x86/boot/compressed: Get rid of GOT fixup code

In a previous patch, we have eliminated GOT entries from the decompressor
binary and added an assertion that the .got section is empty. This means
that the GOT fixup routines that exist in both the 32-bit and 64-bit
startup routines have become dead code, and can be removed.

While at it, drop the KEEP() from the linker script, as it has no effect
on the contents of output sections that are created by the linker itself.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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>
Acked-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200731230820.1742553-4-keescook@chromium.org

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 03557f2..39f0bb4 100644 (file)
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -49,16 +49,13 @@
  * 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, _got, _egot 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, _got, _egot and _end as
- * hidden:
+ * _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:
  */
        .hidden _bss
        .hidden _ebss
-       .hidden _got
-       .hidden _egot
        .hidden _end
 
        __HEAD
@@ -193,19 +190,6 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
        rep     stosl
 
 /*
- * Adjust our own GOT
- */
-       leal    _got(%ebx), %edx
-       leal    _egot(%ebx), %ecx
-1:
-       cmpl    %ecx, %edx
-       jae     2f
-       addl    %ebx, (%edx)
-       addl    $4, %edx
-       jmp     1b
-2:
-
-/*
  * Do the extraction, and jump to the new kernel..
  */
                                /* push arguments for extract_kernel: */

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 97d37f0..bf1ab30 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -40,8 +40,6 @@
  */
        .hidden _bss
        .hidden _ebss
-       .hidden _got
-       .hidden _egot
        .hidden _end
 
        __HEAD
@@ -354,25 +352,6 @@ SYM_CODE_START(startup_64)
        leaq    boot_stack_end(%rbx), %rsp
 
        /*
-        * paging_prepare() and cleanup_trampoline() below can have GOT
-        * references. Adjust the table with address we are running at.
-        *
-        * Zero RAX for adjust_got: the GOT was not adjusted before;
-        * there's no adjustment to undo.
-        */
-       xorq    %rax, %rax
-
-       /*
-        * Calculate the address the binary is loaded at and use it as
-        * a GOT adjustment.
-        */
-       call    1f
-1:     popq    %rdi
-       subq    $1b, %rdi
-
-       call    .Ladjust_got
-
-       /*
         * At this point we are in long mode with 4-level paging enabled,
         * but we might want to enable 5-level paging or vice versa.
         *
@@ -464,21 +443,6 @@ trampoline_return:
        pushq   $0
        popfq
 
-       /*
-        * Previously we've adjusted the GOT with address the binary was
-        * loaded at. Now we need to re-adjust for relocation address.
-        *
-        * Calculate the address the binary is loaded at, so that we can
-        * undo the previous GOT adjustment.
-        */
-       call    1f
-1:     popq    %rax
-       subq    $1b, %rax
-
-       /* The new adjustment is the relocation address */
-       movq    %rbx, %rdi
-       call    .Ladjust_got
-
 /*
  * Copy the compressed kernel to the end of our buffer
  * where decompression in place becomes safe.
@@ -556,27 +520,6 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
        jmp     *%rax
 SYM_FUNC_END(.Lrelocated)
 
-/*
- * Adjust the global offset table
- *
- * RAX is the previous adjustment of the table to undo (use 0 if it's the
- * first time we touch GOT).
- * RDI is the new adjustment to apply.
- */
-.Ladjust_got:
-       /* Walk through the GOT adding the address to the entries */
-       leaq    _got(%rip), %rdx
-       leaq    _egot(%rip), %rcx
-1:
-       cmpq    %rcx, %rdx
-       jae     2f
-       subq    %rax, (%rdx)    /* Undo previous adjustment */
-       addq    %rdi, (%rdx)    /* Apply the new adjustment */
-       addq    $8, %rdx
-       jmp     1b
-2:
-       ret
-
        .code32
 /*
  * This is the 32-bit trampoline that will be copied over to low memory.

diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
index 4bcc943..a4a4a59 100644 (file)
--- a/arch/x86/boot/compressed/vmlinux.lds.S
+++ b/arch/x86/boot/compressed/vmlinux.lds.S
@@ -43,9 +43,7 @@ SECTIONS
                _erodata = . ;
        }
        .got : {
-               _got = .;
-               KEEP(*(.got))
-               _egot = .;
+               *(.got)
        }
        .got.plt : {
                *(.got.plt)