Date:   Wed, 18 Mar 2020 15:47:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Terrell <terrelln@...com>
Cc:     linux-kernel@...r.kernel.org, Chris Mason <clm@...com>,
        linux-kbuild@...r.kernel.org, x86@...nel.org,
        gregkh@...uxfoundation.org, Petr Malat <oss@...at.biz>,
        Kernel Team <Kernel-team@...com>,
        Nick Terrell <nickrterrell@...il.com>,
        Adam Borowski <kilobyte@...band.pl>,
        Patrick Williams <patrickw3@...com>, rmikey@...com,
        mingo@...nel.org
Subject: Re: [PATCH v2 3/7] lib: add zstd support to decompress

On Tue, Mar 17, 2020 at 05:22:59PM -0700, Nick Terrell wrote:
> * Add unzstd() and the zstd decompress interface.
> * Add zstd support to decompress_method().
> 
> The decompress_method() and unzstd() functions are used to decompress
> the initramfs and the initrd. The __decompress() function is used in
> the preboot environment to decompress a zstd compressed kernel.
> 
> The zstd decompression function allows the input and output buffers to
> overlap because that is used by x86 kernel decompression.
> 
> Signed-off-by: Nick Terrell <terrelln@...com>
> ---
>  include/linux/decompress/unzstd.h |  11 +
>  lib/Kconfig                       |   4 +
>  lib/Makefile                      |   1 +
>  lib/decompress.c                  |   5 +
>  lib/decompress_unzstd.c           | 337 ++++++++++++++++++++++++++++++
>  5 files changed, 358 insertions(+)
>  create mode 100644 include/linux/decompress/unzstd.h
>  create mode 100644 lib/decompress_unzstd.c
> 
> diff --git a/include/linux/decompress/unzstd.h b/include/linux/decompress/unzstd.h
> new file mode 100644
> index 000000000000..56d539ae880f
> --- /dev/null
> +++ b/include/linux/decompress/unzstd.h
> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef LINUX_DECOMPRESS_UNZSTD_H
> +#define LINUX_DECOMPRESS_UNZSTD_H
> +
> +int unzstd(unsigned char *inbuf, long len,
> +	   long (*fill)(void*, unsigned long),
> +	   long (*flush)(void*, unsigned long),
> +	   unsigned char *output,
> +	   long *pos,
> +	   void (*error_fn)(char *x));
> +#endif
> diff --git a/lib/Kconfig b/lib/Kconfig
> index bc7e56370129..11de5fa09a52 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -336,6 +336,10 @@ config DECOMPRESS_LZ4
>  	select LZ4_DECOMPRESS
>  	tristate
>  
> +config DECOMPRESS_ZSTD
> +	select ZSTD_DECOMPRESS
> +	tristate
> +
>  #
>  # Generic allocator support is selected if needed
>  #
> diff --git a/lib/Makefile b/lib/Makefile
> index 611872c06926..09ad45ba6883 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -160,6 +160,7 @@ lib-$(CONFIG_DECOMPRESS_LZMA) += decompress_unlzma.o
>  lib-$(CONFIG_DECOMPRESS_XZ) += decompress_unxz.o
>  lib-$(CONFIG_DECOMPRESS_LZO) += decompress_unlzo.o
>  lib-$(CONFIG_DECOMPRESS_LZ4) += decompress_unlz4.o
> +lib-$(CONFIG_DECOMPRESS_ZSTD) += decompress_unzstd.o
>  
>  obj-$(CONFIG_TEXTSEARCH) += textsearch.o
>  obj-$(CONFIG_TEXTSEARCH_KMP) += ts_kmp.o
> diff --git a/lib/decompress.c b/lib/decompress.c
> index 857ab1af1ef3..ab3fc90ffc64 100644
> --- a/lib/decompress.c
> +++ b/lib/decompress.c
> @@ -13,6 +13,7 @@
>  #include <linux/decompress/inflate.h>
>  #include <linux/decompress/unlzo.h>
>  #include <linux/decompress/unlz4.h>
> +#include <linux/decompress/unzstd.h>
>  
>  #include <linux/types.h>
>  #include <linux/string.h>
> @@ -37,6 +38,9 @@
>  #ifndef CONFIG_DECOMPRESS_LZ4
>  # define unlz4 NULL
>  #endif
> +#ifndef CONFIG_DECOMPRESS_ZSTD
> +# define unzstd NULL
> +#endif
>  
>  struct compress_format {
>  	unsigned char magic[2];
> @@ -52,6 +56,7 @@ static const struct compress_format compressed_formats[] __initconst = {
>  	{ {0xfd, 0x37}, "xz", unxz },
>  	{ {0x89, 0x4c}, "lzo", unlzo },
>  	{ {0x02, 0x21}, "lz4", unlz4 },
> +	{ {0x28, 0xb5}, "zstd", unzstd },
>  	{ {0, 0}, NULL, NULL }
>  };
>  
> diff --git a/lib/decompress_unzstd.c b/lib/decompress_unzstd.c
> new file mode 100644
> index 000000000000..f38eba5efe23
> --- /dev/null
> +++ b/lib/decompress_unzstd.c
> @@ -0,0 +1,337 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Important notes about in-place decompression
> + *
> + * At least on x86, the kernel is decompressed in place: the compressed data
> + * is placed to the end of the output buffer, and the decompressor overwrites
> + * most of the compressed data. There must be enough safety margin to
> + * guarantee that the write position is always behind the read position.
> + *
> + * The safety margin for ZSTD with a 128 KB block size is calculated below.
> + * Note that the margin with ZSTD is bigger than with GZIP or XZ!
> + *
> + * The worst case for in-place decompression is that the beginning of
> + * the file is compressed extremely well, and the rest of the file is
> + * uncompressible. Thus, we must look for worst-case expansion when the
> + * compressor is encoding uncompressible data.
> + *
> + * The structure of the .zst file in case of a compresed kernel is as follows.
> + * Maximum sizes (as bytes) of the fields are in parenthesis.
> + *
> + *    Frame Header: (18)
> + *    Blocks: (N)
> + *    Checksum: (4)
> + *
> + * The frame header and checksum overhead is at most 22 bytes.
> + *
> + * ZSTD stores the data in blocks. Each block has a header whose size is
> + * a 3 bytes. After the block header, there is up to 128 KB of payload.
> + * The maximum uncompressed size of the payload is 128 KB. The minimum
> + * uncompressed size of the payload is never less than the payload size
> + * (excluding the block header).
> + *
> + * The assumption, that the uncompressed size of the payload is never
> + * smaller than the payload itself, is valid only when talking about
> + * the payload as a whole. It is possible that the payload has parts where
> + * the decompressor consumes more input than it produces output. Calculating
> + * the worst case for this would be tricky. Instead of trying to do that,
> + * let's simply make sure that the decompressor never overwrites any bytes
> + * of the payload which it is currently reading.
> + *
> + * Now we have enough information to calculate the safety margin. We need
> + *   - 22 bytes for the .zst file format headers;
> + *   - 3 bytes per every 128 KiB of uncompressed size (one block header per
> + *     block); and
> + *   - 128 KiB (biggest possible zstd block size) to make sure that the
> + *     decompressor never overwrites anything from the block it is currently
> + *     reading.
> + *
> + * We get the following formula:
> + *
> + *    safety_margin = 22 + uncompressed_size * 3 / 131072 + 131072
> + *                 <= 22 + (uncompressed_size >> 15) + 131072
> + */
> +
> +#ifdef STATIC
> +	/*
> +	 * Preboot environments #include "path/to/decompress_unzstd.c".
> +	 * All of the source files we depend on must be #included.
> +	 * zstd's only source dependeny is xxhash, which has no source
> +	 * dependencies.
> +	 *
> +	 * zstd and xxhash both avoid declaring themselves as modules
> +	 * when PREBOOT is defined.
> +	 */
> +#	define PREBOOT

Can you use ZSTD_PREBOOT for this? Other decompressors namespace this
define.

(And a style nit: I'd put the comment above the #ifdef STATIC and use a
single space, not a tab, for the "indented" #-lines)

> +#	include "xxhash.c"
> +#	include "zstd/entropy_common.c"
> +#	include "zstd/fse_decompress.c"
> +#	include "zstd/huf_decompress.c"
> +#	include "zstd/zstd_common.c"
> +#	include "zstd/decompress.c"
> +#endif
> +
> +#include <linux/decompress/mm.h>
> +#include <linux/kernel.h>
> +#include <linux/zstd.h>
> +
> +/* 8 MB maximum window size */
> +#define ZSTD_WINDOWSIZE_MAX	(1 << 23)
> +/* Size of the input and output buffers in multi-call mdoe */
> +#define ZSTD_IOBUF_SIZE		4096
> +
> +static int INIT handle_zstd_error(size_t ret, void (*error)(char *x))
> +{
> +	const int err = ZSTD_getErrorCode(ret);
> +
> +	if (!ZSTD_isError(ret))
> +		return 0;
> +
> +	switch (err) {
> +	case ZSTD_error_memory_allocation:
> +		error("ZSTD decompressor ran out of memory");
> +		break;
> +	case ZSTD_error_prefix_unknown:
> +		error("Input is not in the ZSTD format (wrong magic bytes)");
> +		break;
> +	case ZSTD_error_dstSize_tooSmall:
> +	case ZSTD_error_corruption_detected:
> +	case ZSTD_error_checksum_wrong:
> +		error("ZSTD-compressed data is corrupt");
> +		break;
> +	default:
> +		error("ZSTD-compressed data is probably corrupt");
> +		break;
> +	}
> +	return -1;
> +}
> +
> +/*
> + * Handle the case where we have the entire input and output in one segment.
> + * We can allocate less memory (no circular buffer for the sliding window),
> + * and avoid some memcpy() calls.
> + */
> +static int INIT decompress_single(const u8 *in_buf, long in_len, u8 *out_buf,
> +				  long out_len, long *in_pos,
> +				  void (*error)(char *x))
> +{
> +	const size_t wksp_size = ZSTD_DCtxWorkspaceBound();
> +	void *wksp = large_malloc(wksp_size);
> +	ZSTD_DCtx *dctx = ZSTD_initDCtx(wksp, wksp_size);
> +	int err;
> +	size_t ret;
> +
> +	if (dctx == NULL) {
> +		error("Out of memory while allocating ZSTD_DCtx");
> +		err = -1;
> +		goto out;
> +	}
> +	/*
> +	 * Find out how large the frame actually is, there may be junk at
> +	 * the end of the frame that ZSTD_decompressDCtx() can't handle.
> +	 */
> +	ret = ZSTD_findFrameCompressedSize(in_buf, in_len);
> +	err = handle_zstd_error(ret, error);
> +	if (err)
> +		goto out;
> +	in_len = (long)ret;
> +
> +	ret = ZSTD_decompressDCtx(dctx, out_buf, out_len, in_buf, in_len);
> +	err = handle_zstd_error(ret, error);
> +	if (err)
> +		goto out;
> +
> +	if (in_pos != NULL)
> +		*in_pos = in_len;
> +
> +	err = 0;
> +out:
> +	if (wksp != NULL)
> +		large_free(wksp);
> +	return err;
> +}
> +
> +static int INIT __unzstd(unsigned char *in_buf, long in_len,
> +			 long (*fill)(void*, unsigned long),
> +			 long (*flush)(void*, unsigned long),
> +			 unsigned char *out_buf, long out_len,
> +			 long *in_pos,
> +			 void (*error)(char *x))
> +{
> +	ZSTD_inBuffer in;
> +	ZSTD_outBuffer out;
> +	ZSTD_frameParams params;
> +	void *in_allocated = NULL;
> +	void *out_allocated = NULL;
> +	void *wksp = NULL;
> +	size_t wksp_size;
> +	ZSTD_DStream *dstream;
> +	int err;
> +	size_t ret;
> +
> +	if (out_len == 0)
> +		out_len = LONG_MAX; /* no limit */
> +
> +	if (fill == NULL && flush == NULL)
> +		/*
> +		 * We can decompress faster and with less memory when we have a
> +		 * single chunk.
> +		 */
> +		return decompress_single(in_buf, in_len, out_buf, out_len,
> +					 in_pos, error);
> +
> +	/*
> +	 * If in_buf is not provided, we must be using fill(), so allocate
> +	 * a large enough buffer. If it is provided, it must be at least
> +	 * ZSTD_IOBUF_SIZE large.
> +	 */
> +	if (in_buf == NULL) {
> +		in_allocated = malloc(ZSTD_IOBUF_SIZE);
> +		if (in_allocated == NULL) {
> +			error("Out of memory while allocating input buffer");
> +			err = -1;
> +			goto out;
> +		}
> +		in_buf = in_allocated;
> +		in_len = 0;
> +	}
> +	/* Read the first chunk, since we need to decode the frame header. */
> +	if (fill != NULL)
> +		in_len = fill(in_buf, ZSTD_IOBUF_SIZE);
> +	if (in_len < 0) {
> +		error("ZSTD-compressed data is truncated");
> +		err = -1;
> +		goto out;
> +	}
> +	/* Set the first non-empty input buffer. */
> +	in.src = in_buf;
> +	in.pos = 0;
> +	in.size = in_len;
> +	/* Allocate the output buffer if we are using flush(). */
> +	if (flush != NULL) {
> +		out_allocated = malloc(ZSTD_IOBUF_SIZE);
> +		if (out_allocated == NULL) {
> +			error("Out of memory while allocating output buffer");
> +			err = -1;
> +			goto out;
> +		}
> +		out_buf = out_allocated;
> +		out_len = ZSTD_IOBUF_SIZE;
> +	}
> +	/* Set the output buffer. */
> +	out.dst = out_buf;
> +	out.pos = 0;
> +	out.size = out_len;
> +
> +	/*
> +	 * We need to know the window size to allocate the ZSTD_DStream.
> +	 * Since we are streaming, we need to allocate a buffer for the sliding
> +	 * window. The window size varies from 1 KB to ZSTD_WINDOWSIZE_MAX
> +	 * (8 MB), so it is important to use the actual value so as not to
> +	 * waste memory when it is smaller.
> +	 */
> +	ret = ZSTD_getFrameParams(&params, in.src, in.size);
> +	err = handle_zstd_error(ret, error);
> +	if (err)
> +		goto out;
> +	if (ret != 0) {
> +		error("ZSTD-compressed data has an incomplete frame header");
> +		err = -1;
> +		goto out;
> +	}
> +	if (params.windowSize > ZSTD_WINDOWSIZE_MAX) {
> +		error("ZSTD-compressed data has too large a window size");
> +		err = -1;
> +		goto out;
> +	}
> +
> +	/*
> +	 * Allocate the ZSTD_DStream now that we know how much memory is
> +	 * required.
> +	 */
> +	wksp_size = ZSTD_DStreamWorkspaceBound(params.windowSize);
> +	wksp = large_malloc(wksp_size);
> +	dstream = ZSTD_initDStream(params.windowSize, wksp, wksp_size);
> +	if (dstream == NULL) {
> +		error("Out of memory while allocating ZSTD_DStream");
> +		err = -1;
> +		goto out;
> +	}
> +
> +	/*
> +	 * Decompression loop:
> +	 * Read more data if necessary (error if no more data can be read).
> +	 * Call the decompression function, which returns 0 when finished.
> +	 * Flush any data produced if using flush().
> +	 */
> +	if (in_pos != NULL)
> +		*in_pos = 0;
> +	do {
> +		/*
> +		 * If we need to reload data, either we have fill() and can
> +		 * try to get more data, or we don't and the input is truncated.
> +		 */
> +		if (in.pos == in.size) {
> +			if (in_pos != NULL)
> +				*in_pos += in.pos;
> +			in_len = fill ? fill(in_buf, ZSTD_IOBUF_SIZE) : -1;
> +			if (in_len < 0) {
> +				error("ZSTD-compressed data is truncated");
> +				err = -1;
> +				goto out;
> +			}
> +			in.pos = 0;
> +			in.size = in_len;
> +		}
> +		/* Returns zero when the frame is complete. */
> +		ret = ZSTD_decompressStream(dstream, &out, &in);
> +		err = handle_zstd_error(ret, error);
> +		if (err)
> +			goto out;
> +		/* Flush all of the data produced if using flush(). */
> +		if (flush != NULL && out.pos > 0) {
> +			if (out.pos != flush(out.dst, out.pos)) {
> +				error("Failed to flush()");
> +				err = -1;
> +				goto out;
> +			}
> +			out.pos = 0;
> +		}
> +	} while (ret != 0);
> +
> +	if (in_pos != NULL)
> +		*in_pos += in.pos;
> +
> +	err = 0;
> +out:
> +	if (in_allocated != NULL)
> +		free(in_allocated);
> +	if (out_allocated != NULL)
> +		free(out_allocated);
> +	if (wksp != NULL)
> +		large_free(wksp);
> +	return err;
> +}
> +
> +#ifndef PREBOOT
> +STATIC int INIT unzstd(unsigned char *buf, long len,
> +		       long (*fill)(void*, unsigned long),
> +		       long (*flush)(void*, unsigned long),
> +		       unsigned char *out_buf,
> +		       long *pos,
> +		       void (*error)(char *x))
> +{
> +	return __unzstd(buf, len, fill, flush, out_buf, 0, pos, error);
> +}
> +#else
> +STATIC int INIT __decompress(unsigned char *buf, long len,
> +			     long (*fill)(void*, unsigned long),
> +			     long (*flush)(void*, unsigned long),
> +			     unsigned char *out_buf, long out_len,
> +			     long *pos,
> +			     void (*error)(char *x))
> +{
> +	return __unzstd(buf, len, fill, flush, out_buf, out_len, pos, error);
> +}
> +#endif
> -- 
> 2.25.1
> 

-- 
Kees Cook

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