ALSA: seq: oss: Fix Spectre v1 vulnerability

dev is indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

sound/core/seq/oss/seq_oss_synth.c:626 snd_seq_oss_synth_make_info() warn: potential spectre issue 'dp->synths' [w] (local cap)

Fix this by sanitizing dev before using it to index dp->synths.

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://lore.kernel.org/lkml/20180423164740.GY17484@dhcp22.suse.cz/

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
CVE-2017-5753

(cherry picked from commit c709f14f0616482b67f9fbcb965e1493a03ff30b)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/sound/core/seq/oss/seq_oss_synth.c b/sound/core/seq/oss/seq_oss_synth.c
index 278ebb9931225998dd07f0606eeabe289d71aff5..c939459172353dee5ee651ee4694f43f2aa9be7d 100644 (file)
--- a/sound/core/seq/oss/seq_oss_synth.c
+++ b/sound/core/seq/oss/seq_oss_synth.c
@@ -617,13 +617,14 @@ int
 snd_seq_oss_synth_make_info(struct seq_oss_devinfo *dp, int dev, struct synth_info *inf)
 {
        struct seq_oss_synth *rec;
+       struct seq_oss_synthinfo *info = get_synthinfo_nospec(dp, dev);
 
-       if (dev < 0 || dev >= dp->max_synthdev)
+       if (!info)
                return -ENXIO;
 
-       if (dp->synths[dev].is_midi) {
+       if (info->is_midi) {
                struct midi_info minf;
-               snd_seq_oss_midi_make_info(dp, dp->synths[dev].midi_mapped, &minf);
+               snd_seq_oss_midi_make_info(dp, info->midi_mapped, &minf);
                inf->synth_type = SYNTH_TYPE_MIDI;
                inf->synth_subtype = 0;
                inf->nr_voices = 16;