On Wed, 31 Aug 2016 13:15:33 +0200, Takashi Sakamoto wrote:
In hwdep interface of fireworks driver, accessing to user space is in a critical section with disabled local interrupt. Depending on architecture, accessing to user space can cause page fault exception. Then local processor stores machine status and handles the synchronous event. A handler corresponding to the event can call task scheduler to wait for preparing pages. In a case of usage of single core processor, the state to disable local interrupt is worse because it don't handle usual interrupts from hardware.
This commit fixes this bug, performing the accessing outside spinlock.
Reported-by: Vaishali Thakkar vaishali.thakkar@oracle.com Cc: stable@vger.kernel.org Fixes: 555e8a8f7f14('ALSA: fireworks: Add command/response functionality into hwdep interface') Signed-off-by: Takashi Sakamoto o-takashi@sakamocchi.jp
sound/firewire/fireworks/fireworks_hwdep.c | 52 ++++++++++++++++++------ sound/firewire/fireworks/fireworks_transaction.c | 4 +- 2 files changed, 42 insertions(+), 14 deletions(-)
diff --git a/sound/firewire/fireworks/fireworks_hwdep.c b/sound/firewire/fireworks/fireworks_hwdep.c index 33df865..2563490 100644 --- a/sound/firewire/fireworks/fireworks_hwdep.c +++ b/sound/firewire/fireworks/fireworks_hwdep.c @@ -25,6 +25,7 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained, { unsigned int length, till_end, type; struct snd_efw_transaction *t;
u8 *pull_ptr; long count = 0;
if (remained < sizeof(type) + sizeof(struct snd_efw_transaction))
@@ -38,8 +39,17 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained, buf += sizeof(type);
/* write into buffer as many responses as possible */
- spin_lock_irq(&efw->lock);
- /*
* When another task reaches here during this task's access to user* space, it picks up current position in buffer and can read the same* series of responses.*/- pull_ptr = efw->pull_ptr;
- while (efw->resp_queues > 0) {
t = (struct snd_efw_transaction *)(efw->pull_ptr);
t = (struct snd_efw_transaction *)(pull_ptr);length = be32_to_cpu(t->length) * sizeof(__be32);
/* confirm enough space for this response */
@@ -49,16 +59,19 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained, /* copy from ring buffer to user buffer */ while (length > 0) { till_end = snd_efw_resp_buf_size -
(unsigned int)(efw->pull_ptr - efw->resp_buf);
(unsigned int)(pull_ptr - efw->resp_buf); till_end = min_t(unsigned int, length, till_end);
if (copy_to_user(buf, efw->pull_ptr, till_end))
spin_unlock_irq(&efw->lock);if (copy_to_user(buf, pull_ptr, till_end)) return -EFAULT;
efw->pull_ptr += till_end;if (efw->pull_ptr >= efw->resp_buf +snd_efw_resp_buf_size)efw->pull_ptr -= snd_efw_resp_buf_size;
spin_lock_irq(&efw->lock);pull_ptr += till_end;if (pull_ptr >= efw->resp_buf + snd_efw_resp_buf_size)pull_ptr -= snd_efw_resp_buf_size; length -= till_end; buf += till_end;@@ -69,6 +82,18 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained, efw->resp_queues--; }
- /*
* All of tasks can read from the buffer nearly simultaneously, but the* position of each task is different depending on the length of given* buffer. Here, for simplicity, a position of buffer is set by the* latest task. It's better for a listening application to allow one* thread to read from the buffer. Unless, each task can read different* sequence of responses depending on variation of buffer length.*/- efw->pull_ptr = pull_ptr;
- spin_unlock_irq(&efw->lock);
Hrm, I'm afraid that it still doesn't work properly when accessed concurrently. In your code, efw->pull_ptr isn't updated until the end of the loop, while dfw->resp_queues are decremented in the loop.
Suppose resp_queues = 2, and two threads read concurrently. What happens? Both threads read from the first element only once, and resp_queues are decremented twice (one per each). And now both threads go out of the loops, and both set the pull_ptr to the same next item, although the second item hasn't been processed.
Takashi