3.15-stable review patch. If anyone has any objections, please let me know.
------------------
From: Theodore Ts'o <tytso@xxxxxxx>
commit e33ba5fa7afce1a9f159704121d4e4d110df8185 upstream.
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@xxxxxxx>
Cc: Greg Price <price@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/char/random.c | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -979,7 +979,6 @@ static void push_to_pool(struct work_str
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_sto
/* 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;
--
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