On Wed, 23 Jan 2019 13:46:58 +0100, Leo Yan wrote: > > 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. > > I am lacking security related knowledge, especially for SELinux. > So only can give background information but not sure if it's really > helpful for discussion. > > Android web page [1] give some information for this: > > "The shared memory is referenced using a file descriptor that is > generated by the ALSA driver. If the file descriptor is directly > associated with a /dev/snd/ driver file, then it can be used by the > AAudio service in SHARED mode. But the descriptor cannot be passed to > the client code for EXCLUSIVE mode. The /dev/snd/ file descriptor > would provide too broad of access to the client, so it is blocked by > SELinux. > > 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. > > An anon_inode:dmabuffer file descriptor can be generated using the > Android Ion memory library." > > So we work out dmabuf driver for audio buffer, the audio buffer will > be exported and attached by using dma-buf framework; then we can > return one file descriptor which is generated by dma-buf and this > file descriptor is bound with anon inode based on dma-buf core code. > > If we directly use the device node /dev/snd/ as file descriptor, even > though we specify flag O_EXCL when open it, but it still is not an > anon inode file descriptor. Thus this is not safe enough and will be > blocked by SELinux. On the other hand, this patch wants to use > dma-buf framework to provide file descriptor for the audio buffer, and > this audio buffer can be one of mutiple audio buffers in the system > and it can be shared to any audio client program. > > Again, I have no less knowledge for SELinux so sorry if I introduce any > noise at here. And very appreciate any comments for this. Hrm, it sounds like a workaround just to bypass SELinux check... The sound server can open another PCM stream with O_APPEND, and pass that fd to the client, too? thanks, Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
