random: fix BUG_ON caused by accounting simplification

Commit ee1de406ba6eb1 ("random: simplify accounting logic") simplified
things too much, in that it allows the following to trigger an
overflow that results in a BUG_ON crash:

dd if=/dev/urandom of=/dev/zero bs=67108707 count=1

Thanks to Peter Zihlstra for discovering the crash, and Hannes
Frederic for analyizing the root cause.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Greg Price <price@mit.edu>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6b75713d953a4e719cd33610f6034bec430c5d95..102c50d38902ca43fed85641618ba202a7251679 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -995,8 +995,11 @@ retry:
                ibytes = min_t(size_t, ibytes, have_bytes - reserved);
        if (ibytes < min)
                ibytes = 0;
-       entropy_count = max_t(int, 0,
-                             entropy_count - (ibytes << (ENTROPY_SHIFT + 3)));
+       if (have_bytes >= ibytes + reserved)
+               entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
+       else
+               entropy_count = reserved << (ENTROPY_SHIFT + 3);
+
        if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
                goto retry;