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Date: Wed, 05 Jan 2022 17:02:48 +0100
From: Florian Weimer <>
To: "Andy Lutomirski" <>
        "Linux API" <>,,,, "the arch/x86 maintainers" <>,, <>,
        "Dave Hansen" <>,
        "Kees Cook" <>, Andrei Vagin <>
Subject: [PATCH v3 1/3] x86: Implement arch_prctl(ARCH_VSYSCALL_CONTROL) to
 disable vsyscall

Distributions struggle with changing the default for vsyscall
emulation because it is a clear break of userspace ABI, something
that should not happen.

The legacy vsyscall interface is supposed to be used by libcs only,
not by applications.  This commit adds a new arch_prctl request,
ARCH_VSYSCALL_CONTROL, with one argument.  If the argument is 0,
executing vsyscalls will cause the process to terminate.  Argument 1
turns vsyscall back on (this is mostly for a largely theoretical
CRIU use case).

Newer libcs can use a zero ARCH_VSYSCALL_CONTROL at startup to disable
vsyscall for the process.  Legacy libcs do not perform this call, so
vsyscall remains enabled for them.  This approach should achieves
backwards compatibility (perfect compatibility if the assumption that
only libcs use vsyscall is accurate), and it provides full hardening
for new binaries.

The chosen value of ARCH_VSYSCALL_CONTROL should avoid conflicts
with other x86-64 arch_prctl requests.  The fact that with
vsyscall=emulate, reading the vsyscall region is still possible
even after a zero ARCH_VSYSCALL_CONTROL is considered limitation
in the current implementation and may change in a future kernel

Future arch_prctls requests commonly used at process startup can imply
ARCH_VSYSCALL_CONTROL with a zero argument, so that a separate system
call for disabling vsyscall is avoided.

Signed-off-by: Florian Weimer <>
Acked-by: Andrei Vagin <>
v3: Remove warning log message.  Split out test.
    for the toggle behavior.  Implement hiding [vsyscall] in
    /proc/PID/maps and test it.  Various other test fixes cleanups
    (e.g., fixed missing second argument to gettimeofday).

arch/x86/entry/vsyscall/vsyscall_64.c | 7 ++++++-
 arch/x86/include/asm/mmu.h            | 6 ++++++
 arch/x86/include/uapi/asm/prctl.h     | 2 ++
 arch/x86/kernel/process_64.c          | 7 +++++++
 4 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index fd2ee9408e91..6fc524b9f232 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -174,6 +174,9 @@ bool emulate_vsyscall(unsigned long error_code,
 	tsk = current;
+	if (tsk->mm->context.vsyscall_disabled)
+		goto sigsegv;
 	 * Check for access_ok violations and find the syscall nr.
@@ -316,8 +319,10 @@ static struct vm_area_struct gate_vma __ro_after_init = {
 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
+	if (!mm || mm->context.vsyscall_disabled)
+		return NULL;
-	if (!mm || !(mm->context.flags & MM_CONTEXT_HAS_VSYSCALL))
+	if (!(mm->context.flags & MM_CONTEXT_HAS_VSYSCALL))
 		return NULL;
 	if (vsyscall_mode == NONE)
diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index 5d7494631ea9..3934d6907910 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -41,6 +41,12 @@ typedef struct {
 #ifdef CONFIG_X86_64
 	unsigned short flags;
+	/*
+	 * Changed by arch_prctl(ARCH_VSYSCALL_CONTROL).
+	 */
+	bool vsyscall_disabled;
 	struct mutex lock;
 	void __user *vdso;			/* vdso base address */
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 754a07856817..aad0bcfbf49f 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -18,4 +18,6 @@
 #define ARCH_MAP_VDSO_32	0x2002
 #define ARCH_MAP_VDSO_64	0x2003
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 3402edec236c..834bad068211 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -816,6 +816,13 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 		ret = put_user(base, (unsigned long __user *)arg2);
+		if (unlikely(arg2 > 1))
+			return -EINVAL;
+		current->mm->context.vsyscall_disabled = !arg2;
+		break;
 # ifdef CONFIG_X86_X32_ABI

base-commit: c9e6606c7fe92b50a02ce51dda82586ebdf99b48

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