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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

> --- /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?

> --- 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?

> --- 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)?

Jan