Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled
From: Andy Lutomirski <luto@kernel.org>
Date: 2020-09-19 00:12:06
Also in:
linux-arch, linux-doc, linux-mm, lkml
On Fri, Sep 18, 2020 at 12:23 PM Yu-cheng Yu [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- v12: - 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); +#ifdef CONFIG_X86_INTEL_CET + 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.