[LTP] [PATCH] kallsyms: skip user-space mapped addresses

[Li Wang]

On Tue, Jul 9, 2024 at 8:18 PM Jan Stancek <jstancek@redhat.com> wrote:

> RHEL9 s390x with KPTI enabled has been observed to sporadically
> use same address ranges for user-space libraries as the ones
> occupied by kallsyms while in kernel-space.
>

Thanks for the proposal fix, do you think we need to add
a check to see if KPTI is explicitly enabled before doing that
is_address_mapped?
(or, check the configuration for CONFIG_PAGE_TABLE_ISOLATION)



> The test currently (wrongly) assumes, that a write to kallsym
> address should fail and doesn't take into account that same
> memory ranges could be mapped differently in user vs. kernel-space.
>
> Add a check to see if a kallsym address is already mapped by user-space,
> and if it is, skip it to avoid false positives.


> Before:
>     13  kallsyms.c:115: TFAIL: Successfully accessed kernel addr
> 0x3ff8060787d (r) (__kstrtabns_nfs3_set_ds_client)
>     14  kallsyms.c:115: TFAIL: Successfully accessed kernel addr
> 0x3ff806007a0 (t) (nfs_init_server_aclclient)
>     15  kallsyms.c:115: TFAIL: Successfully accessed kernel addr
> 0x3ff8060a058 (d) (nfsacl_version)
>     16  kallsyms.c:115: TFAIL: Successfully accessed kernel addr
> 0x3ff8060a078 (d) (nfsacl_rpcstat)
>     ...
>     kallsyms.c:130: TFAIL: Caught 103102 SIGSEGV but expected 153738
>
> After:
>     kallsyms.c:175: TDEBUG: Skipping userspace mapped address 0x3ff80220024
>     kallsyms.c:175: TDEBUG: Skipping userspace mapped address 0x3ff8022003c
>     kallsyms.c:175: TDEBUG: Skipping userspace mapped address 0x3ff80213b20
>     ...
>     kallsyms.c:184: TPASS: Caught 103473 SIGSEGV in access ksymbols addr,
> skipped 27234
>
> Signed-off-by: Jan Stancek <jstancek@redhat.com>
> ---
>  testcases/kernel/security/kallsyms/kallsyms.c | 68 +++++++++++++++++--
>  1 file changed, 63 insertions(+), 5 deletions(-)
>
> diff --git a/testcases/kernel/security/kallsyms/kallsyms.c
> b/testcases/kernel/security/kallsyms/kallsyms.c
> index 2a5cbaa325ea..b07cb9cda268 100644
> --- a/testcases/kernel/security/kallsyms/kallsyms.c
> +++ b/testcases/kernel/security/kallsyms/kallsyms.c
> @@ -54,6 +54,8 @@ static struct kallsym *sym_table;
>  static unsigned int nr_symbols;
>  static sigjmp_buf jmpbuf;
>  volatile sig_atomic_t segv_caught;
> +static unsigned long *ranges;
> +static int ranges_size, ranges_len;
>
>  static void segv_handler(int sig)
>  {
> @@ -89,6 +91,49 @@ static unsigned int read_kallsyms(struct kallsym
> *table, unsigned int table_size
>         return nr_syms;
>  }
>
> +static void read_proc_self_maps(void)
> +{
> +       FILE *fp;
> +
> +       ranges_len = 0;
> +       fp = fopen("/proc/self/maps", "r");
> +       if (fp == NULL)
> +               tst_brk(TBROK | TERRNO, "Failed to open /proc/self/maps.");
> +
> +       while (!feof(fp)) {
> +               unsigned long start, end;
> +               int ret;
> +
> +               ret = fscanf(fp, "%lx-%lx %*[^\n]\n", &start, &end);
> +               if (ret != 2) {
> +                       fclose(fp);
> +                       tst_brk(TBROK | TERRNO, "Couldn't parse
> /proc/self/maps line.");
> +               }
> +
> +               if (ranges_size < ranges_len + 1) {
> +                       ranges_size += 128;
> +                       ranges = realloc(ranges,
> 2*ranges_size*sizeof(unsigned long));
> +               }
> +               ranges[ranges_len*2] = start;
> +               ranges[ranges_len*2 + 1] = end;
> +
> +               ranges_len++;
> +       }
> +
> +       fclose(fp);
> +}
> +
> +static int is_address_mapped(unsigned long addr)
> +{
> +       int i;
> +
> +       for (i = 0; i < ranges_len; i++) {
> +               if (ranges[i*2] <= addr && addr < ranges[i*2 + 1])
> +                       return 1;
> +       }
> +       return 0;
> +}
> +
>  static void setup(void)
>  {
>         struct sigaction sa;
> @@ -117,17 +162,30 @@ static void access_ksymbols_address(struct kallsym
> *table)
>         }
>  }
>
> +
>  static void test_access_kernel_address(void)
>  {
> -       segv_caught = 0;
> +       int skipped = 0;
>
> -       for (unsigned int i = 0; i < nr_symbols; i++)
> +       segv_caught = 0;
> +       read_proc_self_maps();
> +
> +       for (unsigned int i = 0; i < nr_symbols; i++) {
> +               if (is_address_mapped(sym_table[i].addr)) {
> +                       tst_res(TDEBUG, "Skipping userspace mapped address
> 0x%lx",
> +                               sym_table[i].addr);
> +                       skipped++;
> +                       continue;
> +               }
>                 access_ksymbols_address(&sym_table[i]);
> +       }
>
> -       if (segv_caught == (sig_atomic_t)nr_symbols)
> -               tst_res(TPASS, "Caught %d SIGSEGV in access ksymbols
> addr", segv_caught);
> +       if (segv_caught == (sig_atomic_t)nr_symbols - skipped)
> +               tst_res(TPASS, "Caught %d SIGSEGV in access ksymbols addr,
> skipped %d",
> +                       segv_caught, skipped);
>         else
> -               tst_res(TFAIL, "Caught %d SIGSEGV but expected %d",
> segv_caught, nr_symbols);
> +               tst_res(TFAIL, "Caught %d SIGSEGV but expected %d, skipped
> %d",
> +                       segv_caught, nr_symbols-skipped, skipped);
>  }
>
>  static void cleanup(void)
> --
> 2.39.3
>
>