x86/asm/entry: Clear EXTRA_REGS for all executable formats

On failure, sys_execve() does not clobber EXTRA_REGS, so we can
just return to userpsace without saving/restoring them.

On success, ELF_PLAT_INIT() in sys_execve() clears all these
registers.

On other executable formats:

  - binfmt_flat.c has similar FLAT_PLAT_INIT, but x86 (and everyone
    else except sh) doesn't define it.

  - binfmt_elf_fdpic.c has ELF_FDPIC_PLAT_INIT, but x86 (and most
    others) doesn't define it.

  - There are no such hooks in binfmt_aout.c et al. We inherit
    EXTRA_REGS from the prior executable.

This inconsistency was not intended.

This change removes SAVE/RESTORE_EXTRA_REGS in stub_execve,
removes register clearing in ELF_PLAT_INIT(),
and instead simply clears them on success in stub_execve.

Run-tested.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1428173719-7637-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/calling.h

@@ -151,6 +151,15 @@ For 32-bit we have the following conventions - kernel is built with
 	movq_cfi_restore 5*8+\offset, rbx
 	.endm
 
+	.macro ZERO_EXTRA_REGS
+	xorl	%r15d, %r15d
+	xorl	%r14d, %r14d
+	xorl	%r13d, %r13d
+	xorl	%r12d, %r12d
+	xorl	%ebp, %ebp
+	xorl	%ebx, %ebx
+	.endm
+
 	.macro RESTORE_C_REGS_HELPER rstor_rax=1, rstor_rcx=1, rstor_r11=1, rstor_r8910=1, rstor_rdx=1
 	.if \rstor_r11
 	movq_cfi_restore 6*8, r11

arch/x86/include/asm/elf.h

@@ -171,10 +171,11 @@ do {						\
 static inline void elf_common_init(struct thread_struct *t,
 				   struct pt_regs *regs, const u16 ds)
 {
-	regs->ax = regs->bx = regs->cx = regs->dx = 0;
-	regs->si = regs->di = regs->bp = 0;
+	/* Commented-out registers are cleared in stub_execve */
+	/*regs->ax = regs->bx =*/ regs->cx = regs->dx = 0;
+	regs->si = regs->di /*= regs->bp*/ = 0;
 	regs->r8 = regs->r9 = regs->r10 = regs->r11 = 0;
-	regs->r12 = regs->r13 = regs->r14 = regs->r15 = 0;
+	/*regs->r12 = regs->r13 = regs->r14 = regs->r15 = 0;*/
 	t->fs = t->gs = 0;
 	t->fsindex = t->gsindex = 0;
 	t->ds = t->es = ds;

arch/x86/kernel/entry_64.S

@@ -419,25 +419,27 @@ END(stub_\func)
 
 ENTRY(stub_execve)
 	CFI_STARTPROC
-	addq $8, %rsp
-	DEFAULT_FRAME 0
-	SAVE_EXTRA_REGS
-	call sys_execve
-	movq %rax,RAX(%rsp)
-	RESTORE_EXTRA_REGS
-	jmp int_ret_from_sys_call
+	DEFAULT_FRAME 0, 8
+	call	sys_execve
+return_from_execve:
+	testl	%eax, %eax
+	jz	1f
+	/* exec failed, can use fast SYSRET code path in this case */
+	ret
+1:
+	/* must use IRET code path (pt_regs->cs may have changed) */
+	addq	$8, %rsp
+	ZERO_EXTRA_REGS
+	movq	%rax,RAX(%rsp)
+	jmp	int_ret_from_sys_call
 	CFI_ENDPROC
 END(stub_execve)
 
 ENTRY(stub_execveat)
 	CFI_STARTPROC
-	addq $8, %rsp
-	DEFAULT_FRAME 0
-	SAVE_EXTRA_REGS
-	call sys_execveat
-	movq %rax,RAX(%rsp)
-	RESTORE_EXTRA_REGS
-	jmp int_ret_from_sys_call
+	DEFAULT_FRAME 0, 8
+	call	sys_execveat
+	jmp	return_from_execve
 	CFI_ENDPROC
 END(stub_execveat)
 
@@ -472,25 +474,17 @@ END(stub_x32_rt_sigreturn)
 
 ENTRY(stub_x32_execve)
 	CFI_STARTPROC
-	addq $8, %rsp
-	DEFAULT_FRAME 0
-	SAVE_EXTRA_REGS
-	call compat_sys_execve
-	movq %rax,RAX(%rsp)
-	RESTORE_EXTRA_REGS
-	jmp int_ret_from_sys_call
+	DEFAULT_FRAME 0, 8
+	call	compat_sys_execve
+	jmp	return_from_execve
 	CFI_ENDPROC
 END(stub_x32_execve)
 
 ENTRY(stub_x32_execveat)
 	CFI_STARTPROC
-	addq $8, %rsp
-	DEFAULT_FRAME 0
-	SAVE_EXTRA_REGS
-	call compat_sys_execveat
-	movq %rax,RAX(%rsp)
-	RESTORE_EXTRA_REGS
-	jmp int_ret_from_sys_call
+	DEFAULT_FRAME 0, 8
+	call	compat_sys_execveat
+	jmp	return_from_execve
 	CFI_ENDPROC
 END(stub_x32_execveat)