Hi,
On Fr, 2014-07-18 at 18:07 -0400, Theodore Ts'o wrote:
> From: Hannes Frederic Sowa <han...@stressinduktion.org>
>
> The expression entropy_count -= ibytes << (ENTROPY_SHIFT + 3) could
> actually increase entropy_count if during assignment of the unsigned
> expression on the RHS (mind the -=) we reduce the value modulo
> 2^width(int) and assign it to entropy_count. Trinity found this.
>
> [ Commit modified by tytso to add an additional safety check for a
> negative entropy_count -- which should never happen, and to also add
> an additional paranoia check to prevent overly large count values to
> be passed into urandom_read(). ]
>
> Reported-by: Dave Jones <da...@redhat.com>
> Cc: Greg Price <pr...@mit.edu>
> Signed-off-by: Hannes Frederic Sowa <han...@stressinduktion.org>
> Signed-off-by: Theodore Ts'o <ty...@mit.edu>
> ---
> drivers/char/random.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0a7ac0a..003f744 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -641,7 +641,7 @@ retry:
> } while (unlikely(entropy_count < pool_size-2 && pnfrac));
> }
>
> - if (entropy_count < 0) {
> + if (unlikely(entropy_count < 0)) {
> pr_warn("random: negative entropy/overflow: pool %s count %d\n",
> r->name, entropy_count);
> WARN_ON(1);
> @@ -981,7 +981,7 @@ static size_t account(struct entropy_store *r, size_t
> nbytes, int min,
> int reserved)
> {
> int entropy_count, orig;
> - size_t ibytes;
> + size_t ibytes, nfrac;
>
> BUG_ON(r->entropy_count > r->poolinfo->poolfracbits);
>
> @@ -999,7 +999,17 @@ retry:
> }
> if (ibytes < min)
> ibytes = 0;
> - if ((entropy_count -= ibytes << (ENTROPY_SHIFT + 3)) < 0)
> +
> + nfrac = ibytes << (ENTROPY_SHIFT + 3);
> + if (entropy_count < 0) {
Minor nit: maybe also add an unlikely() here?
> + pr_warn("random: negative entropy count: pool %s count %d\n",
> + r->name, entropy_count);
> + WARN_ON(1);
> + entropy_count = 0;
> + }
> + if ((unsigned) entropy_count > nfrac)
(unsigned) -> (size_t)
size_t could also be (unsigned long) so the plain (unsigned) is
misleading.
(Maybe I wouldn't have done the cast at all, as we compile the kernel
with -Wno-sign-compare and we have the < 0 check right above, but I
don't have a strong opinion on that.)
> + entropy_count -= nfrac;
> + else
> entropy_count = 0;
>
> if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
> @@ -1376,6 +1386,7 @@ urandom_read(struct file *file, char __user *buf,
> size_t nbytes, loff_t *ppos)
> "with %d bits of entropy available\n",
> current->comm, nonblocking_pool.entropy_total);
>
> + nbytes = min_t(size_t, nbytes, INT_MAX >> ENTROPY_SHIFT);
Hmm, not sure, nfracs unit is 1/8 bits, so don't we have to limit nbytes
to INT_MAX >> (ENTROPY_SHIFT + 3) here?
And if we want to be even more correct here, we could switch from
INT_MAX to SIZE_MAX, as we do all nfrac calculations in the size_t
domain.
Maximum read/write size is SSIZE_MAX, so we don't need to care about
that, but if a user on a 64 bit machine requests INT_MAX bytes, we only
account/extract INT_MAX >> (ENTROPY_SHIFT + 3) bytes and cause a partial
read, though we actually could calulcate a correct nfrac for INT_MAX.
Because we don't have such large poolfragbits pools we would still
always end up with 0 while still allowing larger buffers to fill.
Hm, I just see that we should leave the INT_MAX limit just because of
the tracepoint.
Good catch,
Hannes
> ret = extract_entropy_user(&nonblocking_pool, buf, nbytes);
>
> trace_urandom_read(8 * nbytes, ENTROPY_BITS(&nonblocking_pool),
--
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