linux_dsm_epyc7002/arch/x86
Borislav Petkov 31ac34ca56 x86/cpu: Fix MSR value truncation issue
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@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1448273546-2567-6-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-11-24 09:15:55 +01:00
..
boot kasan: move KASAN_SANITIZE in arch/x86/boot/Makefile 2015-11-05 19:34:48 -08:00
configs Merge branch 'drm-next' of git://people.freedesktop.org/~airlied/linux 2015-09-04 15:49:32 -07:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2015-11-04 09:11:12 -08:00
entry mm: mlock: add new mlock system call 2015-11-05 19:34:48 -08:00
ia32 Merge branch 'x86-headers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2015-11-03 21:05:40 -08:00
include x86/cpu/amd, kvm: Satisfy guest kernel reads of IC_CFG MSR 2015-11-24 09:15:54 +01:00
kernel x86/cpu: Fix MSR value truncation issue 2015-11-24 09:15:55 +01:00
kvm x86/cpu/amd, kvm: Satisfy guest kernel reads of IC_CFG MSR 2015-11-24 09:15:54 +01:00
lguest genirq: Remove irq argument from irq flow handlers 2015-09-16 15:47:51 +02:00
lib x86/cpu: Unify CPU family, model, stepping calculation 2015-11-24 09:15:54 +01:00
math-emu Merge branch 'x86-headers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2015-11-03 21:05:40 -08:00
mm Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2015-11-22 12:00:12 -08:00
net ebpf: migrate bpf_prog's flags to bitfield 2015-10-03 05:02:39 -07:00
oprofile x86/asm/entry: Change all 'user_mode_vm()' calls to 'user_mode()' 2015-03-23 11:14:17 +01:00
pci PCI changes for the v4.4 merge window: 2015-11-06 11:29:53 -08:00
platform Merge branch 'x86-platform-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2015-11-03 21:33:18 -08:00
power x86/ldt: Make modify_ldt synchronous 2015-07-31 10:23:23 +02:00
purgatory
ras x86/ras/mce_amd_inj: Inject bank 4 errors on the NBC 2015-10-12 16:15:48 +02:00
realmode Merge branch 'perf-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2015-02-16 14:58:12 -08:00
tools
um Merge branch 'for-linus-4.4-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rw/uml 2015-11-10 16:33:37 -08:00
video
xen xen: features for 4.4-rc0 2015-11-04 17:32:42 -08:00
.gitignore
Kbuild x86/asm/entry, x86/vdso: Move the vDSO code to arch/x86/entry/vdso/ 2015-06-03 18:51:37 +02:00
Kconfig Merge branch 'x86-asm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2015-11-03 18:59:10 -08:00
Kconfig.cpu x86/Kconfig/cpus: Fix/complete CPU type help texts 2015-10-21 11:12:56 +02:00
Kconfig.debug x86: don't make DEBUG_WX default to 'y' even with DEBUG_RODATA 2015-11-06 09:12:41 -08:00
Makefile Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2015-11-04 09:11:12 -08:00
Makefile_32.cpu
Makefile.um kbuild: use relative path more to include Makefile 2015-04-02 16:42:08 +02:00