Messages in this thread Patch in this message | | | Date | Wed, 8 Dec 2004 18:58:31 -0800 | From | Matt Mackall <> | Subject | Re: Concurrent access to /dev/urandom |
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On Wed, Dec 08, 2004 at 08:57:05PM -0500, Theodore Ts'o wrote: > On Wed, Dec 08, 2004 at 01:56:14PM -0800, Matt Mackall wrote: > > > > Ted, I think this is a bit more straightforward than your patch, and > > safer as it protects get_random_bytes() and internal extract_entropy() > > users. And I'd be leery of your get_cpu() trick due to preempt > > issues. > > I'm concerned that turning off interrupts during even a single SHA-1 > transform will put us above the radar with respect to the preempt > latency statistics again. We could use a separate spinlock that only > pretects the mix_ptr and mixing access to the pool, so we're at least > not disabling interrupts, but we still are holding a spinlock across a > cryptographic operation.
A big mixlock was my first thought, but it'd still have to be _irqsave as we can reach extract_entropy from irq handlers.
> So I've come up with another trick which I think avoids needing to add > additional locking altogether. What we do is we diddle the initial > HASH input values with the following values: initial the processor ID, > the current task pointer, and preempt_count(). On an UP system with > preemption, it won't matter if we get preempted, since on a UP system > access to the pool is by definition serialized :-). On a SMP system > with preemption, while we could theoretically get preempted away and > then scheduled on another CPU, just in time for another process to > call extract_entropy(), the task identifier is enough to guarantee a > unique starting point. The reason for adding preempt_count() is so we > can deal with the case where a process gets interrupted, and the > bottom half handler calls get_random_bytes(), and at the same time > said process gets preempted away to another CPU(). I think this > covers all of the cases..... > > Yeah, it would be simper to reason about things if we were to just put > it under the spinlock, but everyone seems tp be on a reduce latency at > all costs kick as of late. :-)
I'd like to combine this with my approach of fiddling with the mixing offset in a similar manner. In the duplicate case, we were basically returning SHA(x[y]) twice and now we're returning SHA(x[y]^knowns). This makes me a bit uneasy. I'd rather do SHA(x[knowns%sizeof(x)]^knowns2), then we've at least got some different _unknowns_ in the hash from the attacker's perspective, yes?
Something like:
Signed-off-by: Matt Mackall <mpm@selenic.com>
Index: random/drivers/char/random.c =================================================================== --- random.orig/drivers/char/random.c 2004-12-08 18:17:21.000000000 -0800 +++ random/drivers/char/random.c 2004-12-08 18:47:17.914493794 -0800 @@ -1343,7 +1343,7 @@ { ssize_t ret, i; __u32 tmp[TMP_BUF_SIZE]; - __u32 x; + __u32 x, offset, wrap; unsigned long cpuflags; @@ -1402,14 +1402,35 @@ sec_random_state->entropy_count); } - /* Hash the pool to get the output */ - tmp[0] = 0x67452301; + /* + * Hash the pool to get the output. + * + * We diddle the initial inputs so that if two + * processors are executing extract_entropy + * concurrently, they will get different results. Even + * if we get preempted and moved to another CPU, the + * combination of initial CPU, task pointer, and + * preempt count is good enough to avoid duplication. + * We could instead use more locking here, but the + * resulting latency is painful. + */ + tmp[0] = 0x67452301 ^ smp_processor_id(); tmp[1] = 0xefcdab89; - tmp[2] = 0x98badcfe; + tmp[2] = 0x98badcfe ^ preempt_count(); tmp[3] = 0x10325476; #ifdef USE_SHA tmp[4] = 0xc3d2e1f0; #endif + + /* + * Generate an offset for mixing (multiple of 16) so + * that we have different unknowns in the mix in the + * concurrent case as well. + */ + + wrap = r->poolinfo.poolwords; + offset = ((__u32)current * 8675309 % wrap) & ~15; + /* * As we hash the pool, we mix intermediate values of * the hash back into the pool. This eliminates @@ -1419,10 +1440,10 @@ * function can be inverted. */ for (i = 0, x = 0; i < r->poolinfo.poolwords; i += 16, x+=2) { - HASH_TRANSFORM(tmp, r->pool+i); + HASH_TRANSFORM(tmp, r->pool + (i + offset) % wrap); add_entropy_words(r, &tmp[x%HASH_BUFFER_SIZE], 1); } - + /* * In case the hash function has some recognizable * output pattern, we fold it in half.
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