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>

2 files changed, 13 insertions, 3 deletions

diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
index 4b5f7bf2b780..1c8b50edb2db 100644
--- a/arch/x86/include/asm/calling.h
+++ b/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
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index ca3347a9dab5..3563107b5060 100644
--- a/arch/x86/include/asm/elf.h
+++ b/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;