Re: [FFmpeg-devel] [PATCH] lavu/avstring: check for overlong encodings

[Stefano Sabatini]

On date Friday 2014-08-29 15:57:32 +0200, wm4 encoded:
> On Thu, 28 Aug 2014 17:39:27 +0200
> Stefano Sabatini <stefa...@gmail.com> wrote:
> 
> > Fix reopened trac ticket #1163.
> > ---
> >  libavutil/avstring.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/libavutil/avstring.c b/libavutil/avstring.c
> > index a63fb84..df27d5e 100644
> > --- a/libavutil/avstring.c
> > +++ b/libavutil/avstring.c
> > @@ -331,7 +331,10 @@ int av_utf8_decode(int32_t *codep, const uint8_t 
> > **bufp, const uint8_t *buf_end,
> >      const uint8_t *p = *bufp;
> >      uint32_t top;
> >      uint64_t code;
> > -    int ret = 0;
> > +    int ret = 0, tail_len;
> > +    uint32_t overlong_encoding_mins[6] = {
> > +        0x00000000, 0x00000080, 0x00000800, 0x00010000, 0x00200000, 
> > 0x04000000,
> > +    };
> >  
> >      if (p >= buf_end)
> >          return 0;
> > @@ -346,8 +349,10 @@ int av_utf8_decode(int32_t *codep, const uint8_t 
> > **bufp, const uint8_t *buf_end,
> >      }
> >      top = (code & 128) >> 1;
> >  
> > +    tail_len = 0;
> >      while (code & top) {
> >          int tmp;
> > +        tail_len++;
> >          if (p >= buf_end) {
> >              (*bufp) ++;
> >              return AVERROR(EILSEQ); /* incomplete sequence */
> > @@ -364,6 +369,12 @@ int av_utf8_decode(int32_t *codep, const uint8_t 
> > **bufp, const uint8_t *buf_end,
> >      }
> >      code &= (top << 1) - 1;
> >  
> > +    /* check for overlong encodings */
> > +    if (code < overlong_encoding_mins[tail_len]) {
> > +        ret = AVERROR(EILSEQ);
> > +        goto end;
> > +    }
> > +
> >      if (code >= 1<<31) {
> >          ret = AVERROR(EILSEQ);  /* out-of-range value */
> >          goto end;
> 

> Looks ok and simple to me. Is there a guarantee tail_len never
> becomes larger than 5?

It is mathematically impossible that the length will be larger than 5.
Added an assert for that though, alternatively I could add a check.

> 
> Also note that libavcodec/utils.c contains the same check (but less
> readable) in utf8_check().
-- 
FFmpeg = Faithful Fundamentalist Most Power Enhancing Goblin

>From 75db7cc2ea2b7b12e611b7b3fd103aee4f707dde Mon Sep 17 00:00:00 2001
From: Stefano Sabatini <stefa...@gmail.com>
Date: Thu, 28 Aug 2014 17:37:27 +0200
Subject: [PATCH] lavu/avstring: check for overlong encodings

Fix reopened trac ticket #1163.
---
 libavutil/avstring.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/libavutil/avstring.c b/libavutil/avstring.c
index a63fb84..fd010e4 100644
--- a/libavutil/avstring.c
+++ b/libavutil/avstring.c
@@ -27,6 +27,7 @@
 #include "config.h"
 #include "common.h"
 #include "mem.h"
+#include "avassert.h"
 #include "avstring.h"
 #include "bprint.h"
 
@@ -331,7 +332,10 @@ int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
     const uint8_t *p = *bufp;
     uint32_t top;
     uint64_t code;
-    int ret = 0;
+    int ret = 0, tail_len;
+    uint32_t overlong_encoding_mins[6] = {
+        0x00000000, 0x00000080, 0x00000800, 0x00010000, 0x00200000, 0x04000000,
+    };
 
     if (p >= buf_end)
         return 0;
@@ -346,8 +350,10 @@ int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
     }
     top = (code & 128) >> 1;
 
+    tail_len = 0;
     while (code & top) {
         int tmp;
+        tail_len++;
         if (p >= buf_end) {
             (*bufp) ++;
             return AVERROR(EILSEQ); /* incomplete sequence */
@@ -364,6 +370,13 @@ int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
     }
     code &= (top << 1) - 1;
 
+    /* check for overlong encodings */
+    av_assert0(tail_len <= 5);
+    if (code < overlong_encoding_mins[tail_len]) {
+        ret = AVERROR(EILSEQ);
+        goto end;
+    }
+
     if (code >= 1<<31) {
         ret = AVERROR(EILSEQ);  /* out-of-range value */
         goto end;
-- 
1.8.3.2


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