[PATCH] kexec: restore functionality to preserve fd xattrs for uncompressed kernel images

[Alejandro Hernandez <alhe@linux.microsoft.com>]

Since commit 714fa115 xattrs attached to a kernel image file (such as IMA
signatures) are not being preserved, do_kexec_file_load() was modified to use
memfd_create() which now passes an anonymous file descriptor to
kexec_file_load(). This change eliminated the filesystem inode identity of the
original kernel file, hence attributes are no longer visible to the kernel IMA
appraisal handler during kexec_file_load, causing IMA policy enforcement to fail
even for validly signed kernel images.

This patch attempts to restore such behavior although, only does it for
uncompressed kernel images. To do this, we first figure out if the image file is
compressed or not and call each method accordingly.

Compressed images continue to use memfd and cannot carry forward the original fd
security attributes, since their decompressed bytes do not match the signed
artifact. Proper handling of compressed images with IMA would require either
signing the decompressed artifact or kernel-side support for decompression
within the kexec_file_load path.

Signed-off-by: Alejandro Hernandez Samaniego <alhe@linux.microsoft.com>
---
 kexec/kexec.c | 42 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 7 deletions(-)

diff --git a/kexec/kexec.c b/kexec/kexec.c
index 08edfca..a388762 100644
--- a/kexec/kexec.c
+++ b/kexec/kexec.c
@@ -1328,17 +1328,41 @@ static int do_kexec_file_load(int fileind, int argc, char **argv,
 
 	kernel = argv[fileind];
 
-	/* slurp in the input kernel */
+        /* Hold original fd with its xattrs */
+	kernel_fd = open(kernel, O_RDONLY);
+	if (kernel_fd == -1) {
+		fprintf(stderr, "Failed to open file %s:%s\n", kernel,
+				strerror(errno));
+		return EFAILED;
+	}
+
+	/* Compressed vs Uncompressed */
+	struct stat kernel_stat;
+	off_t kernel_file_size = -1;
+	if (fstat(kernel_fd, &kernel_stat) == 0)
+		kernel_file_size = kernel_stat.st_size;
+
+        /* slurp in the input kernel */
 	kernel_buf = slurp_decompress_file(kernel, &kernel_size);
 	if (!kernel_buf) {
 		fprintf(stderr, "Failed to decompress file %s:%s\n", kernel,
 				strerror(errno));
+		close(kernel_fd);
 		return EFAILED;
 	}
-	kernel_fd = copybuf_memfd(kernel_buf, kernel_size);
-	if (kernel_fd < 0) {
-		fprintf(stderr, "Failed to copy decompressed buf\n");
-		return EFAILED;
+
+	if (kernel_file_size != kernel_size) {
+		close(kernel_fd);
+		kernel_fd = copybuf_memfd(kernel_buf, kernel_size);
+		if (kernel_fd < 0) {
+			fprintf(stderr, "Failed to copy decompressed buf\n");
+			return EFAILED;
+		}
+		dbgprintf("%s: compressed input, using memfd kernel fd %d\n",
+			  __func__, kernel_fd);
+	} else {
+		dbgprintf("%s: using original kernel file fd %d\n",
+			  __func__, kernel_fd);
 	}
 
 	for (i = 0; i < file_types; i++) {
@@ -1361,11 +1385,15 @@ static int do_kexec_file_load(int fileind, int argc, char **argv,
 	}
 
        /*
-	* image type specific load functioin detect the capsule kernel type
+	* image type specific load function detect the capsule kernel type
 	* and create another fd for file load. For example the zboot kernel.
 	*/
-	if (info.kernel_fd != -1)
+	if (info.kernel_fd != -1) {
+		dbgprintf("%s: using image-specific kernel fd %d\n",
+			  __func__, info.kernel_fd);
+		close(kernel_fd);
 		kernel_fd = info.kernel_fd;
+	}
 
 	/*
 	 * If there is no initramfs, set KEXEC_FILE_NO_INITRAMFS flag so that
-- 
2.43.0