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x86/boot: Remove run-time relocations from .head.text code

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The assembly code in head_{32,64}.S, while meant to be
position-independent, generates run-time relocations because it uses
instructions such as:

	leal	gdt(%edx), %eax

which make the assembler and linker think that the code is using %edx as
an index into gdt, and hence gdt needs to be relocated to its run-time
address.

On 32-bit, with lld Dmitry Golovin reports that this results in a
link-time error with default options (i.e. unless -z notext is
explicitly passed):

  LD      arch/x86/boot/compressed/vmlinux
  ld.lld: error: can't create dynamic relocation R_386_32 against local
  symbol in readonly segment; recompile object files with -fPIC or pass
  '-Wl,-z,notext' to allow text relocations in the output

With the BFD linker, this generates a warning during the build, if
--warn-shared-textrel is enabled, which at least Gentoo enables by
default:

  LD      arch/x86/boot/compressed/vmlinux
  ld: arch/x86/boot/compressed/head_32.o: warning: relocation in read-only section `.head.text'
  ld: warning: creating a DT_TEXTREL in object

On 64-bit, it is not possible to link the kernel as -pie with lld, and
it is only possible with a BFD linker that supports -z noreloc-overflow,
i.e. versions >2.26. This is because these instructions cannot really be
relocated: the displacement field is only 32-bits wide, and thus cannot
be relocated for a 64-bit load address. The -z noreloc-overflow option
simply overrides the linker error, and results in R_X86_64_RELATIVE
relocations that apply a 64-bit relocation to a 32-bit field anyway.
This happens to work because nothing will process these run-time
relocations.

Start fixing this by removing relocations from .head.text:

- On 32-bit, use a base register that holds the address of the GOT and
  reference symbol addresses using @GOTOFF, i.e.
	leal	gdt@GOTOFF(%edx), %eax

- On 64-bit, most of the code can (and already does) use %rip-relative
  addressing, however the .code32 bits can't, and the 64-bit code also
  needs to reference symbol addresses as they will be after moving the
  compressed kernel to the end of the decompression buffer.
  For these cases, reference the symbols as an offset to startup_32 to
  avoid creating relocations, i.e.:

	leal	(gdt-startup_32)(%bp), %eax

  This only works in .head.text as the subtraction cannot be represented
  as a PC-relative relocation unless startup_32 is in the same section
  as the code. Move efi32_pe_entry into .head.text so that it can use
  the same method to avoid relocations.

Reported-by: Dmitry Golovin <dima@golovin.in>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Link: https://lore.kernel.org/r/20200731230820.1742553-6-keescook@chromium.org
This commit is contained in:
Arvind Sankar 2020-07-31 16:07:49 -07:00 committed by Ingo Molnar
parent 2e7a858ba8
commit a2c4fc4d4e
2 changed files with 90 additions and 78 deletions
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64
arch/x86/boot/compressed/head_32.S
View File

@ -33,26 +33,10 @@
#include <asm/bootparam.h>
/*
* The 32-bit x86 assembler in binutils 2.26 will generate R_386_GOT32X
* relocation to get the symbol address in PIC. When the compressed x86
* kernel isn't built as PIC, the linker optimizes R_386_GOT32X
* relocations to their fixed symbol addresses. However, when the
* compressed x86 kernel is loaded at a different address, it leads
* to the following load failure:
*
* Failed to allocate space for phdrs
*
* during the decompression stage.
*
* If the compressed x86 kernel is relocatable at run-time, it should be
* compiled with -fPIE, instead of -fPIC, if possible and should be built as
* Position Independent Executable (PIE) so that linker won't optimize
* R_386_GOT32X relocation to its fixed symbol address. Older
* linkers generate R_386_32 relocations against locally defined symbols,
* _bss, _ebss and _end, in PIE. It isn't wrong, just less optimal than
* R_386_RELATIVE. But the x86 kernel fails to properly handle R_386_32
* relocations when relocating the kernel. To generate R_386_RELATIVE
* relocations, we mark _bss, _ebss and _end as hidden:
* These symbols needed to be marked as .hidden to prevent the BFD linker from
* generating R_386_32 (rather than R_386_RELATIVE) relocations for them when
* the 32-bit compressed kernel is linked as PIE. This is no longer necessary,
* but it doesn't hurt to keep them .hidden.
*/
.hidden _bss
.hidden _ebss
@ -74,10 +58,10 @@ SYM_FUNC_START(startup_32)
leal (BP_scratch+4)(%esi), %esp
call 1f
1: popl %edx
subl $1b, %edx
addl $_GLOBAL_OFFSET_TABLE_+(.-1b), %edx
/* Load new GDT */
leal gdt(%edx), %eax
leal gdt@GOTOFF(%edx), %eax
movl %eax, 2(%eax)
lgdt (%eax)
@ -90,14 +74,16 @@ SYM_FUNC_START(startup_32)
movl %eax, %ss
/*
* %edx contains the address we are loaded at by the boot loader and %ebx
* contains the address where we should move the kernel image temporarily
* for safe in-place decompression. %ebp contains the address that the kernel
* will be decompressed to.
* %edx contains the address we are loaded at by the boot loader (plus the
* offset to the GOT). The below code calculates %ebx to be the address where
* we should move the kernel image temporarily for safe in-place decompression
* (again, plus the offset to the GOT).
*
* %ebp is calculated to be the address that the kernel will be decompressed to.
*/
#ifdef CONFIG_RELOCATABLE
movl %edx, %ebx
leal startup_32@GOTOFF(%edx), %ebx
#ifdef CONFIG_EFI_STUB
/*
@ -108,7 +94,7 @@ SYM_FUNC_START(startup_32)
* image_offset = startup_32 - image_base
* Otherwise image_offset will be zero and has no effect on the calculations.
*/
subl image_offset(%edx), %ebx
subl image_offset@GOTOFF(%edx), %ebx
#endif
movl BP_kernel_alignment(%esi), %eax
@ -125,10 +111,10 @@ SYM_FUNC_START(startup_32)
movl %ebx, %ebp // Save the output address for later
/* Target address to relocate to for decompression */
addl BP_init_size(%esi), %ebx
subl $_end, %ebx
subl $_end@GOTOFF, %ebx
/* Set up the stack */
leal boot_stack_end(%ebx), %esp
leal boot_stack_end@GOTOFF(%ebx), %esp
/* Zero EFLAGS */
pushl $0
@ -139,8 +125,8 @@ SYM_FUNC_START(startup_32)
* where decompression in place becomes safe.
*/
pushl %esi
leal (_bss-4)(%edx), %esi
leal (_bss-4)(%ebx), %edi
leal (_bss@GOTOFF-4)(%edx), %esi
leal (_bss@GOTOFF-4)(%ebx), %edi
movl $(_bss - startup_32), %ecx
shrl $2, %ecx
std
@ -153,14 +139,14 @@ SYM_FUNC_START(startup_32)
* during extract_kernel below. To avoid any issues, repoint the GDTR
* to the new copy of the GDT.
*/
leal gdt(%ebx), %eax
leal gdt@GOTOFF(%ebx), %eax
movl %eax, 2(%eax)
lgdt (%eax)
/*
* Jump to the relocated address.
*/
leal .Lrelocated(%ebx), %eax
leal .Lrelocated@GOTOFF(%ebx), %eax
jmp *%eax
SYM_FUNC_END(startup_32)
@ -170,7 +156,7 @@ SYM_FUNC_START_ALIAS(efi_stub_entry)
add $0x4, %esp
movl 8(%esp), %esi /* save boot_params pointer */
call efi_main
leal startup_32(%eax), %eax
/* efi_main returns the possibly relocated address of startup_32 */
jmp *%eax
SYM_FUNC_END(efi32_stub_entry)
SYM_FUNC_END_ALIAS(efi_stub_entry)
@ -183,8 +169,8 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
* Clear BSS (stack is currently empty)
*/
xorl %eax, %eax
leal _bss(%ebx), %edi
leal _ebss(%ebx), %ecx
leal _bss@GOTOFF(%ebx), %edi
leal _ebss@GOTOFF(%ebx), %ecx
subl %edi, %ecx
shrl $2, %ecx
rep stosl
@ -198,9 +184,9 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
pushl %ebp /* output address */
pushl $z_input_len /* input_len */
leal input_data(%ebx), %eax
leal input_data@GOTOFF(%ebx), %eax
pushl %eax /* input_data */
leal boot_heap(%ebx), %eax
leal boot_heap@GOTOFF(%ebx), %eax
pushl %eax /* heap area */
pushl %esi /* real mode pointer */
call extract_kernel /* returns kernel location in %eax */

104
arch/x86/boot/compressed/head_64.S
View File

@ -43,6 +43,32 @@
.hidden _end
__HEAD
/*
* This macro gives the relative virtual address of X, i.e. the offset of X
* from startup_32. This is the same as the link-time virtual address of X,
* since startup_32 is at 0, but defining it this way tells the
* assembler/linker that we do not want the actual run-time address of X. This
* prevents the linker from trying to create unwanted run-time relocation
* entries for the reference when the compressed kernel is linked as PIE.
*
* A reference X(%reg) will result in the link-time VA of X being stored with
* the instruction, and a run-time R_X86_64_RELATIVE relocation entry that
* adds the 64-bit base address where the kernel is loaded.
*
* Replacing it with (X-startup_32)(%reg) results in the offset being stored,
* and no run-time relocation.
*
* The macro should be used as a displacement with a base register containing
* the run-time address of startup_32 [i.e. rva(X)(%reg)], or as an immediate
* [$ rva(X)].
*
* This macro can only be used from within the .head.text section, since the
* expression requires startup_32 to be in the same section as the code being
* assembled.
*/
#define rva(X) ((X) - startup_32)
.code32
SYM_FUNC_START(startup_32)
/*
@ -65,10 +91,10 @@ SYM_FUNC_START(startup_32)
leal (BP_scratch+4)(%esi), %esp
call 1f
1: popl %ebp
subl $1b, %ebp
subl $ rva(1b), %ebp
/* Load new GDT with the 64bit segments using 32bit descriptor */
leal gdt(%ebp), %eax
leal rva(gdt)(%ebp), %eax
movl %eax, 2(%eax)
lgdt (%eax)
@ -81,7 +107,7 @@ SYM_FUNC_START(startup_32)
movl %eax, %ss
/* setup a stack and make sure cpu supports long mode. */
leal boot_stack_end(%ebp), %esp
leal rva(boot_stack_end)(%ebp), %esp
call verify_cpu
testl %eax, %eax
@ -108,7 +134,7 @@ SYM_FUNC_START(startup_32)
* image_offset = startup_32 - image_base
* Otherwise image_offset will be zero and has no effect on the calculations.
*/
subl image_offset(%ebp), %ebx
subl rva(image_offset)(%ebp), %ebx
#endif
movl BP_kernel_alignment(%esi), %eax
@ -124,7 +150,7 @@ SYM_FUNC_START(startup_32)
/* Target address to relocate to for decompression */
addl BP_init_size(%esi), %ebx
subl $_end, %ebx
subl $ rva(_end), %ebx
/*
* Prepare for entering 64 bit mode
@ -152,19 +178,19 @@ SYM_FUNC_START(startup_32)
1:
/* Initialize Page tables to 0 */
leal pgtable(%ebx), %edi
leal rva(pgtable)(%ebx), %edi
xorl %eax, %eax
movl $(BOOT_INIT_PGT_SIZE/4), %ecx
rep stosl
/* Build Level 4 */
leal pgtable + 0(%ebx), %edi
leal rva(pgtable + 0)(%ebx), %edi
leal 0x1007 (%edi), %eax
movl %eax, 0(%edi)
addl %edx, 4(%edi)
/* Build Level 3 */
leal pgtable + 0x1000(%ebx), %edi
leal rva(pgtable + 0x1000)(%ebx), %edi
leal 0x1007(%edi), %eax
movl $4, %ecx
1: movl %eax, 0x00(%edi)
@ -175,7 +201,7 @@ SYM_FUNC_START(startup_32)
jnz 1b
/* Build Level 2 */
leal pgtable + 0x2000(%ebx), %edi
leal rva(pgtable + 0x2000)(%ebx), %edi
movl $0x00000183, %eax
movl $2048, %ecx
1: movl %eax, 0(%edi)
@ -186,7 +212,7 @@ SYM_FUNC_START(startup_32)
jnz 1b
/* Enable the boot page tables */
leal pgtable(%ebx), %eax
leal rva(pgtable)(%ebx), %eax
movl %eax, %cr3
/* Enable Long mode in EFER (Extended Feature Enable Register) */
@ -211,14 +237,14 @@ SYM_FUNC_START(startup_32)
* We place all of the values on our mini stack so lret can
* used to perform that far jump.
*/
leal startup_64(%ebp), %eax
leal rva(startup_64)(%ebp), %eax
#ifdef CONFIG_EFI_MIXED
movl efi32_boot_args(%ebp), %edi
movl rva(efi32_boot_args)(%ebp), %edi
cmp $0, %edi
jz 1f
leal efi64_stub_entry(%ebp), %eax
movl efi32_boot_args+4(%ebp), %esi
movl efi32_boot_args+8(%ebp), %edx // saved bootparams pointer
leal rva(efi64_stub_entry)(%ebp), %eax
movl rva(efi32_boot_args+4)(%ebp), %esi
movl rva(efi32_boot_args+8)(%ebp), %edx // saved bootparams pointer
cmpl $0, %edx
jnz 1f
/*
@ -229,7 +255,7 @@ SYM_FUNC_START(startup_32)
* the correct stack alignment for entry.
*/
subl $40, %esp
leal efi_pe_entry(%ebp), %eax
leal rva(efi_pe_entry)(%ebp), %eax
movl %edi, %ecx // MS calling convention
movl %esi, %edx
1:
@ -255,18 +281,18 @@ SYM_FUNC_START(efi32_stub_entry)
call 1f
1: pop %ebp
subl $1b, %ebp
subl $ rva(1b), %ebp
movl %esi, efi32_boot_args+8(%ebp)
movl %esi, rva(efi32_boot_args+8)(%ebp)
SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
movl %ecx, efi32_boot_args(%ebp)
movl %edx, efi32_boot_args+4(%ebp)
movb $0, efi_is64(%ebp)
movl %ecx, rva(efi32_boot_args)(%ebp)
movl %edx, rva(efi32_boot_args+4)(%ebp)
movb $0, rva(efi_is64)(%ebp)
/* Save firmware GDTR and code/data selectors */
sgdtl efi32_boot_gdt(%ebp)
movw %cs, efi32_boot_cs(%ebp)
movw %ds, efi32_boot_ds(%ebp)
sgdtl rva(efi32_boot_gdt)(%ebp)
movw %cs, rva(efi32_boot_cs)(%ebp)
movw %ds, rva(efi32_boot_ds)(%ebp)
/* Disable paging */
movl %cr0, %eax
@ -345,11 +371,11 @@ SYM_CODE_START(startup_64)
/* Target address to relocate to for decompression */
movl BP_init_size(%rsi), %ebx
subl $_end, %ebx
subl $ rva(_end), %ebx
addq %rbp, %rbx
/* Set up the stack */
leaq boot_stack_end(%rbx), %rsp
leaq rva(boot_stack_end)(%rbx), %rsp
/*
* At this point we are in long mode with 4-level paging enabled,
@ -423,7 +449,7 @@ SYM_CODE_START(startup_64)
lretq
trampoline_return:
/* Restore the stack, the 32-bit trampoline uses its own stack */
leaq boot_stack_end(%rbx), %rsp
leaq rva(boot_stack_end)(%rbx), %rsp
/*
* cleanup_trampoline() would restore trampoline memory.
@ -435,7 +461,7 @@ trampoline_return:
* this function call.
*/
pushq %rsi
leaq top_pgtable(%rbx), %rdi
leaq rva(top_pgtable)(%rbx), %rdi
call cleanup_trampoline
popq %rsi
@ -449,9 +475,9 @@ trampoline_return:
*/
pushq %rsi
leaq (_bss-8)(%rip), %rsi
leaq (_bss-8)(%rbx), %rdi
movq $_bss /* - $startup_32 */, %rcx
shrq $3, %rcx
leaq rva(_bss-8)(%rbx), %rdi
movl $(_bss - startup_32), %ecx
shrl $3, %ecx
std
rep movsq
cld
@ -462,15 +488,15 @@ trampoline_return:
* during extract_kernel below. To avoid any issues, repoint the GDTR
* to the new copy of the GDT.
*/
leaq gdt64(%rbx), %rax
leaq gdt(%rbx), %rdx
leaq rva(gdt64)(%rbx), %rax
leaq rva(gdt)(%rbx), %rdx
movq %rdx, 2(%rax)
lgdt (%rax)
/*
* Jump to the relocated address.
*/
leaq .Lrelocated(%rbx), %rax
leaq rva(.Lrelocated)(%rbx), %rax
jmp *%rax
SYM_CODE_END(startup_64)
@ -482,7 +508,7 @@ SYM_FUNC_START_ALIAS(efi_stub_entry)
movq %rdx, %rbx /* save boot_params pointer */
call efi_main
movq %rbx,%rsi
leaq startup_64(%rax), %rax
leaq rva(startup_64)(%rax), %rax
jmp *%rax
SYM_FUNC_END(efi64_stub_entry)
SYM_FUNC_END_ALIAS(efi_stub_entry)
@ -645,7 +671,7 @@ SYM_DATA(efi_is64, .byte 1)
#define BS32_handle_protocol 88 // offsetof(efi_boot_services_32_t, handle_protocol)
#define LI32_image_base 32 // offsetof(efi_loaded_image_32_t, image_base)
.text
__HEAD
.code32
SYM_FUNC_START(efi32_pe_entry)
/*
@ -667,12 +693,12 @@ SYM_FUNC_START(efi32_pe_entry)
call 1f
1: pop %ebx
subl $1b, %ebx
subl $ rva(1b), %ebx
/* Get the loaded image protocol pointer from the image handle */
leal -4(%ebp), %eax
pushl %eax // &loaded_image
leal loaded_image_proto(%ebx), %eax
leal rva(loaded_image_proto)(%ebx), %eax
pushl %eax // pass the GUID address
pushl 8(%ebp) // pass the image handle
@ -707,7 +733,7 @@ SYM_FUNC_START(efi32_pe_entry)
* use it before we get to the 64-bit efi_pe_entry() in C code.
*/
subl %esi, %ebx
movl %ebx, image_offset(%ebp) // save image_offset
movl %ebx, rva(image_offset)(%ebp) // save image_offset
jmp efi32_pe_stub_entry
2: popl %edi // restore callee-save registers