Re: [musl] Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit layout for snd_pcm_mmap_status/control
From: Takashi Iwai
Date: Fri Oct 08 2021 - 04:43:28 EST
On Thu, 07 Oct 2021 18:51:58 +0200,
Rich Felker wrote:
>
> On Thu, Oct 07, 2021 at 06:18:52PM +0200, Takashi Iwai wrote:
> > On Thu, 07 Oct 2021 18:06:36 +0200,
> > Rich Felker wrote:
> > >
> > > On Thu, Oct 07, 2021 at 05:33:19PM +0200, Takashi Iwai wrote:
> > > > On Thu, 07 Oct 2021 15:11:00 +0200,
> > > > Arnd Bergmann wrote:
> > > > >
> > > > > On Thu, Oct 7, 2021 at 2:43 PM Takashi Iwai <tiwai@xxxxxxx> wrote:
> > > > > > On Thu, 07 Oct 2021 13:48:44 +0200, Arnd Bergmann wrote:
> > > > > > > On Thu, Oct 7, 2021 at 12:53 PM Takashi Iwai <tiwai@xxxxxxx> wrote:
> > > > > > > > On Wed, 06 Oct 2021 19:49:17 +0200, Michael Forney wrote:
> > > > > > >
> > > > > > > As far as I can tell, the broken interface will always result in
> > > > > > > user space seeing a zero value for "avail_min". Can you
> > > > > > > make a prediction what that would mean for actual
> > > > > > > applications? Will they have no audio output, run into
> > > > > > > a crash, or be able to use recover and appear to work normally
> > > > > > > here?
> > > > > >
> > > > > > No, fortunately it's only about control->avail_min, and fiddling this
> > > > > > value can't break severely (otherwise it'd be a security problem ;)
> > > > > >
> > > > > > In the buggy condition, it's always zero, and the kernel treated as if
> > > > > > 1, i.e. wake up as soon as data is available, which is OK-ish for most
> > > > > > applications. Apps usually don't care about the wake-up condition so
> > > > > > much. There are subtle difference and may influence on the stability
> > > > > > of stream processing, but the stability usually depends more strongly
> > > > > > on the hardware and software configurations.
> > > > > >
> > > > > > That being said, the impact by this bug (from the application behavior
> > > > > > POV) is likely quite small, but the contamination is large; as you
> > > > > > pointed out, it's much larger than I thought.
> > > > >
> > > > > Ok, got it.
> > > > >
> > > > > > The definition in uapi/sound/asound.h is a bit cryptic, but IIUC,
> > > > > > __snd_pcm_mmap_control64 is used for 64bit archs, right? If so, the
> > > > > > problem rather hits more widely on 64bit archs silently. Then, the
> > > > > > influence by this bug must be almost negligible, as we've had no bug
> > > > > > report about the behavior change.
> > > > >
> > > > > While __snd_pcm_mmap_control64 is only used on 32-bit
> > > > > architectures when 64-bit time_t is used. At the moment, this
> > > > > means all users of musl-1.2.x libc, but not glibc.
> > > > >
> > > > > On 64-bit architectures, __snd_pcm_mmap_control and
> > > > > __snd_pcm_mmap_control64 are meant to be identical,
> > > > > and this is actually true regardless of the bug, since
> > > > > __pad_before_uframe and __pad_after_uframe both
> > > > > end up as zero-length arrays here.
> > > > >
> > > > > > We may just fix it in kernel and for new library with hoping that no
> > > > > > one sees the actual problem. Or, we may provide a complete new set of
> > > > > > mmap offsets and ioctl to cover both broken and fixed interfaces...
> > > > > > The decision depends on how perfectly we'd like to address the bug.
> > > > > > As of now, I'm inclined to go for the former, but I'm open for more
> > > > > > opinions.
> > > > >
> > > > > Adding the musl list to Cc for additional testers, anyone interested
> > > > > please see [1] for the original report.
> > > > >
> > > > > It would be good to hear from musl users that are already using
> > > > > audio support with 32-bit applications on 64-bit kernels, which
> > > > > is the case that has the problem today. Have you noticed any
> > > > > problems with audio support here? If not, we can probably
> > > > > "fix" the kernel here and make the existing binaries behave
> > > > > the same way on 32-bit kernels. If there are applications that
> > > > > don't work in that environment today, I think we need to instead
> > > > > change the kernel to accept the currently broken format on
> > > > > both 32-bit and 64-bit kernels, possibly introducing yet another
> > > > > format that works as originally intended but requires a newly
> > > > > built kernel.
> > > >
> > > > Thanks!
> > > >
> > > > And now, looking more deeply, I feel more desperate.
> > > >
> > > > This bug makes the expected padding gone on little-endian.
> > > > On LE 32bit, the buggy definition is:
> > > >
> > > > char __pad1[0];
> > > > u32 appl_ptr;
> > > > char __pad2[0]; // this should have been [4]
> > > > char __pad3[0];
> > > > u32 avail_min;
> > > > char __pad4[4];
> > > >
> > > > When an application issues SYNC_PTR64 ioctl to submit appl_ptr and
> > > > avail_min updates, 64bit kernel (in compat mode) reads directly as:
> > > >
> > > > u64 appl_ptr;
> > > > u64 avail_min;
> > > >
> > > > Hence a bogus appl_ptr would be passed if avail_min != 0.
> > > > And usually application sets non-zero avail_min.
> > > > That is, the bug must hit more severely if the new API were really
> > > > used. It wouldn't crash, but some weird streaming behavior can
> > > > happen like noise, jumping or underruns.
> > > >
> > > > (Reading back avail_min=0 to user-space is rather harmless. Ditto for
> > > > the case of BE, then at least there is no appl_ptr corruption.)
> > > >
> > > > This made me wonder which way to go:
> > > > it's certainly possible to fix the new kernel to treat both buggy and
> > > > sane formats (disabling compat mmap and re-define ioctls, having the
> > > > code for old APIs). The problem is, however, in the case where the
> > > > application needs to run on the older kernel that expects the buggy
> > > > format. Then apps would still have to send in the old buggy format --
> > > > or maybe better in the older 32bit format that won't hit the bug
> > > > above. It makes situation more complicated.
> > >
> > > Can't an ioctl number just be redefined so that, on old kernels with
> > > the buggy one, newly built applications get told that mmap is not
> > > available and use the unaffected non-mmap fallback?
> >
> > The problem is that the SYNC_PTR64 ioctl itself for non-mmap fallback
> > is equally buggy due to this bug, too. So disabling mmap doesn't help
> > alone.
> >
> > And, yes, we can redefine ioctl numbers. But, then, application would
> > have to be bilingual, as well as the kernel; it'll have to switch back
> > to old API when running on older kernel, while the same binary would
> > need to run in a new API for a newer kernel.
> >
> > Maybe we can implement it in alsa-lib, if it really worth for it.
>
> In musl we already have ioctl struct conversion for running on
> time32-only kernels. So it may be practical to convert this too if
> needed.
I guess we can work around without ioctl renumbering. The PCM API has
a protocol version handshaking, and user-space is supposed to tell its
API version to kernel. So the kernel can know in what version
user-space is talking with.
Below is the PoC fix in the kernel side (totally untested).
The fix for alsa-lib will follow.
thanks,
Takashi
---
diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index 5859ca0a1439..dbdbf0c794d8 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -156,7 +156,7 @@ struct snd_hwdep_dsp_image {
* *
*****************************************************************************/
-#define SNDRV_PCM_VERSION SNDRV_PROTOCOL_VERSION(2, 0, 15)
+#define SNDRV_PCM_VERSION SNDRV_PROTOCOL_VERSION(2, 0, 16)
typedef unsigned long snd_pcm_uframes_t;
typedef signed long snd_pcm_sframes_t;
@@ -550,6 +550,7 @@ struct __snd_pcm_sync_ptr {
} s;
union {
struct __snd_pcm_mmap_control control;
+ struct __snd_pcm_mmap_control control_api_2_0_15; /* no bug in 32bit mode */
unsigned char reserved[64];
} c;
};
@@ -557,11 +558,15 @@ struct __snd_pcm_sync_ptr {
#if defined(__BYTE_ORDER) ? __BYTE_ORDER == __BIG_ENDIAN : defined(__BIG_ENDIAN)
typedef char __pad_before_uframe[sizeof(__u64) - sizeof(snd_pcm_uframes_t)];
typedef char __pad_after_uframe[0];
+typedef char __pad_before_u32[4];
+typedef char __pad_after_u32[0];
#endif
#if defined(__BYTE_ORDER) ? __BYTE_ORDER == __LITTLE_ENDIAN : defined(__LITTLE_ENDIAN)
typedef char __pad_before_uframe[0];
typedef char __pad_after_uframe[sizeof(__u64) - sizeof(snd_pcm_uframes_t)];
+typedef char __pad_before_u32[0];
+typedef char __pad_after_u32[4];
#endif
struct __snd_pcm_mmap_status64 {
@@ -579,13 +584,23 @@ struct __snd_pcm_mmap_status64 {
struct __snd_pcm_mmap_control64 {
__pad_before_uframe __pad1;
snd_pcm_uframes_t appl_ptr; /* RW: appl ptr (0...boundary-1) */
- __pad_before_uframe __pad2;
+ __pad_after_uframe __pad2;
__pad_before_uframe __pad3;
snd_pcm_uframes_t avail_min; /* RW: min available frames for wakeup */
__pad_after_uframe __pad4;
};
+/* buggy mmap control definition for 2.0.15 PCM API on 32bit mode */
+struct __snd_pcm_mmap_control64_api_2_0_15 {
+ __pad_before_u32 __pad1;
+ __u32 appl_ptr;
+ __pad_before_u32 __pad2; /* SiC! here is the bug */
+ __pad_before_u32 __pad3;
+ __u32 avail_min;
+ __pad_after_uframe __pad4;
+};
+
struct __snd_pcm_sync_ptr64 {
__u32 flags;
__u32 pad1;
@@ -595,6 +610,7 @@ struct __snd_pcm_sync_ptr64 {
} s;
union {
struct __snd_pcm_mmap_control64 control;
+ struct __snd_pcm_mmap_control64_api_2_0_15 control_api_2_0_15;
unsigned char reserved[64];
} c;
};
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 46c643db18eb..1f26dd2a2525 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2958,8 +2958,19 @@ static int snd_pcm_delay(struct snd_pcm_substream *substream,
return err;
}
+/* PCM 2.0.15 API definition had a bug in mmap control; it puts the avail_min
+ * at the wrong offset due to a typo in padding type.
+ * The bug hits only on 32bit, either on 32bit arch or in 32bit compat mode.
+ */
+static bool is_buggy_control(struct snd_pcm_file *pcm_file)
+{
+ return pcm_file->user_pversion == SNDRV_PROTOCOL_VERSION(2, 0, 15) &&
+ (in_compat_syscall() || !IS_ENABLED(CONFIG_64BIT));
+}
+
static int snd_pcm_sync_ptr(struct snd_pcm_substream *substream,
- struct snd_pcm_sync_ptr __user *_sync_ptr)
+ struct snd_pcm_sync_ptr __user *_sync_ptr,
+ bool buggy_control)
{
struct snd_pcm_runtime *runtime = substream->runtime;
struct snd_pcm_sync_ptr sync_ptr;
@@ -2970,8 +2981,17 @@ static int snd_pcm_sync_ptr(struct snd_pcm_substream *substream,
memset(&sync_ptr, 0, sizeof(sync_ptr));
if (get_user(sync_ptr.flags, (unsigned __user *)&(_sync_ptr->flags)))
return -EFAULT;
- if (copy_from_user(&sync_ptr.c.control, &(_sync_ptr->c.control), sizeof(struct snd_pcm_mmap_control)))
- return -EFAULT;
+ if (buggy_control) {
+ if (copy_from_user(&sync_ptr.c.control_api_2_0_15,
+ &(_sync_ptr->c.control_api_2_0_15),
+ sizeof(sync_ptr.c.control_api_2_0_15)))
+ return -EFAULT;
+ } else {
+ if (copy_from_user(&sync_ptr.c.control,
+ &(_sync_ptr->c.control),
+ sizeof(sync_ptr.c.control)))
+ return -EFAULT;
+ }
status = runtime->status;
control = runtime->control;
if (sync_ptr.flags & SNDRV_PCM_SYNC_PTR_HWSYNC) {
@@ -2981,19 +3001,34 @@ static int snd_pcm_sync_ptr(struct snd_pcm_substream *substream,
}
snd_pcm_stream_lock_irq(substream);
if (!(sync_ptr.flags & SNDRV_PCM_SYNC_PTR_APPL)) {
- err = pcm_lib_apply_appl_ptr(substream,
- sync_ptr.c.control.appl_ptr);
+ if (buggy_control) {
+ err = pcm_lib_apply_appl_ptr(substream,
+ sync_ptr.c.control_api_2_0_15.appl_ptr);
+ } else {
+ err = pcm_lib_apply_appl_ptr(substream,
+ sync_ptr.c.control.appl_ptr);
+ }
if (err < 0) {
snd_pcm_stream_unlock_irq(substream);
return err;
}
} else {
- sync_ptr.c.control.appl_ptr = control->appl_ptr;
+ if (buggy_control)
+ sync_ptr.c.control_api_2_0_15.appl_ptr = control->appl_ptr;
+ else
+ sync_ptr.c.control.appl_ptr = control->appl_ptr;
+ }
+ if (!(sync_ptr.flags & SNDRV_PCM_SYNC_PTR_AVAIL_MIN)) {
+ if (buggy_control)
+ control->avail_min = sync_ptr.c.control_api_2_0_15.avail_min;
+ else
+ control->avail_min = sync_ptr.c.control.avail_min;
+ } else {
+ if (buggy_control)
+ sync_ptr.c.control_api_2_0_15.avail_min = control->avail_min;
+ else
+ sync_ptr.c.control.avail_min = control->avail_min;
}
- if (!(sync_ptr.flags & SNDRV_PCM_SYNC_PTR_AVAIL_MIN))
- control->avail_min = sync_ptr.c.control.avail_min;
- else
- sync_ptr.c.control.avail_min = control->avail_min;
sync_ptr.s.status.state = status->state;
sync_ptr.s.status.hw_ptr = status->hw_ptr;
sync_ptr.s.status.tstamp = status->tstamp;
@@ -3289,7 +3324,8 @@ static int snd_pcm_common_ioctl(struct file *file,
case __SNDRV_PCM_IOCTL_SYNC_PTR32:
return snd_pcm_ioctl_sync_ptr_compat(substream, arg);
case __SNDRV_PCM_IOCTL_SYNC_PTR64:
- return snd_pcm_sync_ptr(substream, arg);
+ return snd_pcm_sync_ptr(substream, arg,
+ is_buggy_control(pcm_file));
#ifdef CONFIG_SND_SUPPORT_OLD_API
case SNDRV_PCM_IOCTL_HW_REFINE_OLD:
return snd_pcm_hw_refine_old_user(substream, arg);
@@ -3851,6 +3887,8 @@ static int snd_pcm_mmap(struct file *file, struct vm_area_struct *area)
return -ENXIO;
fallthrough;
case SNDRV_PCM_MMAP_OFFSET_CONTROL_NEW:
+ if (is_buggy_control(pcm_file))
+ return -ENXIO;
if (!pcm_control_mmap_allowed(pcm_file))
return -ENXIO;
return snd_pcm_mmap_control(substream, file, area);