[PATCH] x86/asm/entry/64: Fix comment about SYSENTER MSRs

[Denys Vlasenko <dvlasenk@redhat.com>]

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 Indel 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: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
 arch/x86/kernel/cpu/common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index b5ad0fa..7c57b12 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1212,8 +1212,10 @@ 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 for Intel CPUs.
+	 * On AMD, these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP.
+	 * This does not cause SYSENTER to jump to 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);
-- 
1.8.1.4