[PATCH] random: do not allow user to keep crng key around on stack

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

The fast key erasure RNG design relies on the key that's used to be used
and then discarded. We do this, making judicious use of
memzero_explicit().  However, reads to /dev/urandom and calls to
getrandom() involve a copy_to_user(), and userspace can use FUSE or
userfaultfd, or make a massive call, dynamically remap memory addresses
as it goes, and set the process priority to idle, in order to keep a
kernel stack alive indefinitely. By probing
/proc/sys/kernel/random/entropy_avail to learn when the crng key is
refreshed, a malicious userspace could mount this attack every 5 minutes
thereafter, breaking the crng's forward secrecy.

In order to fix this, we just overwrite the stack's key with the first
32 bytes of the "free" fast key erasure output. This makes short reads a
tiny bit slower, since they no longer get 32 bytes for free, but short
reads are already faster than syscall overhead, so it doesn't matter.
And for long reads, the difference is lost in the amortization, so it
doesn't change anything.

We don't need to do this for get_random_bytes() and the various
kernel-space callers, and later, if we ever switch to always batching,
this won't be necessary either, so there's no need to change the API of
these functions.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jann Horn <jannh@google.com>
Fixes: c92e040d575a ("random: add backtracking protection to the CRNG")
Fixes: 186873c549df ("random: use simpler fast key erasure flow on per-cpu keys")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 388025d6d38d..1eb220a5f44f 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -532,19 +532,20 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 	if (!nbytes)
 		return 0;
 
-	len = min_t(size_t, 32, nbytes);
-	crng_make_state(chacha_state, output, len);
-
-	if (copy_to_user(buf, output, len))
-		return -EFAULT;
-	nbytes -= len;
-	buf += len;
-	ret += len;
+	/*
+	 * Immediately overwrite the ChaCha key at index 4, in case userspace
+	 * causes copy_to_user() below to sleep forever, so that we still
+	 * retain forward secrecy in that case.
+	 */
+	crng_make_state(chacha_state, (u8 *)&chacha_state[4], CHACHA_KEY_SIZE);
 
-	while (nbytes) {
+	do {
 		if (large_request && need_resched()) {
-			if (signal_pending(current))
+			if (signal_pending(current)) {
+				if (!ret)
+					ret = -ERESTARTSYS;
 				break;
+			}
 			schedule();
 		}
 
@@ -561,7 +562,7 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 		nbytes -= len;
 		buf += len;
 		ret += len;
-	}
+	} while (nbytes);
 
 	memzero_explicit(chacha_state, sizeof(chacha_state));
 	memzero_explicit(output, sizeof(output));
-- 
2.35.1