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    Subject[PATCH] x86-64, vsyscalls: Rename UNSAFE_VSYSCALLS to COMPAT_VSYSCALLS

    * Linus Torvalds <> wrote:

    > On Mon, Jun 6, 2011 at 2:50 AM, Andy Lutomirski <> wrote:
    > > CONFIG_UNSAFE_VSYSCALLS was added in the previous patch as a
    > > temporary hack to avoid penalizing users who don't build glibc from
    > > git.
    > I really hate that name.
    > Do you have *any* reason to call this "unsafe"?

    No, there's no reason at all for that. That naming is borderline
    security FUD and last time i saw the series i considered renaming
    it but got distracted :-)

    How about the patch below? COMPAT_VSYSCALLS looks like a good logical
    extension to the COMPAT_VDSO we already have.

    CONFIG_FIXED_VSYSCALLS seemed a bit awkward to me nor does it carry
    the compat nature of them.



    From 1593843e2ada6d6832d0de4d633aacd997dc3a45 Mon Sep 17 00:00:00 2001
    From: Ingo Molnar <>
    Date: Mon, 6 Jun 2011 12:13:40 +0200
    Subject: [PATCH] x86-64, vsyscalls: Rename UNSAFE_VSYSCALLS to COMPAT_VSYSCALLS

    Linus pointed out that the UNSAFE_VSYSCALL naming was inherently
    bad: it suggests that there's something unsafe about enabling them,
    while in reality they only have any security effect in the presence
    of some *other* security hole.

    So rename it to CONFIG_COMPAT_VSYSCALL and fix the documentation
    and Kconfig text to correctly explain the purpose of this change.

    Reported-by: Linus Torvalds <>
    Cc: Andy Lutomirski <>
    Cc: Jesper Juhl <>
    Cc: Borislav Petkov <>
    Cc: Arjan van de Ven <>
    Cc: Jan Beulich <>
    Cc: richard -rw- weinberger <>
    Cc: Mikael Pettersson <>
    Cc: Andi Kleen <>
    Cc: Brian Gerst <>
    Cc: Louis Rilling <>
    Signed-off-by: Ingo Molnar <>
    Documentation/feature-removal-schedule.txt | 7 ++++---
    arch/x86/Kconfig | 17 ++++++++++-------
    arch/x86/kernel/vsyscall_64.c | 8 ++++----
    arch/x86/kernel/vsyscall_emu_64.S | 2 +-
    4 files changed, 19 insertions(+), 15 deletions(-)

    diff --git a/Documentation/feature-removal-schedule.txt b/Documentation/feature-removal-schedule.txt
    index 94b4470..4282ab2 100644
    --- a/Documentation/feature-removal-schedule.txt
    +++ b/Documentation/feature-removal-schedule.txt
    @@ -601,10 +601,11 @@ Who: Laurent Pinchart <>


    When: When glibc 2.14 or newer is ubitquitous. Perhaps mid-2012.
    -Why: Having user-executable code at a fixed address is a security problem.
    - Turning off CONFIG_UNSAFE_VSYSCALLS mostly removes the risk but will
    +Why: Having user-executable syscall invoking code at a fixed addresses makes
    + it easier for attackers to exploit security holes.
    + Turning off CONFIG_COMPAT_VSYSCALLS mostly removes the risk but will
    make the time() function slower on glibc versions 2.13 and below.
    Who: Andy Lutomirski <>

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index 79e5d8a..30041d8 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -1646,20 +1646,23 @@ config COMPAT_VDSO

    If unsure, say Y.

    def_bool y
    - prompt "Unsafe fast legacy vsyscalls"
    + prompt "Fixed address legacy vsyscalls"
    depends on X86_64
    Legacy user code expects to be able to issue three syscalls
    - by calling fixed addresses in kernel space. If you say N,
    - then the kernel traps and emulates these calls. If you say
    - Y, then there is actual executable code at a fixed address
    - to implement time() efficiently.
    + by calling a fixed addresses. If you say N, then the kernel
    + traps and emulates these calls. If you say Y, then there is
    + actual executable code at a fixed address to implement time()
    + efficiently.

    On a system with recent enough glibc (probably 2.14 or
    newer) and no static binaries, you can say N without a
    - performance penalty to improve security
    + performance penalty to improve security: having no fixed
    + address userspace-executable syscall invoking code makes
    + it harder for both remote and local attackers to exploit
    + security holes.

    If unsure, say Y.

    diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
    index 285af7a..27d49b7 100644
    --- a/arch/x86/kernel/vsyscall_64.c
    +++ b/arch/x86/kernel/vsyscall_64.c
    @@ -116,7 +116,7 @@ static int al_to_vsyscall_nr(u8 al)
    return -1;


    /* This will break when the xtime seconds get inaccurate, but that is
    * unlikely */
    @@ -138,9 +138,9 @@ vtime(time_t *t)
    return result;


    -/* If CONFIG_UNSAFE_VSYSCALLS=y, then this is incorrect for vsyscall_nr == 1. */
    +/* If CONFIG_COMPAT_VSYSCALLS=y, then this is incorrect for vsyscall_nr == 1. */
    static inline unsigned long vsyscall_intcc_addr(int vsyscall_nr)
    return VSYSCALL_START + 1024*vsyscall_nr + 2;
    @@ -202,7 +202,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)

    case 1:
    warn_bad_vsyscall(KERN_WARNING, regs, "bogus time() vsyscall "
    "emulation (exploit attempt?)");
    goto sigsegv;
    diff --git a/arch/x86/kernel/vsyscall_emu_64.S b/arch/x86/kernel/vsyscall_emu_64.S
    index 7ebde61..2d53e26 100644
    --- a/arch/x86/kernel/vsyscall_emu_64.S
    +++ b/arch/x86/kernel/vsyscall_emu_64.S
    @@ -25,7 +25,7 @@ ENTRY(vsyscall_0)

    .section .vsyscall_1, "a"
    movb $0xce, %al

     \ /
      Last update: 2011-06-06 12:27    [W:0.028 / U:5.560 seconds]
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