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>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: stable@vger.kernel.org
---
 arch/x86/include/asm/elf.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -287,7 +287,7 @@ struct task_struct;
 
 #define	ARCH_DLINFO_IA32						\
 do {									\
-	if (vdso32_enabled) {						\
+	if (VDSO_CURRENT_BASE) {					\
 		NEW_AUX_ENT(AT_SYSINFO,	VDSO_ENTRY);			\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);	\
 	}								\