[PATCH] Fill in ELF image holes with PROT_NONE to prevent mapping to the hole

[blackzert]

From: Ilya Smith <blackzert@xxxxxxxxx>

Hello,

In the current code, when loading an ELF file with a hole, the loader leaves an
unmapped memory region within the loaded ELF file. The unmapped memory region
can be used to bypass ASLR. The ASLR can be bypassed as follows:

1. Allocate memory using an mmap system call whose size is smaller or equal to
the hole size.

2. Use the obtained address to find loaded libraries and their data.

To avoid exploitation of ELF images with holes, a hole can be mapped with
PROT_NONE permissions.

By the way, the GNU C Library maps such a hole with PROT_NONE.


Example application:

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>

int main() {
	long len;
	long offset;
	unsigned long value;
	char c;
	printf("This is a simple example how to use hole in ld to bypass aslr\n");
	printf("First we get length of array\n");
	scanf("%"SCNu64,&len);
	unsigned long *ptr = malloc(len);
	printf("%p\n", ptr);

	while (1) {
		printf("Now lets use this array to read/write values. Read or Write?\n");

		scanf("%c", &c);
		switch(c) {
			case 'r':
				scanf("%"SCNi64, &offset);
				if (offset > len)
					printf("too big\n");
				else
					printf("%llx\n", *(unsigned long *)((char*)ptr + offset));
			break;
			case 'w':
				scanf("%"SCNi64" %"SCNi64, &offset, &value);
				if (offset > len)
					printf("too big\n");
				else
					ptr[offset] = value;
			break;
			case '\n':
				break;
			default:
				goto finish;
			break;
		}
	}

finish:
	free(ptr);
	printf("Bye.\n");
}

Example of usage:

$ gcc -g ld-hole.c
$ ./a.out 
This is a simple example how to use hole in ld to bypass aslr
First we get length of array
1341500
0x7ff1ade24010
Now lets use this array to read/write values. Read or Write?
Now lets use this array to read/write values. Read or Write?
r
-4722704
3010102464c457f

Last output value is ELF header of libc.

Signed-off-by: Ilya Smith <blackzert@xxxxxxxxx>
---
 fs/binfmt_elf.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 879ff9c..99cb121 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -346,6 +346,7 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
 		unsigned long total_size)
 {
 	unsigned long map_addr;
+	unsigned long map_none_addr;
 	unsigned long size = eppnt->p_filesz + ELF_PAGEOFFSET(eppnt->p_vaddr);
 	unsigned long off = eppnt->p_offset - ELF_PAGEOFFSET(eppnt->p_vaddr);
 	addr = ELF_PAGESTART(addr);
@@ -361,14 +362,22 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
 	* The _first_ mmap needs to know the full size, otherwise
 	* randomization might put this image into an overlapping
 	* position with the ELF binary image. (since size < total_size)
-	* So we first map the 'big' image - and unmap the remainder at
-	* the end. (which unmap is needed for ELF images with holes.)
+	* So we first map the 'big' image - and remmap the remainder at
+	* the end with PROT_NONE. (which remmap is needed for ELF images
+	* with holes.)
 	*/
 	if (total_size) {
 		total_size = ELF_PAGEALIGN(total_size);
 		map_addr = vm_mmap(filep, addr, total_size, prot, type, off);
-		if (!BAD_ADDR(map_addr))
-			vm_munmap(map_addr+size, total_size-size);
+		if (!BAD_ADDR(map_addr)) {
+			map_none_addr =
+				vm_mmap(NULL, map_addr + size,
+					total_size - size, PROT_NONE,
+					MAP_PRIVATE | MAP_ANONYMOUS |
+					MAP_FIXED, off);
+			if (BAD_ADDR(map_none_addr))
+				return map_none_addr;
+		}
 	} else
 		map_addr = vm_mmap(filep, addr, size, prot, type, off);
 
-- 
2.7.4