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Date: Sun, 15 Jun 2014 21:47:26 -0400 From: Theodore Ts'o <tytso@....edu> To: Linux Kernel Developers List <linux-kernel@...r.kernel.org> Cc: Theodore Ts'o <tytso@....edu>, Greg Price <price@....edu> Subject: [PATCH] random: fix nasty entropy accounting bug Commit 0fb7a01af5b0 "random: simplify accounting code", introduced in v3.15, has a very nasty accounting problem when the entropy pool has has fewer bytes of entropy than the number of requested reserved bytes. In that case, "have_bytes - reserved" goes negative, and since size_t is unsigned, the expression: ibytes = min_t(size_t, ibytes, have_bytes - reserved); ... does not do the right thing. This is rather bad, because it defeats the catastrophic reseeding feature in the xfer_secondary_pool() path. It also can cause the "BUG: spinlock trylock failure on UP" for some kernel configurations when prandom_reseed() calls get_random_bytes() in the early init, since when the entropy count gets corrupted, credit_entropy_bits() erroneously believes that the nonblocking pool has been fully initialized (when in fact it is not), and so it calls prandom_reseed(true) recursively leading to the spinlock BUG. The logic is *not* the same it was originally, but in the cases where it matters, the behavior is the same, and the resulting code is hopefully easier to read and understand. Fixes: 0fb7a01af5b0 "random: simplify accounting code" Signed-off-by: Theodore Ts'o <tytso@....edu> Cc: Greg Price <price@....edu> Cc: stable@...r.kernel.org #v3.15 --- drivers/char/random.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 102c50d..2b6e4cd 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -979,7 +979,6 @@ static void push_to_pool(struct work_struct *work) static size_t account(struct entropy_store *r, size_t nbytes, int min, int reserved) { - int have_bytes; int entropy_count, orig; size_t ibytes; @@ -988,17 +987,19 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min, /* Can we pull enough? */ retry: entropy_count = orig = ACCESS_ONCE(r->entropy_count); - have_bytes = entropy_count >> (ENTROPY_SHIFT + 3); ibytes = nbytes; /* If limited, never pull more than available */ - if (r->limit) - ibytes = min_t(size_t, ibytes, have_bytes - reserved); + if (r->limit) { + int have_bytes = entropy_count >> (ENTROPY_SHIFT + 3); + + if ((have_bytes -= reserved) < 0) + have_bytes = 0; + ibytes = min_t(size_t, ibytes, have_bytes); + } if (ibytes < min) ibytes = 0; - if (have_bytes >= ibytes + reserved) - entropy_count -= ibytes << (ENTROPY_SHIFT + 3); - else - entropy_count = reserved << (ENTROPY_SHIFT + 3); + if ((entropy_count -= ibytes << (ENTROPY_SHIFT + 3)) < 0) + entropy_count = 0; if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig) goto retry; -- 2.0.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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