From: "Jason A. Donenfeld" Jason@zx2c4.com
commit aba120cc101788544aa3e2c30c8da88513892350 upstream.
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. If we're returning <= 32 bytes to the user, then we can still return those bytes directly, so that short reads don't become slower. And for long reads, the difference is hopefully lost in the amortization, so it doesn't change much, with that amortization helping variously for medium reads.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/char/random.c | 35 +++++++++++++++++++++++------------ 1 file changed, 23 insertions(+), 12 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -533,19 +533,29 @@ static ssize_t get_random_bytes_user(voi 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 with random + * bytes, 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); + /* + * However, if we're doing a read of len <= 32, we don't need to + * use chacha_state after, so we can simply return those bytes to + * the user directly. + */ + if (nbytes <= CHACHA_KEY_SIZE) { + ret = copy_to_user(buf, &chacha_state[4], nbytes) ? -EFAULT : nbytes; + goto out_zero_chacha; + }
- while (nbytes) { + do { if (large_request && need_resched()) { - if (signal_pending(current)) + if (signal_pending(current)) { + if (!ret) + ret = -ERESTARTSYS; break; + } schedule(); }
@@ -562,10 +572,11 @@ static ssize_t get_random_bytes_user(voi nbytes -= len; buf += len; ret += len; - } + } while (nbytes);
- memzero_explicit(chacha_state, sizeof(chacha_state)); memzero_explicit(output, sizeof(output)); +out_zero_chacha: + memzero_explicit(chacha_state, sizeof(chacha_state)); return ret; }