Linux kernel EFI stub bug?

Michael Brown
2014-07-09T15:31:12Z

I think I've found a bug in the kernel's EFI boot stub. Specifically,
in arch/x86/boot/compressed/eboot.c, in make_boot_params():

sys_table = (efi_system_table_t *)(unsigned long)efi_early->table;

This compiles and links (on my system) to

mov %rdi,0xe658(%rip) # 0x3d16f0

The problem is that address 0x3d16f0 is beyond the end of the loaded
kernel image:

objdump -x shows:

Sections:
Idx Name Size VMA LMA File off Algn
0 .setup 000041e0 0000000000000200 0000000000000200 00000200 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .reloc 00000020 00000000000043e0 00000000000043e0 000043e0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .text 003c0e90 0000000000004400 0000000000004400 00004400 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE

giving an image end address of 0x4400+0x3c0e90=0x3c5290 (which matches
the size of the bzImage file).

The upshot is that the kernel writes beyond the end of allocated memory,
producing undefined behaviour. (In my test case, it ends up corrupting
the initramfs image, resulting in an unbootable kernel.)

The same problem seems to exist in efi_main():

sys_table = _table;

mov %rdi,0xe1bb(%rip) # 0x3d16f0

As far as I can tell, the underlying problem is that .bss variables in
eboot.o end up with addresses beyond the end of the loaded kernel.

Any ideas?

Michael