From a43bed8220f26178f0f63bf4c0fc23b7b8b76763 Mon Sep 17 00:00:00 2001 From: Dominik Brodowski Date: Fri, 28 Jan 2022 08:02:54 +0100 Subject: [PATCH] hwrng: core - credit entropy for low quality sources of randomness In case the entropy quality is low, there may be less than one bit to credit in the call to add_hwgenerator_randomness(): The number of bytes returned by rng_get_data() multiplied by the current quality (in entropy bits per 1024 bits of input) must be larger than 128 to credit at least one bit. However, imx-rngc.c sets the quality to 19, but may return less than 32 bytes; hid_u2fzero.c sets the quality to 1; and users may override the quality setting manually. In case there is less than one bit to credit, keep track of it and add that credit to the next iteration. Cc: Herbert Xu Cc: Jason A. Donenfeld Signed-off-by: Dominik Brodowski Signed-off-by: Herbert Xu --- drivers/char/hw_random/core.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c index bc9f95cbac92..f327f7493585 100644 --- a/drivers/char/hw_random/core.c +++ b/drivers/char/hw_random/core.c @@ -424,6 +424,7 @@ static int __init register_miscdev(void) static int hwrng_fillfn(void *unused) { + size_t entropy, entropy_credit = 0; /* in 1/1024 of a bit */ long rc; while (!kthread_should_stop()) { @@ -445,9 +446,17 @@ static int hwrng_fillfn(void *unused) msleep_interruptible(10000); continue; } + + /* If we cannot credit at least one bit of entropy, + * keep track of the remainder for the next iteration + */ + entropy = rc * current_quality * 8 + entropy_credit; + if ((entropy >> 10) == 0) + entropy_credit = entropy; + /* Outside lock, sure, but y'know: randomness. */ add_hwgenerator_randomness((void *)rng_fillbuf, rc, - rc * current_quality * 8 >> 10); + entropy >> 10); } hwrng_fill = NULL; return 0; -- 2.11.0