On Fri, 25 Jan 2019 11:10:25 +0100, Takashi Iwai wrote: > > On Fri, 25 Jan 2019 10:25:37 +0100, > Baolin Wang wrote: > > > > Hi Jaroslav, > > On Thu, 24 Jan 2019 at 21:43, Jaroslav Kysela <perex@xxxxxxxx> wrote: > > > > > > Dne 23.1.2019 v 13:46 Leo Yan napsal(a): > > > > Hi all, > > > > > > > > On Wed, Jan 23, 2019 at 12:58:51PM +0100, Takashi Iwai wrote: > > > >> On Tue, 22 Jan 2019 21:25:35 +0100, > > > >> Mark Brown wrote: > > > >>> > > > >>> On Mon, Jan 21, 2019 at 03:15:43PM +0100, Jaroslav Kysela wrote: > > > >>>> Dne 21.1.2019 v 13:40 Mark Brown napsal(a): > > > >>> > > > >>>>> It was the bit about adding more extended permission control that I was > > > >>>>> worried about there, not the initial O_APPEND bit. Indeed the O_APPEND > > > >>>>> bit sounds like it might also work from the base buffer sharing point of > > > >>>>> view, I have to confess I'd not heard of that feature before (it didn't > > > >>>>> come up in the discussion when Eric raised this in Prague). > > > >>> > > > >>>> With permissions, I meant to make possible to restrict the file > > > >>>> descriptor operations (ioctls) for the depending task (like access to > > > >>>> the DMA buffer, synchronize it for the non-coherent platforms and maybe > > > >>>> read/write the actual position, delay etc.). It should be relatively > > > >>>> easy to implement using the snd_pcm_file structure. > > > >>> > > > >>> Right, that's what I understood you to mean. If you want to have a > > > >>> policy saying "it's OK to export a PCM file descriptor if it's only got > > > >>> permissions X and Y" the security module is going to need to know about > > > >>> the mechanism for setting those permissions. With dma_buf that's all a > > > >>> bit easier as there's less new stuff, though I've no real idea how much > > > >>> of a big deal that actually is. > > > >> > > > >> There are many ways to implement such a thing, yeah. If we'd need an > > > >> implementation that is done solely in the sound driver layer, I can > > > >> imagine to introduce either a new ioctl or an open flag (like O_EXCL) > > > >> to specify the restricted sharing. That is, a kind of master / slave > > > >> model where only the master is allowed to manipulate the stream while > > > >> the slave can mmap, read/write and get status. > > > > > > > > In order to support EXCLUSIVE mode, it is necessary to convert the > > > > /dev/snd/ descriptor to an anon_inode:dmabuffer file descriptor. > > > > SELinux allows that file descriptor to be passed to the client. It can > > > > also be used by the AAudioService. > > > > > > Okay, so this is probably the only point which we should resolve for the > > > already available DMA buffer sharing in ALSA (the O_APPEND flag). > > > > > > I had another glance to your dma-buf implementation and I see many > > > things which might cause problems: > > > > > > - allow to call dma-buf ioctls only when the audio device is in specific > > > state (stream is not running) > > > > Right. Will fix. > > > > > - as Takashi mentioned, if we return another file-descriptor (dma-buf > > > export) to the user space and the server closes the main pcm > > > file-descriptor (the client does not) - the result will be a crash (dma > > > buffer will be freed, but referenced through the dma-buf interface) > > > > Yes, will fix. > > There are a few more overlooked problems. A part of them was already > mentioned in my previous reply, but let me repeat: > > - The racy ioctls have to be considered; you can perform this export > ioctl concurrently, and both of them write and mix up the setup, > which is obviously broken. > > - The PCM buffer can be re-allocated on the fly. If the current > buffer is abandoned while exporting, it leads to the UAF. > > - Similarly, what if the PCM stream that is attached is closed without > detaching itself? Or, what if the PCM stream attaches itself twice > without detaching? > > - The driver may provide its own mmap method, and you can't hard-code > the mmap implementation as currently in snd_pcm_dmabuf_mmap(). > > I suppose you can drop of most of the code in snd_pcm_dmabuf_map(), > instead, assign PCM substream in obj, and call snd_pcm_mmap_data() > with the given VMA. If this really works, it manages the mmap > refcount, so the previous two issues should be covered there. > But it needs more consideration... Erm, obviously it's not enough. Each attach / detach needs to manage the refcount, too, for covering the cases above. It can re-use the PCM mmap_refount, though. Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
