cbfs: Allow controlling decompression of unverified files

This patch adds a new Kconfig that controls whether CBFS APIs for
unverified areas will allow file decompression when CBFS verification is
enabled. This should be disallowed by default because it exposes the
attack surface of all supported decompression algorithms. Make
allowances for one legacy use case with CONFIG_SOC_INTEL_CSE_LITE_
COMPRESS_ME_RW that should become obsolete with VBOOT_CBFS_INTEGRATION.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Ieae420f51cbc01dae2ab265414219cc9c288087b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75457
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/lib/Kconfig.cbfs_verification

@@ -37,6 +37,20 @@ config TOCTOU_SAFETY
 	  bootblock is also safe against these vulnerabilities (i.e. there's no
 	  point in enabling this when you just rely on flash write-protection).
 
+config CBFS_ALLOW_UNVERIFIED_DECOMPRESSION
+	bool "Run decompression algorithms on potentially untrusted code"
+	default n
+	help
+	  This controls whether cbfs_unverified_area_...() access functions may
+	  decompress files. This exposes the attack surface of all supported
+	  decompression algorithms. Even if you don't compress the files you are
+	  planning to load with these functions, since file metadata is also
+	  unverified, an attacker can potentially replace them with compressed
+	  files to access a vulnerability in the decompression code.
+
+	  If you don't need to load compressed files from unverified areas, say
+	  no here for tighter security.
+
 config CBFS_HASH_ALGO
 	int
 	default 1 if CBFS_HASH_SHA1

src/lib/cbfs.c

@@ -208,6 +208,13 @@ static size_t cbfs_load_and_decompress(const struct region_device *rdev, void *b
 	DEBUG("Decompressing %zu bytes from '%s' to %p with algo %d\n",
 	      in_size, mdata->h.filename, buffer, compression);
 
+	if (CONFIG(CBFS_VERIFICATION) && !CONFIG(CBFS_ALLOW_UNVERIFIED_DECOMPRESSION) &&
+	    skip_verification && compression != CBFS_COMPRESS_NONE) {
+		ERROR("Refusing to decompress unverified file '%s' with algo %d\n",
+		      mdata->h.filename, compression);
+		return 0;
+	}
+
 	switch (compression) {
 	case CBFS_COMPRESS_NONE:
 		if (buffer_size < in_size)

src/soc/intel/common/block/cse/Kconfig

@@ -223,6 +223,7 @@ config SOC_INTEL_CSE_LITE_COMPRESS_ME_RW
 	bool
 	default n
 	depends on SOC_INTEL_CSE_LITE_SKU
+	select CBFS_ALLOW_UNVERIFIED_DECOMPRESSION if CBFS_VERIFICATION && !VBOOT_CBFS_INTEGRATION
 	help
 	 Enable compression on Intel CSE CBFS RW blob