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    Subject[PATCH 4.19 089/234] random: remove outdated INT_MAX >> 6 check in urandom_read()
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit 434537ae54ad37e93555de21b6ac8133d6d773a9 upstream.

    In 79a8468747c5 ("random: check for increase of entropy_count because of
    signed conversion"), a number of checks were added around what values
    were passed to account(), because account() was doing fancy fixed point
    fractional arithmetic, and a user had some ability to pass large values
    directly into it. One of things in that commit was limiting those values
    to INT_MAX >> 6. The first >> 3 was for bytes to bits, and the next >> 3
    was for bits to 1/8 fractional bits.

    However, for several years now, urandom reads no longer touch entropy
    accounting, and so this check serves no purpose. The current flow is:

    urandom_read_nowarn()-->get_random_bytes_user()-->chacha20_block()

    Of course, we don't want that size_t to be truncated when adding it into
    the ssize_t. But we arrive at urandom_read_nowarn() in the first place
    either via ordinary fops, which limits reads to MAX_RW_COUNT, or via
    getrandom() which limits reads to INT_MAX.

    Cc: Theodore Ts'o <tytso@mit.edu>
    Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
    Reviewed-by: Jann Horn <jannh@google.com>
    Reviewed-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -1284,9 +1284,8 @@ void rand_initialize_disk(struct gendisk
    static ssize_t urandom_read_nowarn(struct file *file, char __user *buf,
    size_t nbytes, loff_t *ppos)
    {
    - int ret;
    + ssize_t ret;

    - nbytes = min_t(size_t, nbytes, INT_MAX >> 6);
    ret = get_random_bytes_user(buf, nbytes);
    trace_urandom_read(nbytes, input_pool.entropy_count);
    return ret;

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