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Date:   Mon, 9 Oct 2023 10:29:25 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Nick Terrell <terrelln@fb.com>,
        syzbot <syzbot+1f2eb3e8cd123ffce499@syzkaller.appspotmail.com>,
        clm@fb.com, dsterba@suse.com, josef@toxicpanda.com,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        linux-hardening@vger.kernel.org
Subject: Re: [syzbot] [zstd] UBSAN: array-index-out-of-bounds in
 FSE_decompress_wksp_body_bmi2
Message-ID: <202310091025.4939AEBC9@keescook>
References: <00000000000049964e06041f2cbf@google.com>
 <20231007210556.GA174883@sol.localdomain>
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On Sat, Oct 07, 2023 at 02:05:56PM -0700, Eric Biggers wrote:
> Hi Nick,
> 
> On Wed, Aug 30, 2023 at 12:49:53AM -0700, syzbot wrote:
> > UBSAN: array-index-out-of-bounds in lib/zstd/common/fse_decompress.c:345:30
> > index 33 is out of range for type 'FSE_DTable[1]' (aka 'unsigned int[1]')
> 
> Zstandard needs to be converted to use C99 flex-arrays instead of length-1
> arrays.  https://github.com/facebook/zstd/pull/3785 would fix this in upstream
> Zstandard, though it doesn't work well with the fact that upstream Zstandard
> supports C90.  Not sure how you want to handle this.

For the kernel, we just need:

diff --git a/lib/zstd/common/fse_decompress.c b/lib/zstd/common/fse_decompress.c
index a0d06095be83..b11e87fff261 100644
--- a/lib/zstd/common/fse_decompress.c
+++ b/lib/zstd/common/fse_decompress.c
@@ -312,7 +312,7 @@ size_t FSE_decompress_wksp(void* dst, size_t dstCapacity, const void* cSrc, size
 
 typedef struct {
     short ncount[FSE_MAX_SYMBOL_VALUE + 1];
-    FSE_DTable dtable[1]; /* Dynamically sized */
+    FSE_DTable dtable[]; /* Dynamically sized */
 } FSE_DecompressWksp;
 
 
And if upstream wants to stay C89 compat, perhaps:

#if __STDC_VERSION__ >= 199901L
# define __FLEX_ARRAY_DIM	/*C99*/
#else
# define __FLEX_ARRAY_DIM	0
#endif

and then use __FLEX_ARRAY_DIM as needed (and keep the other "-1" changes
in the github commit):

 typedef struct {
     short ncount[FSE_MAX_SYMBOL_VALUE + 1];
-    FSE_DTable dtable[1]; /* Dynamically sized */
+    FSE_DTable dtable[__FLEX_ARRAY_DIM]; /* Dynamically sized */
 } FSE_DecompressWksp;
 

-- 
Kees Cook