[PATCH 81/82] lib: zstd: Refactor intentional wrap-around test

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Switch to a more regular type for a 64-bit value and refactor the
open-coded wrap-around addition test to use subtraction from the type max
(since add_would_overflow() may not be defined in early boot code). This
paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Nick Terrell <terrelln@xxxxxx>
Cc: Paul Jones <paul@xxxxxxxxxxxxxxx>
Cc: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Cc: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
Cc: Xin Gao <gaoxin@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 lib/zstd/decompress/zstd_decompress.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/zstd/decompress/zstd_decompress.c b/lib/zstd/decompress/zstd_decompress.c
index 6b3177c94711..2c87cf702ad6 100644
--- a/lib/zstd/decompress/zstd_decompress.c
+++ b/lib/zstd/decompress/zstd_decompress.c
@@ -585,7 +585,7 @@ ZSTDLIB_API size_t ZSTD_readSkippableFrame(void* dst, size_t dstCapacity, unsign
  *  @return : decompressed size of the frames contained */
 unsigned long long ZSTD_findDecompressedSize(const void* src, size_t srcSize)
 {
-    unsigned long long totalDstSize = 0;
+    U64 totalDstSize = 0;
 
     while (srcSize >= ZSTD_startingInputLength(ZSTD_f_zstd1)) {
         U32 const magicNumber = MEM_readLE32(src);
@@ -606,7 +606,7 @@ unsigned long long ZSTD_findDecompressedSize(const void* src, size_t srcSize)
             if (ret >= ZSTD_CONTENTSIZE_ERROR) return ret;
 
             /* check for overflow */
-            if (totalDstSize + ret < totalDstSize) return ZSTD_CONTENTSIZE_ERROR;
+            if (U64_MAX - totalDstSize < ret) return ZSTD_CONTENTSIZE_ERROR;
             totalDstSize += ret;
         }
         {   size_t const frameSrcSize = ZSTD_findFrameCompressedSize(src, srcSize);
-- 
2.34.1