Messages in this thread Patch in this message | | | From | "Jason A. Donenfeld" <> | Subject | [PATCH] random: allow partial reads if later user copies fail | Date | Thu, 7 Apr 2022 21:34:33 +0200 |
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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_READ | 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> --- drivers/char/random.c | 21 ++++++++++----------- 1 file changed, 10 insertions(+), 11 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c index e15063d61460..dd31fddecd17 100644 --- 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, ret = 0; u32 chacha_state[CHACHA_STATE_WORDS]; u8 output[CHACHA_BLOCK_SIZE]; @@ -543,37 +542,37 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) * the user directly. */ if (nbytes <= CHACHA_KEY_SIZE) { - ret = copy_to_user(buf, &chacha_state[4], nbytes) ? -EFAULT : nbytes; + ret = copy_to_user(buf, &chacha_state[4], nbytes) ? 0 : 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; + if (copy_to_user(buf, output, len)) 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)) { 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; } /* -- 2.35.1
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