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

commit 6fdc6dd90272ce7e75d744f71535cfbd8d77da81 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.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 01f15b227d7ed49ec2c8a9ce68c4c6a2e3fca6b1..2fa7f4f6ecb31416b96efccb38fa548645d6710d 100644 (file)
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -272,7 +272,7 @@ struct task_struct;
 
 #define        ARCH_DLINFO_IA32(vdso_enabled)                                  \
 do {                                                                   \
-       if (vdso_enabled) {                                             \
+       if (VDSO_CURRENT_BASE) {                                        \
                NEW_AUX_ENT(AT_SYSINFO, VDSO_ENTRY);                    \
                NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);        \
        }                                                               \