linux-hardening - Re: Coverity: HUF_buildCTableFromTree(): Memory

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Date:   Thu, 27 Oct 2022 15:43:08 +0200
From:   Jann Horn <jannh@...gle.com>
To:     coverity-bot <keescook@...omium.org>
Cc:     Nick Terrell <terrelln@...com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-next@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: Coverity: HUF_buildCTableFromTree(): Memory - corruptions

On Thu, Oct 27, 2022 at 2:06 AM coverity-bot <keescook@...omium.org> wrote:
>
> Hello!
>
> This is an experimental semi-automated report about issues detected by
> Coverity from a scan of next-20221026 as part of the linux-next scan project:
> https://scan.coverity.com/projects/linux-next-weekly-scan
>
> You're getting this email because you were associated with the identified
> lines of code (noted below) that were touched by commits:
>
>   Mon Oct 24 12:12:32 2022 -0700
>     2aa14b1ab2c4 ("zstd: import usptream v1.5.2")
>
> Coverity reported the following:
>
> *** CID 1525550:  Memory - corruptions  (OVERRUN)
> /lib/zstd/compress/huf_compress.c: 673 in HUF_buildCTableFromTree()
> 667                 min += nbPerRank[n];
> 668                 min >>= 1;
> 669         }   }
> 670         for (n=0; n<alphabetSize; n++)
> 671             HUF_setNbBits(ct + huffNode[n].byte, huffNode[n].nbBits);   /* push nbBits per symbol, symbol order */
> 672         for (n=0; n<alphabetSize; n++)
> vvv     CID 1525550:  Memory - corruptions  (OVERRUN)
> vvv     Overrunning array "valPerRank" of 13 2-byte elements at element index 255 (byte offset 511) using index "HUF_getNbBits(ct[n])" (which evaluates to 255).
> 673             HUF_setValue(ct + n, valPerRank[HUF_getNbBits(ct[n])]++);   /* assign value within rank, symbol order */
> 674         CTable[0] = maxNbBits;
> 675     }
> 676
> 677     size_t HUF_buildCTable_wksp (HUF_CElt* CTable, const unsigned* count, U32 maxSymbolValue, U32 maxNbBits, void* workSpace, size_t wkspSize)
> 678     {

I haven't looked at the other warnings, but from a glance this code
looks fine to me. Coverity is claiming that some symbols can have 255
bits, but we just went through HUF_setMaxHeight(), which enforced that
no symbols have more bits than HUF_TABLELOG_MAX.

Heuristic checks like this are likely to generate lots of false
positives in compression code, I think.