Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

From: Yu, Yu-cheng
Date: Mon Sep 21 2020 - 12:22:32 EST

On 9/18/2020 5:11 PM, Andy Lutomirski wrote:
On Fri, Sep 18, 2020 at 12:23 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:

Emulation of the legacy vsyscall page is required by some programs
built before 2013. Newer programs after 2013 don't use it.
Disable vsyscall emulation when Control-flow Enforcement (CET) is
enabled to enhance security.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
- Disable vsyscall emulation only when it is attempted (vs. at compile time).

arch/x86/entry/vsyscall/vsyscall_64.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index 44c33103a955..3196e963e365 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -150,6 +150,15 @@ bool emulate_vsyscall(unsigned long error_code,

WARN_ON_ONCE(address != regs->ip);

+ if (current->thread.cet.shstk_size ||
+ current->thread.cet.ibt_enabled) {
+ warn_bad_vsyscall(KERN_INFO, regs,
+ "vsyscall attempted with cet enabled");
+ return false;
+ }

Nope, try again. Having IBT on does *not* mean that every library in
the process knows that we have indirect branch tracking. The legacy
bitmap exists for a reason. Also, I want a way to flag programs as
not using the vsyscall page, but that flag should not be called CET.
And a process with vsyscalls off should not be able to read the
vsyscall page, and /proc/self/maps should be correct.

So you have some choices:

1. Drop this patch and make it work.

2. Add a real per-process vsyscall control. Either make it depend on
vsyscall=xonly and wire it up correctly or actually make it work
correctly with vsyscall=emulate.

NAK to any hacks in this space. Do it right or don't do it at all.

We can drop this patch, and bring back the previous patch that fixes up shadow stack and ibt. That makes vsyscall emulation work correctly, and does not force the application to do anything different from what is working now. I will post the previous patch as a reply to this thread so that people can make comments on it.