumed by crng_fast_load(), but won't continue onward feeding in bytes
to progress to crng_init==2. This commit fixes the issue by feeding
any leftover bytes into the next phase in add_hwgenerator_randomness().

[linux@dominikbrodowski.net: rewrite commit message]
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/char/random.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 60b39af1279a4..19bfbaf135989 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -975,12 +975,14 @@ static struct crng_state *select_crng(void)

/*
* crng_fast_load() can be called by code in the interrupt service
- * path. So we can't afford to dilly-dally.
+ * path. So we can't afford to dilly-dally. Returns the number of
+ * bytes processed from cp.
*/
-static int crng_fast_load(const char *cp, size_t len)
+static size_t crng_fast_load(const char *cp, size_t len)
{
unsigned long flags;
char *p;
+ size_t ret = 0;

if (!spin_trylock_irqsave(&primary_crng.lock, flags))
return 0;
@@ -991,7 +993,7 @@ static int crng_fast_load(const char *cp, size_t len)
p = (unsigned char *) &primary_crng.state[4];
while (len > 0 && crng_init_cnt < CRNG_INIT_CNT_THRESH) {
p[crng_init_cnt % CHACHA_KEY_SIZE] ^= *cp;
- cp++; crng_init_cnt++; len--;
+ cp++; crng_init_cnt++; len--; ret++;
}
spin_unlock_irqrestore(&primary_crng.lock, flags);
if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
@@ -1000,7 +1002,7 @@ static int crng_fast_load(const char *cp, size_t len)
wake_up_interruptible(&crng_init_wait);
pr_notice("random: fast init done\n");
}
- return 1;
+ return ret;
}

/*
@@ -1353,7 +1355,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
if (unlikely(crng_init == 0)) {
if ((fast_pool->count >= 64) &&
crng_fast_load((char *) fast_pool->pool,
- sizeof(fast_pool->pool))) {
+ sizeof(fast_pool->pool)) > 0) {
fast_pool->count = 0;
fast_pool->last = now;
}
@@ -2501,8 +2503,11 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
struct entropy_store *poolp = &input_pool;

if (unlikely(crng_init == 0)) {
- crng_fast_load(buffer, count);
- return;
+ size_t ret = crng_fast_load(buffer, count);
+ count -= ret;
+ buffer += ret;
+ if (!count || crng_init == 0)
+ return;
}

/* Suspend writing if we're above the trickle threshold.
--
2.34.1