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From bb11ffcff5ae2f25bead921c2a299e7e63d8a759 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20P=C3=ADsa=C5=99?= <ppisar@redhat.com>
Date: Thu, 14 Feb 2019 16:51:54 +0100
Subject: [PATCH] CVE-2019-7572: Fix a buffer overread in IMA_ADPCM_nibble
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If an IMA ADPCM block contained an initial index out of step table
range (loaded in IMA_ADPCM_decode()), IMA_ADPCM_nibble() blindly used
this bogus value and that lead to a buffer overread.

This patch fixes it by moving clamping the index value at the
beginning of IMA_ADPCM_nibble() function instead of the end after
an update.

CVE-2019-7572
https://bugzilla.libsdl.org/show_bug.cgi?id=4495

Signed-off-by: Petr Písař <ppisar@redhat.com>
---
 src/audio/SDL_wave.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/audio/SDL_wave.c b/src/audio/SDL_wave.c
index 2968b3d..69d62dc 100644
--- a/src/audio/SDL_wave.c
+++ b/src/audio/SDL_wave.c
@@ -275,6 +275,14 @@ static Sint32 IMA_ADPCM_nibble(struct IMA_ADPCM_decodestate *state,Uint8 nybble)
 	};
 	Sint32 delta, step;
 
+	/* Clamp index value. The inital value can be invalid. */
+	if ( state->index > 88 ) {
+		state->index = 88;
+	} else
+	if ( state->index < 0 ) {
+		state->index = 0;
+	}
+
 	/* Compute difference and new sample value */
 	step = step_table[state->index];
 	delta = step >> 3;
@@ -286,12 +294,6 @@ static Sint32 IMA_ADPCM_nibble(struct IMA_ADPCM_decodestate *state,Uint8 nybble)
 
 	/* Update index value */
 	state->index += index_table[nybble];
-	if ( state->index > 88 ) {
-		state->index = 88;
-	} else
-	if ( state->index < 0 ) {
-		state->index = 0;
-	}
 
 	/* Clamp output sample */
 	if ( state->sample > max_audioval ) {
-- 
2.20.1