Re: [PATCH v3 2/4] xen/x86: introduce self modifying code test

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Dec 14, 2023 at 12:55:22PM +0100, Jan Beulich wrote:
> On 14.12.2023 11:17, Roger Pau Monne wrote:
> > --- a/xen/arch/x86/setup.c
> > +++ b/xen/arch/x86/setup.c
> > @@ -58,6 +58,7 @@
> >  #include <asm/microcode.h>
> >  #include <asm/prot-key.h>
> >  #include <asm/pv/domain.h>
> > +#include <asm/test-smoc.h>
> >  
> >  /* opt_nosmp: If true, secondary processors are ignored. */
> >  static bool __initdata opt_nosmp;
> > @@ -1951,6 +1952,8 @@ void asmlinkage __init noreturn __start_xen(unsigned 
> > long mbi_p)
> >  
> >      alternative_branches();
> >  
> > +    test_smoc(XEN_SYSCTL_TEST_SMOC_ALL, NULL);
> 
> I realize I'm at risk of causing scope creep, but I'd still like to at
> least ask: As further self-tests are added, we likely don't want to
> alter __start_xen() every time. Should there perhaps better be a wrapper
> (going forward: multiple ones, depending on the time tests want invoking),
> together with a Kconfig control to allow suppressing all of these tests in
> at least release builds?

Right now I only had in mind that livepatch related tests won't be
executed as part of the call in __start_xen(), but all the other ones
would, and hence wasn't expecting the code to change from the form in
the next patch.

> > --- /dev/null
> > +++ b/xen/arch/x86/test/smoc.c
> > @@ -0,0 +1,68 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#include <xen/errno.h>
> > +
> > +#include <asm/alternative.h>
> > +#include <asm/cpufeature.h>
> > +#include <asm/test-smoc.h>
> > +
> > +static bool cf_check test_insn_replacement(void)
> > +{
> > +#define EXPECTED_VALUE 2
> > +    unsigned int r = ~EXPECTED_VALUE;
> > +
> > +    alternative_io("", "mov %1, %0", X86_FEATURE_ALWAYS,
> > +                   "+r" (r), "i" (EXPECTED_VALUE));
> > +
> > +    return r == EXPECTED_VALUE;
> > +#undef EXPECTED_VALUE
> > +}
> > +
> > +int test_smoc(uint32_t selection, uint32_t *results)
> > +{
> > +    struct {
> > +        unsigned int mask;
> > +        bool (*test)(void);
> > +        const char *name;
> > +    } static const tests[] = {
> > +        { XEN_SYSCTL_TEST_SMOC_INSN_REPL, &test_insn_replacement,
> > +          "alternative instruction replacement" },
> > +    };
> > +    unsigned int i;
> > +
> > +    if ( selection & ~XEN_SYSCTL_TEST_SMOC_ALL )
> > +        return -EINVAL;
> > +
> > +    if ( results )
> > +        *results = 0;
> > +
> > +    printk(XENLOG_INFO "Checking Self Modify Code\n");
> > +
> > +    for ( i = 0; i < ARRAY_SIZE(tests); i++ )
> > +    {
> > +        if ( !(selection & tests[i].mask) )
> > +            continue;
> > +
> > +        if ( tests[i].test() )
> > +        {
> > +            if ( results )
> > +                *results |= tests[i].mask;
> > +            continue;
> > +        }
> > +
> > +        add_taint(TAINT_ERROR_SMOC);
> > +        printk(XENLOG_ERR "%s test failed\n", tests[i].name);
> 
> Do we really want both of these even when coming here from the sysctl?

So only print the messages if system_state < SYS_STATE_active?

That would be OK by me.

> > --- a/xen/common/kernel.c
> > +++ b/xen/common/kernel.c
> > @@ -386,13 +386,14 @@ char *print_tainted(char *str)
> >  {
> >      if ( tainted )
> >      {
> > -        snprintf(str, TAINT_STRING_MAX_LEN, "Tainted: %c%c%c%c%c%c",
> > +        snprintf(str, TAINT_STRING_MAX_LEN, "Tainted: %c%c%c%c%c%c%c",
> >                   tainted & TAINT_MACHINE_INSECURE ? 'I' : ' ',
> >                   tainted & TAINT_MACHINE_CHECK ? 'M' : ' ',
> >                   tainted & TAINT_SYNC_CONSOLE ? 'C' : ' ',
> >                   tainted & TAINT_ERROR_INJECT ? 'E' : ' ',
> >                   tainted & TAINT_HVM_FEP ? 'H' : ' ',
> > -                 tainted & TAINT_CPU_OUT_OF_SPEC ? 'S' : ' ');
> > +                 tainted & TAINT_CPU_OUT_OF_SPEC ? 'S' : ' ',
> > +                 tainted & TAINT_ERROR_SMOC ? 'A' : ' ');
> 
> How well is this going to scale as other selftests are added? IOW should
> this taint really be self-modifying-code-specific?

I'm afraid I'm not sure I'm following.  Would you instead like to make
the taint per-test selectable?

> > --- a/xen/include/public/sysctl.h
> > +++ b/xen/include/public/sysctl.h
> > @@ -1180,6 +1180,7 @@ struct xen_sysctl_cpu_policy {
> >  };
> >  typedef struct xen_sysctl_cpu_policy xen_sysctl_cpu_policy_t;
> >  DEFINE_XEN_GUEST_HANDLE(xen_sysctl_cpu_policy_t);
> > +
> >  #endif
> >  
> >  #if defined(__arm__) || defined (__aarch64__)
> 
> Stray change (perhaps leftover from moving code around)?

Indeed, sorry.

Thanks, Roger.