Commit 487d1edb authored Mar 27, 2015 by Denys Vlasenko Committed by Ingo Molnar Mar 27, 2015

x86/asm/entry/64: Fix comment about SYSENTER MSRs



The comment is ancient, it dates to the time when only AMD's
x86_64 implementation existed. AMD wasn't (and still isn't)
supporting SYSENTER, so these writes were "just in case" back
then.

This has changed: Intel's x86_64 appeared, and Intel does
support SYSENTER in long mode. "Some future 64-bit CPU" is here
already.

The code may appear "buggy" for AMD as it stands, since
MSR_IA32_SYSENTER_EIP is only 32-bit for AMD CPUs. Writing a
kernel function's address to it would drop high bits. Subsequent
use of this MSR for branch via SYSENTER seem to allow user to
transition to CPL0 while executing his code. Scary, eh?

Explain why that is not a bug: because SYSENTER insn would not
work on AMD CPU.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1427453956-21931-1-git-send-email-dvlasenk@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent aa6d9a12

arch/x86/kernel/cpu/common.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -1173,8 +1173,10 @@ void syscall_init(void)
		#ifdef CONFIG_IA32_EMULATION
		wrmsrl(MSR_CSTAR, ia32_cstar_target);
		/*
		* Always load these, in case some future 64-bit CPU supports
		* SYSENTER from compat mode too:
		* This only works on Intel CPUs.
		* On AMD CPUs these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP.
		* This does not cause SYSENTER to jump to the wrong location, because
		* AMD doesn't allow SYSENTER in long mode (either 32- or 64-bit).
		*/
		wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
		wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);