[PATCH 5.10.y] 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 context for relocating the is_oss_stream macro because surrounding helpers are 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 bec81f6ba6ba5..d4eb842784143 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 b3e8be5aeafb3..353f675724337 100644
--- a/sound/core/pcm_local.h
+++ b/sound/core/pcm_local.h
@@ -72,4 +72,10 @@ struct page *snd_pcm_sgbuf_ops_page(struct snd_pcm_substream *substream,
 
 #define PCM_RUNTIME_CHECK(sub) snd_BUG_ON(!(sub) || !(sub)->runtime)
 
+#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 40020c5dceda0..7b6902bf0fd12 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -702,12 +702,6 @@ 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 int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 			     struct snd_pcm_hw_params *params)
 {
-- 
2.53.0