Rick's b.log - entry 2020/09/08

Tracking down a buffer overflow

Manga would start, it would download the main manga index, as normal, and then the machine would stiff.
Completely.
No mouse pointer movement, nothing.

Infuriatingly, whatever the code was actually doing would not be replicated in the debugger builds, as programs built with debugging have all of the optimisation turned off, to do exactly what the code says, rather than taking shortcuts.

Also, infuriatingly, adding some of my own logging and debug code made the problem go away.

To try to understand what was happening, let's look at the code. I have removed all comments to keep things simpler.

static BOOL mangareader_parselist(void)
{
   file_handle ifp;
   char inputline[1024];
   int  thissect = -1;
   int  thismanga = 0;
   char *scan = NULL;
   char *poke = NULL;
   int  ttllen = 0;
   int  ttlcrc = 0;
   [etc]

Next we set up the environment (pop up a blank sliding-bar window, paste a message into it, set the slider to 0%), then we check and open the file. I don't need to bore you with the code for that.

Then we start to parse, as follows:

   while ( !File_EOF(ifp) )
   {
      File_ReadLine(ifp, inputline);

      scan = strstr(inputline, "<div class=\"d43\">");
      if (scan != NULL)
      {
         section = scan[17];
         thissect++;

         if (thissect == SECTMAX)
         {
            Error_ReportWithType( [error message here] );
            File_Seek(ifp, File_ReadExtent(ifp));
            goto skipout; // argh! argh! zombies! argh!
         }

         sections[thissect].sectext = section;
         sections[thissect].objects = 0;
         sections[thissect].arrayoffset = thismanga;

         sprintf(event_lastevent.data.bytes, message_read("mread.lpsec"), section, thismanga);
         percent = ( File_ReturnPos(ifp) * 100 ) / File_ReadExtent(ifp);
         busy_setstatus(message_read("mread.lparse"), event_lastevent.data.bytes, TRUE, percent);
         wimp_poll_now();
         wimp_poll_now();
      }

      scan = strstr(inputline, "<li><a href=\"/");
      if ( (scan != NULL) && (section != 0) )
      {

If the divider class "d43" is found, then this marks the beginning of a new section. Because there are over four thousand manga, we use the sections to organise the menu structure for choosing Manga. For example, Sakura Wars (I made that up, but it sounds like something a manga would be called) is listed under the menu entry 'S', and not after two or three thousand others.

If we find a new section, then the section name is easily read out as the 17th byte from the 'div' point. It's a single character, being either '#' or a number, or the letters from A to Z.

We then increment thissect in order that it points to a valid array entry. And then we sanity check it to check it isn't too big (SECTMAX is 40, the letters A-Z, numbers 0-9, and # equal 37; leaving space for three in case they add something else).
If the section maximum has been reached, then we report a warning and force the end of the file to be reached, and goto (yes, a goto!) the end of the scanning routine. The logic here is to basically give up and roll with what we have.

Assuming we continue, then we set up the status for the section. Each array consists of three words. The first is the character of the section. This is only a byte, but things on the ARM are word aligned, so might as well treat it as a word. The next word is a count of how many manga in the section (initialised to zero), and the final word is the number of the next manga to be added (as it is incremented after a manga has been read in).
The logic for this is that if we want to find the 20th manga in 'S', then we simply look for the arrayoffset of 'S' and add 19 to it (19, not 20, as we begin counting from zero).

Every time a new section is read, we update the sliding bar thingy and poll twice. This is a useful tradeoff between parsing speed (we don't want to update/poll too often) versus system responsiveness (we don't want to poll too infrequently). On my Pi1 and Pi2 (sort of average machines these days, I would imagine), you can clearly see it's doing something and it doesn't take too long.

The final snippet is to look for the beginning of an entry. It's a list containing a link. There follow two screenfuls of code to parse that and set up the manga array... but this isn't relevant to the discussion.

It was, as I mentioned, totally stiffing the machine.

By a process of trial and error, I located it down to the three lines that set up the section.

This failed:

         sections[thissect].sectext = section;
         sections[thissect].objects = 0;
         sections[thissect].arrayoffset = thismanga;

This worked:

         printf("Work, you ****ing piece of ****!");
         sections[thissect].sectext = section;
         sections[thissect].objects = 0;
         sections[thissect].arrayoffset = thismanga;

Now is where it starts to get messy. Fasten your seatbelts.

The function entry looks like this:

mangareader_parselist
        MOV      ip,sp
        STMDB    sp!,{v1-v6,fp,ip,lr,pc}
        SUB      fp,ip,#4
        SUB      ip,sp,#&410
        CMP      ip,sl
        BLMI     __rt_stkovf_split_big
        SUB      sp,sp,#&400
        SUB      sp,sp,#&c
        MVN      v4,#0
        MOV      v3,#0
        MOV      v6,#0

It's a pretty standard APCS entry. SP is copied to IP, a bunch of stuff is written to the stack (including the value of the stack pointer on entry, now in IP). Then we muck about to get the size of the stack that we require, and if we cannot, call a function to make it happen.
Then an interesting part. We add &40C to the stack pointer. In other words, 1024 bytes plus 12 bytes.

In looking at the code, it would appear that SP+0 is used for storing the value of the length of the manga title, ttllen, and it looks like SP+4 is used for the result of scan.
The next 1024 bytes are the input buffer to be used by the line reading routine. And finally, SP+408 looks to be a pointer to the data of the event_lastevent block. This is the poll block, and it is used as temporary workspace.

Now, the interesting thing is that I found the bug in an entirely incorrect roundabout way. As I said, removing the lines assigning sections[thissect].whatever = something; led me to suspect that it might have been thissect that was getting corrupted. The section array is set up elsewhere and it is a fixed size (not malloc), so "logically" if something was corrupting thissect, then it might end up writing to a nonsense array offset, like sections[-1389834] or something.
This led me to look at what was defined prior to thisect. That being the big old inputbuffer string space.

I was on the right track... for completely the wrong reason!

Looking at the File_ReadLine() routine, it had an error. It was supposed to read, at most, 1023 bytes. Instead, it read while the offset was <1024.
This might look correct. Certainly, you're going until 1023 and then stopping, right? Wrong. Because you're counting from zero and stuffing a terminating NULL to the end.
In other words, what we want to do is read 1023 bytes and then add a terminator. What actually happened was 1024 bytes were read, and then the terminator was added... to make a total of 1025 bytes.

Now what actually happened here is that the address of that poll block structure was corrupted by overshooting the buffer, and we then use that messed up address with sprintf() to generate the "Reading section 'X', xxxx manga indexed" message. Where? Well...

I'm not entirely certain what bit of memory was being trampled on, as the machine froze, as I said. It was only in that specific build, too. Oh, sure, the problem would always have potentially happened, but until Cloudflare get involved, no lines came anywhere near the 1K string limit. Indeed, given that the fetcher breaks up the HTML (it's either that or completely rewrite the parser!), the only thing that remains "big" is that wodge of CSS at the start of each page. And why this version? For the sme reason that deleting the sections[] assignments or adding debug code made the problem seemingly go away... it simply shifted the position of event_lastevent in the executable, which gave it a different address, which meant that whatever was being trampled on this time wasn't something that was going to instantly stiff the machine. Who knows, given that the mouse pointer froze, I might have just crapped on the interrupt vector chain or something. Arguably one shouldn't be able to write that stuff from user mode code... but this is RISC OS we're talking about... 😔

Oh, one final thing. Remember that I said that code built in debugging mode was very different as optimisations were turned off?

Compare function entry start:

mangareader_parselist                       mangareader_parselist
        MOV      ip,sp                              MOV      ip,sp
        STMDB    sp!,{v1-v6,fp,ip,lr,pc}            STMDB    sp!,{v1-v6,fp,ip,lr,pc}
        SUB      fp,ip,#4                           SUB      fp,ip,#4
        SUB      ip,sp,#&410                        SUB      ip,sp,#&410
        CMP      ip,sl                              CMP      ip,sl
        BLMI     __rt_stkovf_split_big              BLMI     __rt_stkovf_split_big
        SUB      sp,sp,#&400                        SUB      sp,sp,#&400
        SUB      sp,sp,#&c                          SUB      sp,sp,#&c
        MVN      v4,#0                              MVN      v2,#0
        MOV      v3,#0                              MOV      v3,#0
        MOV      v6,#0                              MOV      v4,#0
        ADD      a1,pc,#L00063c-.-8                 MOV      v5,#0
        BL       message_read                       MOV      v6,#0
                                                    MOV      a2,#0
                                                    STR      a2,[sp,#8]
                                                    MOV      a3,#0
                                                    STR      a3,[sp,#4]
                                                    MOV      a4,#0
                                                    STR      a4,[sp]
                                                    ADD      a1,pc,#L0006ec-.-8
                                                    BL       message_read

Now, finally, let's compare the sections[] assignment lines:

|L0003f4.J13.mangareader_parselist|         |L000418.J28.mangareader_parselist|
        LDR      a3,[pc, #L000698-.-8]              LDR      a2,[pc, #L000744-.-8]
        MOV      a4,#0                              STR      v4,[a2,v2,LSL #4]
        ADD      a1,pc,#L00069c-.-8                 MOV      a4,#0
        STR      v6,[a3,v4,LSL #4]                  LDR      a1,[pc, #L000744-.-8]
        ADD      a3,a3,v4,LSL #4                    ADD      a3,a1,v2,LSL #4
        STR      a4,[a3,#4]!                        STR      a4,[a3,#4]
        STR      v3,[a3,#8]!                        LDR      a1,[pc, #L000744-.-8]
        BL       message_read                       ADD      a3,a1,v2,LSL #4
                                                    STR      v3,[a3,#&c]
                                                    ADD      a1,pc,#L000748-.-8
                                                    BL       message_read

Dehumidifier unboxing

And finally... your dose of kitten...

Anna susses ladders

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David Pilling, 11th September 2020, 01:43

@25C and 100% humidity "one liter of air can hold 0.941mmol of water vapor, which is 0.017 mL of liquid water"

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