[PATCH v7 06/33] arm64: kernel: Add relocation check to code built under pi/

[Ard Biesheuvel]

The mini C runtime runs before relocations are processed, and so it
cannot rely on statically initialized pointer variables.

Add a check to ensure that such code does not get introduced by
accident, by going over the relocations in each object that operate on
data sections that are part of the executable image, and raising an
error if any relocations of type R_AARCH64_ABS64 exist. Note that such
relocations are permitted in other places (e.g., debug section) and can
never occur in compiler generated code sections, so only check sections
that have SHF_ALLOC set and SHF_EXECINSTR cleared.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/pi/Makefile    |   9 +-
 arch/arm64/kernel/pi/relacheck.c | 104 ++++++++++++++++++++
 2 files changed, 111 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/pi/Makefile b/arch/arm64/kernel/pi/Makefile
index c844a0546d7f0e62..810fdae897601e88 100644
--- a/arch/arm64/kernel/pi/Makefile
+++ b/arch/arm64/kernel/pi/Makefile
@@ -22,11 +22,16 @@ KCSAN_SANITIZE	:= n
 UBSAN_SANITIZE	:= n
 KCOV_INSTRUMENT	:= n
 
+hostprogs	:= relacheck
+
+quiet_cmd_piobjcopy = $(quiet_cmd_objcopy)
+      cmd_piobjcopy = $(obj)/relacheck $< && $(cmd_objcopy)
+
 $(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_ \
 			       --remove-section=.note.gnu.property \
 			       --prefix-alloc-sections=.init
-$(obj)/%.pi.o: $(obj)/%.o FORCE
-	$(call if_changed,objcopy)
+$(obj)/%.pi.o: $(obj)/%.o $(obj)/relacheck FORCE
+	$(call if_changed,piobjcopy)
 
 $(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
 	$(call if_changed_rule,cc_o_c)
diff --git a/arch/arm64/kernel/pi/relacheck.c b/arch/arm64/kernel/pi/relacheck.c
new file mode 100644
index 0000000000000000..1039259360c735d2
--- /dev/null
+++ b/arch/arm64/kernel/pi/relacheck.c
@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022 - Google LLC
+ * Author: Ard Biesheuvel <ardb at google.com>
+ */
+
+#include <elf.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define HOST_ORDER ELFDATA2LSB
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define HOST_ORDER ELFDATA2MSB
+#endif
+
+static bool swap;
+
+static uint64_t swab_elfxword(uint64_t val)
+{
+	return swap ? __builtin_bswap64(val) : val;
+}
+
+static Elf64_Ehdr *ehdr;
+static Elf64_Shdr *shdr;
+
+static uint32_t swab_elfword(uint32_t val)
+{
+	return swap ? __builtin_bswap32(val) : val;
+}
+
+static uint16_t swab_elfhword(uint16_t val)
+{
+	return swap ? __builtin_bswap16(val) : val;
+}
+
+int main(int argc, char *argv[])
+{
+	struct stat stat;
+	int fd, ret;
+
+	if (argc < 2) {
+		fprintf(stderr, "file argument missing\n");
+		exit(EXIT_FAILURE);
+	}
+
+	fd = open(argv[1], O_RDWR);
+	if (fd < 0) {
+		fprintf(stderr, "failed to open %s\n", argv[1]);
+		exit(EXIT_FAILURE);
+	}
+
+	ret = fstat(fd, &stat);
+	if (ret < 0) {
+		fprintf(stderr, "failed to stat() %s\n", argv[1]);
+		exit(EXIT_FAILURE);
+	}
+
+	ehdr = mmap(0, stat.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (ehdr == MAP_FAILED) {
+		fprintf(stderr, "failed to mmap() %s\n", argv[1]);
+		exit(EXIT_FAILURE);
+	}
+
+	swap = ehdr->e_ident[EI_DATA] != HOST_ORDER;
+	shdr = (void *)ehdr + swab_elfxword(ehdr->e_shoff);
+
+	for (int i = 0; i < swab_elfhword(ehdr->e_shnum); i++) {
+		unsigned long info, flags;
+		const Elf64_Rela *rela;
+		int numrela;
+
+		if (swab_elfword(shdr[i].sh_type) != SHT_RELA)
+			continue;
+
+		/* only consider RELA sections operating on data */
+		info = swab_elfword(shdr[i].sh_info);
+		flags = swab_elfxword(shdr[info].sh_flags);
+		if ((flags & (SHF_ALLOC | SHF_EXECINSTR)) != SHF_ALLOC)
+			continue;
+
+		rela = (void *)ehdr + swab_elfxword(shdr[i].sh_offset);
+		numrela = swab_elfxword(shdr[i].sh_size) / sizeof(*rela);
+
+		for (int j = 0; j < numrela; j++) {
+			uint64_t info = swab_elfxword(rela[j].r_info);
+
+			if (ELF64_R_TYPE(info) == R_AARCH64_ABS64) {
+				fprintf(stderr,
+					"Absolute relocations detected in %s\n",
+					argv[1]);
+				exit(EXIT_FAILURE);
+			}
+		}
+	}
+	return 0;
+}
-- 
2.35.1