[PATCH 5.15.y 2/2] ALSA: pcm: Prevent O_APPEND open on OSS streams

[Sasha Levin <sashal@kernel.org>]

From: Takashi Iwai <tiwai@suse.de>

[ Upstream commit 9017bc3d13da0ea4e63be618c87143a7b69185e6 ]

ALSA PCM core accepts to re-use the existing stream when O_APPEND is
passed, and it's been used for dmix, etc.  However, this mode isn't
supposed to be used for OSS: the OSS stream allows re-configuration
via its hw_params/hw_refine procedure even while it's being mmapped,
so it implies the usage of only a single stream.  Because of this
assumption, the sanity check is skipped for OSS streams, and if user
may attach to an existing OSS stream with O_APPEND, it may result in a
UAF of the mapped pages which could be used as a privilege escalation
exploit.

For addressing it, this patch simply prohibits the O_APPEND behavior
on the OSS stream.  O_APPEND is never used for OSS layer itself, so
this can't break for the right applications.

Fixes: 0df63e44c3e3 ("[ALSA] Add O_APPEND flag support to PCM")
Cc: stable@vger.kernel.org
Reported-by: Bo Cao <treebacker@foxmail.com>
Link: https://lore.kernel.org/tencent_E90BB4F086165FAA3313723567A5C675EA06@qq.com
Link: https://patch.msgid.link/20261103155844.120838-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[ sashal: Adjusted the is_oss_stream insertion context because snd_pcm_dma_buffer_sync() is absent. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 sound/core/pcm.c        | 2 ++
 sound/core/pcm_local.h  | 6 ++++++
 sound/core/pcm_native.c | 6 ------
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/sound/core/pcm.c b/sound/core/pcm.c
index 9df2e819035f2..8bad50bf00e13 100644
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -930,6 +930,8 @@ int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,
 			return -ENODEV;
 		if (! SUBSTREAM_BUSY(substream))
 			return -EBADFD;
+		if (is_oss_stream(substream))
+			return -EINVAL;
 		substream->ref_count++;
 		*rsubstream = substream;
 		return 0;
diff --git a/sound/core/pcm_local.h b/sound/core/pcm_local.h
index fe9689b8a6a64..f43ecb904069c 100644
--- a/sound/core/pcm_local.h
+++ b/sound/core/pcm_local.h
@@ -73,4 +73,10 @@ void snd_pcm_sync_stop(struct snd_pcm_substream *substream, bool sync_irq);
 		for ((subs) = (pcm)->streams[str].substream; (subs); \
 		     (subs) = (subs)->next)
 
+#if IS_ENABLED(CONFIG_SND_PCM_OSS)
+#define is_oss_stream(substream)	((substream)->oss.oss)
+#else
+#define is_oss_stream(substream)	false
+#endif
+
 #endif	/* __SOUND_CORE_PCM_LOCAL_H */
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index e861404d393b6..c6ae68f35f2f8 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -720,12 +720,6 @@ int snd_pcm_runtime_buffer_set_silence(struct snd_pcm_runtime *runtime)
 }
 EXPORT_SYMBOL_GPL(snd_pcm_runtime_buffer_set_silence);
 
-#if IS_ENABLED(CONFIG_SND_PCM_OSS)
-#define is_oss_stream(substream)	((substream)->oss.oss)
-#else
-#define is_oss_stream(substream)	false
-#endif
-
 static void pcm_qos_req_clear(struct snd_pcm_substream *substream)
 {
 	if (cpu_latency_qos_request_active(&substream->latency_pm_qos_req))
-- 
2.53.0