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    Subject[PATCH 5.15 099/145] random: allow partial reads if later user copies fail
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit 5209aed5137880fa229746cb521f715e55596460 upstream.

    Rather than failing entirely if a copy_to_user() fails at some point,
    instead we should return a partial read for the amount that succeeded
    prior, unless none succeeded at all, in which case we return -EFAULT as
    before.

    This makes it consistent with other reader interfaces. For example, the
    following snippet for /dev/zero outputs "4" followed by "1":

    int fd;
    void *x = mmap(NULL, 4096, PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    assert(x != MAP_FAILED);
    fd = open("/dev/zero", O_RDONLY);
    assert(fd >= 0);
    printf("%zd\n", read(fd, x, 4));
    printf("%zd\n", read(fd, x + 4095, 4));
    close(fd);

    This brings that same standard behavior to the various RNG reader
    interfaces.

    While we're at it, we can streamline the loop logic a little bit.

    Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Jann Horn <jannh@google.com>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 22 ++++++++++++----------
    1 file changed, 12 insertions(+), 10 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -525,8 +525,7 @@ EXPORT_SYMBOL(get_random_bytes);

    static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
    {
    - ssize_t ret = 0;
    - size_t len;
    + size_t len, left, ret = 0;
    u32 chacha_state[CHACHA_STATE_WORDS];
    u8 output[CHACHA_BLOCK_SIZE];

    @@ -545,37 +544,40 @@ static ssize_t get_random_bytes_user(voi
    * the user directly.
    */
    if (nbytes <= CHACHA_KEY_SIZE) {
    - ret = copy_to_user(buf, &chacha_state[4], nbytes) ? -EFAULT : nbytes;
    + ret = nbytes - copy_to_user(buf, &chacha_state[4], nbytes);
    goto out_zero_chacha;
    }

    - do {
    + for (;;) {
    chacha20_block(chacha_state, output);
    if (unlikely(chacha_state[12] == 0))
    ++chacha_state[13];

    len = min_t(size_t, nbytes, CHACHA_BLOCK_SIZE);
    - if (copy_to_user(buf, output, len)) {
    - ret = -EFAULT;
    + left = copy_to_user(buf, output, len);
    + if (left) {
    + ret += len - left;
    break;
    }

    - nbytes -= len;
    buf += len;
    ret += len;
    + nbytes -= len;
    + if (!nbytes)
    + break;

    BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0);
    - if (!(ret % PAGE_SIZE) && nbytes) {
    + if (ret % PAGE_SIZE == 0) {
    if (signal_pending(current))
    break;
    cond_resched();
    }
    - } while (nbytes);
    + }

    memzero_explicit(output, sizeof(output));
    out_zero_chacha:
    memzero_explicit(chacha_state, sizeof(chacha_state));
    - return ret;
    + return ret ? ret : -EFAULT;
    }

    /*

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