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Donenfeld" Cc: tytso@mit.edu, hch@lst.de, linux-kernel@vger.kernel.org References: <20220519193133.194138-1-axboe@kernel.dk> <20220519193133.194138-2-axboe@kernel.dk> From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/19/22 5:20 PM, Jason A. Donenfeld wrote: > On Fri, May 20, 2022 at 01:12:04AM +0200, Jason A. Donenfeld wrote: >> Hi Jens, >> >> On Thu, May 19, 2022 at 01:31:32PM -0600, Jens Axboe wrote: >>> for (;;) { >>> chacha20_block(chacha_state, output); >>> if (unlikely(chacha_state[12] == 0)) >>> ++chacha_state[13]; >>> >>> block_len = min_t(size_t, len, CHACHA_BLOCK_SIZE); >>> - left = copy_to_user(ubuf, output, block_len); >>> - if (left) { >>> - ret += block_len - left; >>> + block_len = copy_to_iter(output, block_len, to); >>> + if (!block_len) >>> break; >>> - } >>> >>> - ubuf += block_len; >>> ret += block_len; >>> len -= block_len; >>> - if (!len) >>> - break; >>> >>> BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0); >>> if (ret % PAGE_SIZE == 0) { >>> if (signal_pending(current)) >>> break; >>> cond_resched(); >>> } >>> } >> >> This isn't quite the same, is it? Before, it would immediately break out >> of the loop on any short copy. Now, it will only break out on a zero >> copy, which means it's possible that ret % PAGE_SIZE == 0, and there'll >> be an unnecessary cond_resched() before copy_to_iter() runs again and >> then breaks. > > Maybe something like the below would do the trick? > > > static ssize_t get_random_bytes_user(struct iov_iter *to) > { > size_t block_len, copied, ret = 0, len = iov_iter_count(to); > u32 chacha_state[CHACHA_STATE_WORDS]; > u8 output[CHACHA_BLOCK_SIZE]; > > if (!len) > return 0; > > /* > * 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 (len <= CHACHA_KEY_SIZE) { > ret = copy_to_iter(&chacha_state[4], len, to); > goto out_zero_chacha; > } > > for (;;) { > chacha20_block(chacha_state, output); > if (unlikely(chacha_state[12] == 0)) > ++chacha_state[13]; > > block_len = min_t(size_t, len, CHACHA_BLOCK_SIZE); > copied = copy_to_iter(output, block_len, to); > ret += copied; > if (block_len != copied) > break; > len -= copied; Yep, that looks good! Do you still want a v2? -- Jens Axboe