From 9207dc3b0db368bb9cf5eb295cbc1129c2975e31 Mon Sep 17 00:00:00 2001 From: Andreas Rheinhardt Date: Sun, 21 Nov 2021 01:57:41 +0100 Subject: [PATCH] avcodec/h263: Fix global-buffer-overflow with noout flag2 set h263_get_motion_length() forgot to take an absolute value; as a consequence, a negative index was used to access an array. This leads to potential crashes, but mostly it just accesses what is to the left of ff_mvtab (unless one uses ASAN), thereby defeating the purpose of the AV_CODEC_FLAG2_NO_OUTPUT because the sizes of the returned packets differ from the sizes the encoder would actually have produced. Signed-off-by: Andreas Rheinhardt --- libavcodec/h263.h | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/libavcodec/h263.h b/libavcodec/h263.h index 70fd1ffdc0..d6bef8318d 100644 --- a/libavcodec/h263.h +++ b/libavcodec/h263.h @@ -100,15 +100,16 @@ void ff_h263_encode_motion(PutBitContext *pb, int val, int f_code); static inline int h263_get_motion_length(int val, int f_code){ - int l, bit_size, code; + int bit_size, code, sign; if (val == 0) { return 1; /* ff_mvtab[0][1] */ } else { bit_size = f_code - 1; /* modulo encoding */ - l= INT_BIT - 6 - bit_size; - val = (val<>l; + val = sign_extend(val, 6 + bit_size); + sign = val >> 31; + val = (val ^ sign) - sign; /* val = FFABS(val) */ val--; code = (val >> bit_size) + 1;