[RFC PATCH] x86/cpu: Fix MSR value truncation issue

[Borislav Petkov <bp@alien8.de>]

From: Borislav Petkov <bp@suse.de>

So sparse rightfully complains that the u64 MSR value we're writing into
the STAR MSR, i.e. 0xc0000081, is being truncated:

./arch/x86/include/asm/msr.h:193:36: warning: cast truncates bits from constant value (23001000000000 becomes 0)

because the actual value doesn't fit into the unsigned 32-bit quantity
which are the @low and @high wrmsrl() parameters.

This is not a problem, practically, because gcc is actually being smart
enough here and does the right thing:

  .loc 3 87 0
  xorl    %esi, %esi		# we needz a 32-bit zero
  movl    $2293776, %edx	# 0x00230010 == (__USER32_CS << 16) | __KERNEL_CS go into the high bits
  movl    $-1073741695, %ecx	# MSR_STAR, i.e., 0xc0000081
  movl    %esi, %eax		# low order 32 bits in the MSR which are 0
  #APP
  # 87 "./arch/x86/include/asm/msr.h" 1
          wrmsr

More specifically, MSR_STAR[31:0] is being set to 0. That field is
reserved on Intel and on AMD it is 32-bit SYSCALL Target EIP.

I'd strongly guess because Intel doesn't have SYSCALL in compat/legacy
mode and we're using SYSENTER and INT80 there. And for compat syscalls
in long mode we use CSTAR.

So let's fix the sparse warning by writing SYSRET and SYSCALL CS and SS
into the high 32-bit half of STAR and 0 in the low half explicitly.

 [ Actually, if we had to be precise, we would have to read what's in
   STAR[31:0] and write it back unchanged on Intel and write 0 on AMD. I
   guess the current writing to 0 is still ok since Intel can apparently
   stomach it. ]

The resulting code is identical to what we have above:

  .loc 3 87 0
  xorl    %esi, %esi      # tmp104
  movl    $2293776, %eax  #, tmp103
  movl    $-1073741695, %ecx      #, tmp102
  movl    %esi, %edx      # tmp104, tmp104

  ...

        wrmsr

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/kernel/cpu/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4ddd780aeac9..42cfb0b8249c 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1186,7 +1186,7 @@ void syscall_init(void)
 	 * They both write to the same internal register. STAR allows to
 	 * set CS/DS but only a 32bit target. LSTAR sets the 64bit rip.
 	 */
-	wrmsrl(MSR_STAR,  ((u64)__USER32_CS)<<48  | ((u64)__KERNEL_CS)<<32);
+	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
 	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
 
 #ifdef CONFIG_IA32_EMULATION
-- 
2.3.5