[PATCH 5.17 064/111] random: allow partial reads if later user copies fail

Greg Kroah-Hartman
2022-05-27T11:47:21Z

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
@@ -523,8 +523,7 @@ 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];

@@ -543,37 +542,40 @@ 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;
}

/*