Re: [PATCH 3/6] x86/entry_32: Add VERW just before userspace transition

Pawan Gupta
2023-10-21T01:28:27Z

On Fri, Oct 20, 2023 at 04:49:34PM -0700, Andi Kleen wrote:
> On Fri, Oct 20, 2023 at 01:45:09PM -0700, Pawan Gupta wrote:
> > As done for entry_64, add support for executing VERW late in exit to
> > user path for 32-bit mode.
> >
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > ---
> > arch/x86/entry/entry_32.S | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> > index 6e6af42e044a..bbf77d2aab2e 100644
> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -886,6 +886,9 @@ SYM_FUNC_START(entry_SYSENTER_32)
> > popfl
> > popl %eax
> >
> > + /* Mitigate CPU data sampling attacks .e.g. MDS */
> > + USER_CLEAR_CPU_BUFFERS
> > +
> > /*
> > * Return back to the vDSO, which will pop ecx and edx.
> > * Don't bother with DS and ES (they already contain __USER_DS).
>
> Did you forget the INT 0x80 entry point?

I do have VERW in the INT80 path, the diff is showing just the label
restore_all_switch_stack. Below is the sequence:

SYM_FUNC_START(entry_INT80_32)
ASM_CLAC
pushl %eax /* pt_regs->orig_ax */

SAVE_ALL pt_regs_ax=$-ENOSYS switch_stacks=1 /* save rest */

movl %esp, %eax
call do_int80_syscall_32
.Lsyscall_32_done:
STACKLEAK_ERASE

restore_all_switch_stack:
SWITCH_TO_ENTRY_STACK
CHECK_AND_APPLY_ESPFIX

/* Switch back to user CR3 */
SWITCH_TO_USER_CR3 scratch_reg=%eax

BUG_IF_WRONG_CR3

/* Restore user state */
RESTORE_REGS pop=4 # skip orig_eax/error_code

/* Mitigate CPU data sampling attacks .e.g. MDS */
USER_CLEAR_CPU_BUFFERS
^^^^^^^^^^^^^^^^^^^^^^