linux-hardening - Re: [PATCH v2] lib: zstd: Fix -Wstringop-overflow warning

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Date:   Sat, 14 Jan 2023 01:06:07 +0000
From:   Nick Terrell <terrelln@...a.com>
To:     Kees Cook <keescook@...omium.org>
CC:     Nick Terrell <terrelln@...a.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v2] lib: zstd: Fix -Wstringop-overflow warning



> On Jan 13, 2023, at 4:48 PM, Kees Cook <keescook@...omium.org> wrote:
> 
> !-------------------------------------------------------------------|
>  This Message Is From an External Sender
> 
> |-------------------------------------------------------------------!
> 
> On Tue, Jan 10, 2023 at 11:10:08PM +0000, Nick Terrell wrote:
>> 
>> 
>>> On Jan 4, 2023, at 1:20 PM, Kees Cook <keescook@...omium.org> wrote:
>>> 
>>> !-------------------------------------------------------------------|
>>> This Message Is From an External Sender
>>> 
>>> |-------------------------------------------------------------------!
>>> 
>>> Fix the following -Wstringop-overflow warning when building with GCC 11+:
>>> 
>>> lib/zstd/decompress/huf_decompress.c: In function ‘HUF_readDTableX2_wksp’:
>>> lib/zstd/decompress/huf_decompress.c:700:5: warning: ‘HUF_fillDTableX2.constprop’ accessing 624 bytes in a region of size 52 [-Wstringop-overflow=]
>>> 700 |     HUF_fillDTableX2(dt, maxTableLog,
>>>     |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> 701 |                    wksp->sortedSymbol, sizeOfSort,
>>>     |                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> 702 |                    wksp->rankStart0, wksp->rankVal, maxW,
>>>     |                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> 703 |                    tableLog+1,
>>>     |                    ~~~~~~~~~~~
>>> 704 |                    wksp->calleeWksp, sizeof(wksp->calleeWksp) / sizeof(U32));
>>>     |
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> lib/zstd/decompress/huf_decompress.c:700:5: note: referencing argument 6 of type ‘U32 (*)[13]’ {aka ‘unsigned int (*)[13]’}
>>> lib/zstd/decompress/huf_decompress.c:571:13: note: in a call to function ‘HUF_fillDTableX2.constprop’
>>> 571 | static void HUF_fillDTableX2(HUF_DEltX2* DTable, const U32 targetLog,
>>>     |             ^~~~~~~~~~~~~~~~
>>> 
>>> by using pointer notation instead of array notation.
>>> 
>>> This is one of the last remaining warnings to be fixed before globally
>>> enabling -Wstringop-overflow.
>> 
>> The patch looks correct to me, thanks for reviving it. But, I was attempting to reproduce the issue,
>> so I could better understand what's going on, and I wasn't able to reproduce it myself.
>> 
>> To attempt to reproduce, I applied this patch
>> 
>> ---
>> diff --git a/lib/zstd/Makefile b/lib/zstd/Makefile
>> index 20f08c644b71..190d3d5ab4be 100644
>> --- a/lib/zstd/Makefile
>> +++ b/lib/zstd/Makefile
>> @@ -12,6 +12,8 @@ obj-$(CONFIG_ZSTD_COMPRESS) += zstd_compress.o
>> obj-$(CONFIG_ZSTD_DECOMPRESS) += zstd_decompress.o
>> obj-$(CONFIG_ZSTD_COMMON) += zstd_common.o
>> 
>> +ccflags-y := -Wstringop-overflow=4 -Werror
>> +
>> zstd_compress-y := \
>>                zstd_compress_module.o \
>>                compress/fse_compress.o \
>> ---
>> 
>> Then compiled on x86-64 with gcc 12.2.0 on tag v6.2-rc3. I saw no errors.
>> I also tried with just `-Wstringop-overflow`, and on upstream zstd. I tried to
>> make a minimal reproducer on godbolt, so I could see if it was the gcc version,
>> but wasn't able to make it fail with any of them https://gcc.godbolt.org/z/Exzq9arMr .
>> 
>> Could you please tell me how to reproduce this warning?
> 
> I saw it like so with next-20230113 on x86_64:
> 
> $ gcc --version
> gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
> ...
> $ make KCFLAGS=-Wstringop-overflow allmodconfig lib/zstd/decompress/huf_decompress.o

Thanks, I was able to repro it! I will merge this patch into my tree.

If you would like to submit the same patch upstream yourself, I will accept the PR, otherwise I can submit an upstream PR.

Just to be certain, this patch is to work around a shortcoming in -Wstringop-overflow, but the code was otherwise correct?

Reviewed-by: Nick Terrell <terrelln@...com <mailto:terrelln@...com>>

Best,
Nick Terrell

> -- 
> Kees Cook