ALSA: emux: Fix potential Spectre v1 vulnerabilities

info.mode and info.port are indirectly controlled by user-space,
hence leading to a potential exploitation of the Spectre variant 1
vulnerability.

These issues were detected with the help of Smatch:

sound/synth/emux/emux_hwdep.c:72 snd_emux_hwdep_misc_mode() warn: potential spectre issue 'emu->portptrs[i]->ctrls' [w] (local cap)
sound/synth/emux/emux_hwdep.c:75 snd_emux_hwdep_misc_mode() warn: potential spectre issue 'emu->portptrs' [w] (local cap)
sound/synth/emux/emux_hwdep.c:75 snd_emux_hwdep_misc_mode() warn: potential spectre issue 'emu->portptrs[info.port]->ctrls' [w] (local cap)

Fix this by sanitizing both info.mode and info.port before using them
to index emu->portptrs[i]->ctrls, emu->portptrs[info.port]->ctrls and
emu->portptrs.

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 5 insertions, 2 deletions

diff --git a/sound/synth/emux/emux_hwdep.c b/sound/synth/emux/emux_hwdep.c
index e557946718a9..d9fcae071b47 100644
--- a/sound/synth/emux/emux_hwdep.c
+++ b/sound/synth/emux/emux_hwdep.c
@@ -22,9 +22,9 @@
 #include <sound/core.h>
 #include <sound/hwdep.h>
 #include <linux/uaccess.h>
+#include <linux/nospec.h>
 #include "emux_voice.h"
 
-
 #define TMP_CLIENT_ID   0x1001
 
 /*
@@ -66,13 +66,16 @@ snd_emux_hwdep_misc_mode(struct snd_emux *emu, void __user *arg)
                 return -EFAULT;
         if (info.mode < 0 || info.mode >= EMUX_MD_END)
                 return -EINVAL;
+        info.mode = array_index_nospec(info.mode, EMUX_MD_END);
 
         if (info.port < 0) {
                 for (i = 0; i < emu->num_ports; i++)
                         emu->portptrs[i]->ctrls[info.mode] = info.value;
         } else {
-                if (info.port < emu->num_ports)
+                if (info.port < emu->num_ports) {
+                        info.port = array_index_nospec(info.port, emu->num_ports);
                         emu->portptrs[info.port]->ctrls[info.mode] = info.value;
+                }
         }
         return 0;
 }