ALSA: seq: Fix kernel heap address leak in bounce_error_event()

Overview

In the Linux kernel, the following vulnerability has been resolved:

ALSA: seq: Fix kernel heap address leak in bounce_error_event()

The comment above bounce_error_event() documents that user clients should receive SNDRV_SEQ_EVENT_BOUNCE with the original event embedded as variable-length data, while kernel clients should receive SNDRV_SEQ_EVENT_KERNEL_ERROR with a quoted kernel pointer.

However, the implementation unconditionally uses SNDRV_SEQ_EVENT_KERNEL_ERROR with data.quote.event set to the raw struct snd_seq_event pointer for all clients. When a bounce error event is delivered to a USER_CLIENT via snd_seq_read(), the kernel heap address in data.quote.event is exposed to userspace through copy_to_user() in the fixed-length branch.

This is a distinct leak path from the one addressed by commit 705dd6dcbc0e ("ALSA: seq: Clear variable event pointer on read"), which sanitizes data.ext.ptr in the variable-length branch of snd_seq_read(). The bounce_error_event() leak uses fixed-length events that take the else branch where no sanitization occurs.

Differentiate the bounce event by client type. For USER_CLIENT, send SNDRV_SEQ_EVENT_BOUNCE with SNDRV_SEQ_EVENT_LENGTH_VARIABLE and data.ext pointing to the original event. The variable-length path in snd_seq_event_dup() copies the event data into chained cells, and snd_seq_expand_var_event() copies only the content -- never the pointer -- to userspace. For KERNEL_CLIENT, keep the existing SNDRV_SEQ_EVENT_KERNEL_ERROR behavior with the quoted pointer.

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < eae296065ede41a24040e6a7c909ec970f78b4eb
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 93d260ce43a81df579c98bee92b91316df9c1c57
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < dae23c545eb5a2be3b27a82fd0f611894fb8ab69
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 0527a56cb327021cb73167cfddf0e49efa043500
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < efc86691e4d8083d9e380ea95042c2cf679f65fd
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.