x86/vdso: Plug race between mapping and ELF header setup

commit 6fdc6dd9 upstream.

The vsyscall32 sysctl can racy against a concurrent fork when it switches
from disabled to enabled:

    arch_setup_additional_pages()
	if (vdso32_enabled)
           --> No mapping
                                        sysctl.vsysscall32()
                                          --> vdso32_enabled = true
    create_elf_tables()
      ARCH_DLINFO_IA32
        if (vdso32_enabled) {
           --> Add VDSO entry with NULL pointer

Make ARCH_DLINFO_IA32 check whether the VDSO mapping has been set up for
the newly forked process or not.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mathias Krause <minipli@googlemail.com>
Link: http://lkml.kernel.org/r/20170410151723.602367196@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 3.2: change the flag passed to ARCH_DLINFO_IA32()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/include/asm/elf.h

@@ -298,7 +298,7 @@ do {									\
 
 #define AT_SYSINFO		32
 
-#define COMPAT_ARCH_DLINFO	ARCH_DLINFO_IA32(sysctl_vsyscall32)
+#define COMPAT_ARCH_DLINFO	ARCH_DLINFO_IA32(VDSO_CURRENT_BASE)
 
 #define COMPAT_ELF_ET_DYN_BASE	(TASK_UNMAPPED_BASE + 0x1000000)