On Wed, Mar 7, 2018 at 7:31 PM, Dominik Brodowski
<li...@dominikbrodowski.net> wrote:
> On Wed, Mar 07, 2018 at 11:21:46AM -0800, Kees Cook wrote:
>> On Wed, Mar 7, 2018 at 11:12 AM, Andy Lutomirski <l...@kernel.org> wrote:
>> > Since Linux 3.2, vsyscalls have been deprecated and slow. From 3.2
>> > on, Linux had three vsyscall modes: "native", "emulate", and "none".
>> >
>> > "emulate" is the default. All known user programs work correctly in
>> > emulate mode, but vsyscalls turn into page faults and are emulated.
>> > This is very slow. In "native" mode, the vsyscall page is easily
>> > usable as an exploit gadget, but vsyscalls are a bit faster -- they
>> > turn into normal syscalls. (This is in contrast to vDSO functions,
>> > which can be much faster than syscalls.) In "none" mode, there are
>> > no vsyscalls.
>> >
>> > For all practical purposes, "native" was really just a chicken bit
>> > in case something went wrong with the emulation. It's been over six
>> > years, and nothing has gone wrong. Delete it.
>> >
>> > Signed-off-by: Andy Lutomirski <l...@kernel.org>
>>
>> Acked-by: Kees Cook <keesc...@chromium.org>
>>
>> I related news, I wonder how long before we can switch from EMULATE to
>> NONE as the default? glibc 2.15 (which stopped using vsyscall) is (not
>> coincidentally) 6 years old too...
>
> Or, in the meantime, add a warning
>
> if (vsyscall_mode == EMULATE) {
> warn_bad_vsyscall(KERN_INFO, regs,
> "vsyscalls are deprecated -- use vDSO
> instead");
> }
>
> Otherwise, the patch looks good.
>
I'm not sure the warning buys us anything. AFAICT there are no modern
toolchains in any language that will use vsyscalls. The problem is
old binaries.
