On Wed, 1 May 2013, Theodore Ts'o wrote: > > > ------------------------------------------------------ > > > From: Jiri Kosina <[email protected]> > > > Subject: random: fix accounting race condition with lockless irq > > > entropy_count update > > > > I see this hasn't been included even in the third (last?) batch of your > > pile going to Linus for rc1 ... was this intentional, please? > > > > Hi Jiri, > > Can you send me a copy of the patch?
Hi Ted, absolutely, please find it below. > I can't find it in my inbox... Odd. I can see you CCed in both cases when I have been sending it, and when Andrew has been sending his "added to mm tree" message. Something went wrong somewhere ... Thanks. ------------------------------------------------------ From: Jiri Kosina <[email protected]> Subject: random: fix accounting race condition with lockless irq entropy_count update Commit 902c098a3663 ("random: use lockless techniques in the interrupt path") turned IRQ path from being spinlock protected into lockless cmpxchg-retry update. That commit removed r->lock serialization between crediting entropy bits from IRQ context and accounting when extracting entropy on userspace read path, but didn't turn the r->entropy_count reads/updates in account() to use cmpxchg as well. It has been observed, that under certain circumstances this leads to read() on /dev/urandom to return 0 (EOF), as r->entropy_count gets corrupted and becomes negative, which in turn results in propagating 0 all the way from account() to the actual read() call. Convert the accounting code to be the proper lockless counterpart of what has been partially done by 902c098a3663. Signed-off-by: Jiri Kosina <[email protected]> Cc: Theodore Ts'o <[email protected]> Cc: Greg KH <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- drivers/char/random.c | 22 +++++++++++++++------- 1 file changed, 15 insertions(+), 7 deletions(-) diff -puN drivers/char/random.c~random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update drivers/char/random.c --- a/drivers/char/random.c~random-fix-accounting-race-condition-with-lockless-irq-entropy_count-update +++ a/drivers/char/random.c @@ -865,16 +865,24 @@ static size_t account(struct entropy_sto if (r->entropy_count / 8 < min + reserved) { nbytes = 0; } else { + int entropy_count, orig; +retry: + entropy_count = orig = ACCESS_ONCE(r->entropy_count); /* If limited, never pull more than available */ - if (r->limit && nbytes + reserved >= r->entropy_count / 8) - nbytes = r->entropy_count/8 - reserved; + if (r->limit && nbytes + reserved >= entropy_count / 8) + nbytes = entropy_count/8 - reserved; - if (r->entropy_count / 8 >= nbytes + reserved) - r->entropy_count -= nbytes*8; - else - r->entropy_count = reserved; + if (entropy_count / 8 >= nbytes + reserved) { + entropy_count -= nbytes*8; + if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig) + goto retry; + } else { + entropy_count = reserved; + if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig) + goto retry; + } - if (r->entropy_count < random_write_wakeup_thresh) + if (entropy_count < random_write_wakeup_thresh) wakeup_write = 1; } -- Jiri Kosina SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
