Date: Thu, 13 Jan 2022 13:47:26 -0800 From: Andy Lutomirski <luto@...nel.org> To: Florian Weimer <fweimer@...hat.com> Cc: linux-arch@...r.kernel.org, Linux API <linux-api@...r.kernel.org>, linux-x86_64@...r.kernel.org, kernel-hardening@...ts.openwall.com, linux-mm@...ck.org, the arch/x86 maintainers <x86@...nel.org>, musl@...ts.openwall.com, libc-alpha@...rceware.org, linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>, Kees Cook <keescook@...omium.org>, Andrei Vagin <avagin@...il.com> Subject: Re: [PATCH v3 1/3] x86: Implement arch_prctl(ARCH_VSYSCALL_CONTROL) to disable vsyscall On 1/5/22 08:02, Florian Weimer wrote: > 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 > version. > > 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 <fweimer@...hat.com> > Acked-by: Andrei Vagin <avagin@...il.com> > --- > v3: Remove warning log message. Split out test. > v2: ARCH_VSYSCALL_CONTROL instead of ARCH_VSYSCALL_LOCKOUT. New tests > 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; > + Is there a reason you didn't just change the check earlier in the function to: if (vsyscall_mode == NONE || current->mm->context.vsyscall_disabled) Also, I still think the prctl should not be available if vsyscall=emulate. Either we should fully implement it or we should not implement. We could even do: pr_warn_once("userspace vsyscall hardening request ignored because you have vsyscall=emulate. Unless you absolutely need vsyscall=emulate, update your system to use vsyscall=xonly.\n"); and thus encourage good behavior. --Andy
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