ELF32: stack canary

main.c

@@ -33,11 +33,17 @@
 #include <string.h>
 #include <stdbool.h>
 
+// Data structures as per ELF specification:
+// https://refspecs.linuxfoundation.org/elf/elf.pdf
+// Only what we need (no elf.h)
+
 #define MAX_FILENAME   256
 #define EI_NIDENT      16
 #define MAX_HEADER_LEN 2048
 #define PF_X           1
 #define PT_GNU_STACK   0x6474e551
+#define SHT_DYNSYM     11
+#define SHT_SYMTAB     2
 
 #define Elf32_Half uint16_t
 #define Elf32_Word uint32_t
@@ -52,6 +58,7 @@
 bool verbose = false;
 bool pie = false;
 bool nx = false;
+bool canary = false;
 
 struct Elf32_Ehdr
 {
@@ -83,6 +90,30 @@ struct Elf32_Phdr
 	Elf32_Word p_align;
 };
 
+struct Elf32_Shdr
+{
+	Elf32_Word sh_name;
+	Elf32_Word sh_type;
+	Elf32_Word sh_flags;
+	Elf32_Addr sh_addr;
+	Elf32_Off sh_offset;
+	Elf32_Word sh_size;
+	Elf32_Word sh_link;
+	Elf32_Word sh_info;
+	Elf32_Word sh_addralign;
+	Elf32_Word sh_entsize;
+};
+
+struct Elf32_Sym
+{
+	Elf32_Word st_name;
+	Elf32_Addr st_value;
+	Elf32_Word st_size;
+	unsigned char st_info;
+	unsigned char st_other;
+	Elf32_Half st_shndx;
+};
+
 struct Elf64_Ehdr
 {
 	unsigned char e_ident[EI_NIDENT];
@@ -155,29 +186,35 @@ void display_elf32(struct Elf32_Ehdr* header)
 	printf("32-bit ELF ");
 	display_elf_common(header->e_ident);
 
-	switch (header->e_type)
+	if (verbose)
 	{
-		case 0x00:
+		switch (header->e_type)
-			printf("(no file type),");
+		{
-			break;
+			case 0x00:
-		case 0x01:
+				printf("(no file type),");
-			printf("(relocatable file),");
+				break;
-			break;
+			case 0x01:
-		case 0x02:
+				printf("(relocatable file),");
-			printf("(executable file),");
+				break;
-			break;
+			case 0x02:
-		case 0x03: // (ET_DYN, prob means PIE)
+				printf("(executable file),");
-			printf("(shared object file),");
+				break;
-			pie = true;
+			case 0x03: // (ET_DYN, prob means PIE)
-			break;
+				printf("(shared object file),");
-		case 0x04:
+				pie = true;
-			printf("(core file),");
+				break;
-			break;
+			case 0x04:
-		default:
+				printf("(core file),");
-			printf("(unknown),");
+				break;
-			break;
+			default:
+				printf("(unknown),");
+				break;
+		}
+		printf(" ");
+	} else if (header->e_type == 0x03)
+	{
+		pie = true;
 	}
-	printf(" ");
 
 	switch (header->e_machine)
 	{
@@ -231,13 +268,13 @@ void display_elf32(struct Elf32_Ehdr* header)
 
 void check_sec32(struct Elf32_Ehdr* header, FILE* fp)
 {
-	fseek(fp, header->e_phoff, SEEK_SET);
 
 	/*
 	 * Browse thru program header table (e_phnum items) from e_phoff (program header offset)
 	 * if its stack and has executable bit then we know NX isnt there
 	*/
-	for (size_t i=0; i < header->e_phnum; i++)
+	fseek(fp, header->e_phoff, SEEK_SET);
+	for (size_t i=0; i<header->e_phnum; i++)
 	{
 		struct Elf32_Phdr p_header;
 		fread (&p_header, 1, sizeof(struct Elf32_Phdr), fp);
@@ -253,6 +290,51 @@ void check_sec32(struct Elf32_Ehdr* header, FILE* fp)
 			}
 		}
 	}
+
+	/*
+	 * Same for section header table (e_shnum items) from e_shoff (section header offset)
+	 * find sections .dynsym or .symtab:
+	 * and look for __stack_chk_fail (call for stack smashing)
+	*/
+	fseek(fp, header->e_shoff, SEEK_SET);
+
+	// Read section headers
+	struct Elf32_Shdr *s_headers = malloc(header->e_shnum * sizeof(struct Elf32_Shdr));
+	fread(s_headers, sizeof(struct Elf32_Shdr), header->e_shnum, fp);
+
+	// Find section name string table (at e_shstrndx) and read it
+	struct Elf32_Shdr s_header_str = s_headers[header->e_shstrndx];
+	char* s_header_strtab = malloc(s_header_str.sh_size);
+	fseek(fp, s_header_str.sh_offset, SEEK_SET);
+	fread(s_header_strtab, 1, s_header_str.sh_size, fp);
+
+	for (size_t i=0; i<header->e_shnum; i++)
+	{
+		if (s_headers[i].sh_type == SHT_DYNSYM || s_headers[i].sh_type == SHT_SYMTAB)
+		{
+			// load symbol table
+			unsigned int nsyms = s_headers[i].sh_size / s_headers[i].sh_entsize;
+			struct Elf32_Sym *syms = malloc(s_headers[i].sh_size);
+			fseek(fp, s_headers[i].sh_offset, SEEK_SET);
+			fread(syms, s_headers[i].sh_size, 1, fp);
+
+			// load string table
+			struct Elf32_Shdr str_header = s_headers[s_headers[i].sh_link];
+			char* strtab = malloc(str_header.sh_size);
+			fseek(fp, str_header.sh_offset, SEEK_SET);
+			fread(strtab, 1, str_header.sh_size, fp);
+
+			// iterate through symbols, check for __stack_chk_fail
+			for (size_t j=0; j<nsyms; j++)
+			{
+				const char* name = strtab + syms[j].st_name;
+				if (strcmp(name, "__stack_chk_fail") == 0)
+				{
+					canary = true;	
+				}
+			}
+		}
+	}
 }
 
 void display_elf64(struct Elf64_Ehdr* header)
@@ -274,6 +356,7 @@ void display_sec_common()
 {
 	printf(" %s, ", nx ? "\x1b[32mNX enabled\x1b[0m" : "\x1b[31mNX disabled\x1b[0m");
 	printf("%s, ", pie ? "\x1b[32mPIE enabled\x1b[0m" : "\x1b[31mPIE disabled\x1b[0m");
+	printf("%s\n", canary ? "\x1b[32mStack canary enabled\x1b[0m" : "\x1b[31mStack canary disabled\x1b[0m");
 }
 
 int main(int argc, char* argv[])

makefile

@@ -6,7 +6,7 @@ install:
 
 test:
 	make
-	gcc -m32 main.c
+	gcc -m32 -fstack-protector main.c
 	./helpelf a.out
 
 clean: