[PATCH] random: check for signals every PAGE_SIZE chunk of /dev/[u]random

["Jason A. Donenfeld" <Jason@zx2c4.com>]

In a651a7ba294c ("random: check for signal_pending() outside of
need_resched() check"), Jann pointed out that we previously were only
checking the TIF_NOTIFY_SIGNAL and TIF_SIGPENDING flags if the process
had TIF_NEED_RESCHED set, which meant in practice, super long reads to
/dev/[u]random would delay signal handling by a long time. I tried this,
and indeed I wasn't able to interrupt a /dev/urandom read until after
several megabytes had been read. The bug he fixed has always been there,
and so code that reads from /dev/urandom without checking the return
value of read() has mostly worked for a long time, for most sizes, not
just for <= 256.

Maybe it makes sense to keep that code working. The reason it was so
small prior, ignoring the fact that it didn't work anyway, was likely
because /dev/random used to block, and that could happen for pretty
large lengths of time while entropy was gathered. But now, it's just a
chacha20 call, which is extremely fast and is just operating on pure
data, without having to wait for some external event. In that sense,
/dev/[u]random is a lot more like /dev/zero.

Taking a page out of /dev/zero's read_zero() function, it always returns
at least one chunk, and then checks for signals after each chunk. Chunk
sizes there are of length PAGE_SIZE. Let's just copy the same thing for
/dev/[u]random, and check for signals and cond_resched() for every
PAGE_SIZE amount of data. This makes the behavior more consistent with
expectations, and should mitigate the impact of Jann's fix for the
age-old signal check bug.

Cc: Jann Horn <jannh@google.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7f0253455d4e..e8a578af619a 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -523,7 +523,6 @@ EXPORT_SYMBOL(get_random_bytes);
 
 static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 {
-	bool large_request = nbytes > 256;
 	ssize_t ret = 0;
 	size_t len;
 	u32 chacha_state[CHACHA_STATE_WORDS];
@@ -540,15 +539,6 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 	crng_make_state(chacha_state, (u8 *)&chacha_state[4], CHACHA_KEY_SIZE);
 
 	do {
-		if (large_request) {
-			if (signal_pending(current)) {
-				if (!ret)
-					ret = -ERESTARTSYS;
-				break;
-			}
-			cond_resched();
-		}
-
 		chacha20_block(chacha_state, output);
 		if (unlikely(chacha_state[12] == 0))
 			++chacha_state[13];
@@ -562,6 +552,13 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 		nbytes -= len;
 		buf += len;
 		ret += len;
+
+		BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0);
+		if (!(ret % PAGE_SIZE) && nbytes) {
+			if (signal_pending(current))
+				break;
+			cond_resched();
+		}
 	} while (nbytes);
 
 	memzero_explicit(chacha_state, sizeof(chacha_state));
-- 
2.35.1