Re: [PATCH 4/4] elf: Make glibc.rtld.enable_secure ignore alias environment variables

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

On 2024-05-01 14:00, Adhemerval Zanella Netto wrote:
> 
> 
> On 01/05/24 14:40, Siddhesh Poyarekar wrote:
>> On 2024-04-30 15:25, Adhemerval Zanella wrote:
>>> So tunable with environment variables aliases are also ignored if
>>> glibc.rtld.enable_secure is enabled.  The tunable parsing is also
>>> optimized a bit, where the loop that checks each environment variable
>>> only checks for the tunables with aliases instead of all tables.
>>>
>>> Checked on aarch64-linux-gnu and x86_64-linux-gnu.
>>
>> Changing the sorting of the list of tunables ends up breaking internal ABI.  It's only a problem that crops up intermittently, e.g. when doing updates where a system may, for a very short time, have a different dynamic linker and libc.
>>
>> How about, instead, something like this:
>>
>> 1. Run through envp twice, first time just for tunables and the second time for envvar aliases.
>>
>> 2. Generate a second, separate array of only the tunables with env aliases for the second envvar alias run.  It could be simply:
>>
>> tunable_envalias_t
>> {
>>    const char *name;
>>    tunable_t *t;
>> };
>>
>> 3. Skip the second run if enable_secure is set.
> 
> I though about it and I decided to change tunables sorting because this is
> add extra overhead to a routine that is issue for every new process.  Updating
> the system libc was never atomically due the current constraints (maybe in
> the future if/when we move all libc to ld.so), so I am not really convinced
> that we should make the tunables ordering list an ABI constraint.

I don't have a really strong opinion either way, but it's something 
we've run into often enough in Fedora for it to be pointed out as a 
potential problem.  The right solution for this problem is in fact to 
make the TUNABLE_GET API independent of the tunable ID so that the 
tunables can move around as needed, but I don't think this patch needs 
to block on that.

Florian, do you have an opinion on this?

> 
>>
>> Thanks,
>> Sid
>>
>>> ---
>>>    elf/dl-tunables.c                |  34 ++++++--
>>>    elf/tst-tunables-enable_secure.c | 131 +++++++++++++++++++++++++++----
>>>    scripts/gen-tunables.awk         |  64 ++++++++++-----
>>>    3 files changed, 189 insertions(+), 40 deletions(-)
>>>
>>> diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
>>> index 63cf8c7ab5..c1a1d1a2e3 100644
>>> --- a/elf/dl-tunables.c
>>> +++ b/elf/dl-tunables.c
>>> @@ -300,6 +300,10 @@ __tunables_init (char **envp)
>>>      if (__libc_enable_secure)
>>>        return;
>>>    +  /* The tunable with environment variable alias are placed at the start of
>>> +     tunable array.  */
>>> +  struct tunable_toset_t tunables_env_alias[TUNABLE_NUM_ENV_ALIAS] = { 0 };
>>> +
>>>      while ((envp = get_next_env (envp, &envname, &envval, &prev_envp)) != NULL)
>>>        {
>>>          /* The environment variable is allocated on the stack by the kernel, so
>>> @@ -311,29 +315,43 @@ __tunables_init (char **envp)
>>>          continue;
>>>        }
>>>    -      for (int i = 0; i < tunables_list_size; i++)
>>> +      for (int i = 0; i < TUNABLE_NUM_ENV_ALIAS; i++)
>>>        {
>>>          tunable_t *cur = &tunable_list[i];
>>> +      const char *name = cur->env_alias;
>>>    -      /* Skip over tunables that have either been set already or should be
>>> -         skipped.  */
>>> -      if (cur->initialized || cur->env_alias[0] == '\0')
>>> +      if (name[0] == '\0')
>>>            continue;
>>>    -      const char *name = cur->env_alias;
>>> -
>>> -      /* We have a match.  Initialize and move on to the next line.  */
>>>          if (tunable_is_name (name, envname))
>>>            {
>>>              size_t envvallen = 0;
>>>              /* The environment variable is always null-terminated.  */
>>>              for (const char *p = envval; *p != '\0'; p++, envvallen++);
>>>    -          tunable_initialize (cur, envval, envvallen);
>>> +          tunables_env_alias[i] =
>>> +        (struct tunable_toset_t) { cur, envval, envvallen };
>>>              break;
>>>            }
>>>        }
>>>        }
>>> +
>>> +  /* Check if glibc.rtld.enable_secure was set and skip over the environment
>>> +     variables aliases.  */
>>> +  if (__libc_enable_secure)
>>> +    return;
>>> +
>>> +  for (int i = 0; i < TUNABLE_NUM_ENV_ALIAS; i++)
>>> +    {
>>> +      if (tunables_env_alias[i].t == NULL
>>> +      || tunables_env_alias[i].t->initialized)
>>> +    continue;
>>> +
>>> +      if (!tunable_initialize (tunables_env_alias[i].t,
>>> +                   tunables_env_alias[i].value,
>>> +                   tunables_env_alias[i].len))
>>> +    parse_tunable_print_error (&tunables_env_alias[i]);
>>> +    }
>>>    }
>>>      void
>>> diff --git a/elf/tst-tunables-enable_secure.c b/elf/tst-tunables-enable_secure.c
>>> index f5db1c84e9..d4938a2e5c 100644
>>> --- a/elf/tst-tunables-enable_secure.c
>>> +++ b/elf/tst-tunables-enable_secure.c
>>> @@ -17,6 +17,7 @@
>>>       <https://www.gnu.org/licenses/>.  */
>>>      #include <array_length.h>
>>> +#define TUNABLES_INTERNAL 1
>>>    #include <dl-tunables.h>
>>>    #include <getopt.h>
>>>    #include <intprops.h>
>>> @@ -34,6 +35,8 @@ static int restart;
>>>    static const struct test_t
>>>    {
>>>      const char *env;
>>> +  const char *extraenv;
>>> +  bool check_multiple;
>>>      int32_t expected_malloc_check;
>>>      int32_t expected_enable_secure;
>>>    } tests[] =
>>> @@ -41,39 +44,124 @@ static const struct test_t
>>>      /* Expected tunable format.  */
>>>      /* Tunables should be ignored if enable_secure is set. */
>>>      {
>>> -    "glibc.malloc.check=2:glibc.rtld.enable_secure=1",
>>> +    "GLIBC_TUNABLES=glibc.malloc.check=2:glibc.rtld.enable_secure=1",
>>> +    NULL,
>>> +    false,
>>>        0,
>>>        1,
>>>      },
>>>      /* Tunables should be ignored if enable_secure is set. */
>>>      {
>>> -    "glibc.rtld.enable_secure=1:glibc.malloc.check=2",
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=1:glibc.malloc.check=2",
>>> +    NULL,
>>> +    false,
>>>        0,
>>>        1,
>>>      },
>>>      /* Tunables should be set if enable_secure is unset. */
>>>      {
>>> -    "glibc.rtld.enable_secure=0:glibc.malloc.check=2",
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=0:glibc.malloc.check=2",
>>> +    NULL,
>>> +    false,
>>>        2,
>>>        0,
>>>      },
>>> +  /* Tunables should be ignored if enable_secure is set. */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.malloc.check=2:glibc.rtld.enable_secure=1",
>>> +    "MALLOC_CHECK_=2",
>>> +    false,
>>> +    0,
>>> +    1,
>>> +  },
>>> +  /* Same as before, but with enviroment alias prior GLIBC_TUNABLES.  */
>>> +  {
>>> +    "MALLOC_CHECK_=2",
>>> +    "GLIBC_TUNABLES=glibc.malloc.check=2:glibc.rtld.enable_secure=1",
>>> +    false,
>>> +    0,
>>> +    1,
>>> +  },
>>> +  /* Tunables should be ignored if enable_secure is set. */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=1:glibc.malloc.check=2",
>>> +    "MALLOC_CHECK_=2",
>>> +    false,
>>> +    0,
>>> +    1,
>>> +  },
>>> +  {
>>> +    "MALLOC_CHECK_=2",
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=1:glibc.malloc.check=2",
>>> +    false,
>>> +    0,
>>> +    1,
>>> +  },
>>> +  /* Tunables should be set if enable_secure is unset. */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=0:glibc.malloc.check=2",
>>> +    /* Tunable have precedence over the environment variable.  */

s/Tunable/Tunables/

>>> +    "MALLOC_CHECK_=1",
>>> +    false,
>>> +    2,
>>> +    0,
>>> +  },
>>> +  {
>>> +    "MALLOC_CHECK_=1",
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=0:glibc.malloc.check=2",
>>> +    /* Tunable have precedence over the environment variable.  */
>>> +    false,
>>> +    2,
>>> +    0,
>>> +  },
>>> +  /* Tunables should be set if enable_secure is unset. */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=0",

This gives the false impression that enable_secure can be disabled.  The 
only valid value to set this tunable is 1; 0 should be ignored.  In 
fact, now I wonder if setting enable_secure to 0 should emit a warning, 
like for other invalid tunables.  That's independent of this change of 
course.

>>> +    /* Tunable have precedence over the environment variable.  */
>>> +    "MALLOC_CHECK_=1",
>>> +    false,
>>> +    1,
>>> +    0,
>>> +  },
>>> +  /* Tunables should be set if enable_secure is unset. */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=0",
>>> +    /* Tunable have precedence over the environment variable.  */
>>> +    "MALLOC_CHECK_=1",
>>> +    false,
>>> +    1,
>>> +    0,
>>> +  },
>>> +  /* Check with tunables environment variable alias set multiple times.  */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=1:glibc.malloc.check=2",
>>> +    "MALLOC_CHECK_=2",
>>> +    true,
>>> +    0,
>>> +    1,
>>> +  },
>>> +  /* Tunables should be set if enable_secure is unset. */
>>> +  {
>>> +    "GLIBC_TUNABLES=glibc.rtld.enable_secure=0",
>>> +    /* Tunable have precedence over the environment variable.  */
>>> +    "MALLOC_CHECK_=1",
>>> +    true,
>>> +    1,
>>> +    0,
>>> +  },
>>>    };
>>>      static int
>>>    handle_restart (int i)
>>>    {
>>>      if (tests[i].expected_enable_secure == 1)
>>> -    {
>>> -      TEST_COMPARE (1, __libc_enable_secure);
>>> -    }
>>> +    TEST_COMPARE (1, __libc_enable_secure);
>>>      else
>>> -    {
>>> -      TEST_COMPARE (tests[i].expected_malloc_check,
>>> -            TUNABLE_GET_FULL (glibc, malloc, check, int32_t, NULL));
>>> -      TEST_COMPARE (tests[i].expected_enable_secure,
>>> -            TUNABLE_GET_FULL (glibc, rtld, enable_secure, int32_t,
>>> -            NULL));
>>> -    }
>>> +    TEST_COMPARE (tests[i].expected_enable_secure,
>>> +          TUNABLE_GET_FULL (glibc, rtld, enable_secure, int32_t,
>>> +                     NULL));
>>> +  TEST_COMPARE (tests[i].expected_malloc_check,
>>> +        TUNABLE_GET_FULL (glibc, malloc, check, int32_t, NULL));
>>>      return 0;
>>>    }
>>>    @@ -112,8 +200,23 @@ do_test (int argc, char *argv[])
>>>            printf ("[%d] Spawned test for %s\n", i, tests[i].env);
>>>          setenv ("GLIBC_TUNABLES", tests[i].env, 1);
>>> +
>>> +      char *envp[2 + TUNABLE_NUM_ENV_ALIAS + 1] =
>>> +      {
>>> +    (char *) tests[i].env,
>>> +    (char *) tests[i].extraenv,
>>> +    NULL,
>>> +      };
>>> +      if (tests[i].check_multiple)
>>> +    {
>>> +      int j;
>>> +      for (j=0; j < TUNABLE_NUM_ENV_ALIAS; j++)
>>> +        envp[j + 2] = (char *) tests[i].extraenv;
>>> +      envp[j + 2] = NULL;
>>> +    }
>>> +
>>>          struct support_capture_subprocess result
>>> -    = support_capture_subprogram (spargv[0], spargv, NULL);
>>> +    = support_capture_subprogram (spargv[0], spargv, envp);
>>>          support_capture_subprocess_check (&result, "tst-tunables-enable_secure",
>>>                                    0, sc_allow_stderr);
>>>          support_capture_subprocess_free (&result);
>>> diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
>>> index 9f5336381e..9a18aa6861 100644
>>> --- a/scripts/gen-tunables.awk
>>> +++ b/scripts/gen-tunables.awk
>>> @@ -14,6 +14,7 @@ BEGIN {
>>>      top_ns=""
>>>      max_name_len=0
>>>      max_alias_len=0
>>> +  num_env_alias=0
>>>    }
>>>      # Skip over blank lines and comments.
>>> @@ -60,6 +61,8 @@ $1 == "}" {
>>>        }
>>>        if (!env_alias[top_ns,ns,tunable]) {
>>>          env_alias[top_ns,ns,tunable] = "{0}"
>>> +    } else {
>>> +      num_env_alias = num_env_alias + 1
>>>        }
>>>        len = length(top_ns"."ns"."tunable)
>>>        if (len > max_name_len)
>>> @@ -125,6 +128,39 @@ $1 == "}" {
>>>      }
>>>    }
>>>    +function print_tunable_id_t (envfirst)
>>> +{
>>> +  for (tnm in types) {
>>> +    split (tnm, indices, SUBSEP);
>>> +    t = indices[1];
>>> +    n = indices[2];
>>> +    m = indices[3];
>>> +    if ((envfirst && env_alias[t,n,m] == "{0}") \
>>> +    || (!envfirst && env_alias[t,n,m] != "{0}")) {
>>> +      continue;
>>> +    }
>>> +    printf ("  TUNABLE_ENUM_NAME(%s, %s, %s),\n", t, n, m);
>>> +  }
>>> +}
>>> +
>>> +function print_tunable_entry (envfirst)
>>> +{
>>> +  for (tnm in types) {
>>> +    split (tnm, indices, SUBSEP);
>>> +    t = indices[1];
>>> +    n = indices[2];
>>> +    m = indices[3];
>>> +    if ((envfirst && env_alias[t,n,m] == "{0}") \
>>> +    || (!envfirst && env_alias[t,n,m] != "{0}")) {
>>> +      continue;
>>> +    }
>>> +    printf ("  {TUNABLE_NAME_S(%s, %s, %s)", t, n, m)
>>> +    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, {%s}, false, %s},\n",
>>> +        types[t,n,m], minvals[t,n,m], maxvals[t,n,m], default_val[t,n,m],
>>> +        default_val[t,n,m], env_alias[t,n,m]);
>>> +  }
>>> +}
>>> +
>>>    END {
>>>      if (ns != "") {
>>>        print "Unterminated namespace.  Is a closing brace missing?"
>>> @@ -138,35 +174,27 @@ END {
>>>      print "#endif"
>>>      print "#include <dl-procinfo.h>\n"
>>>    +  # asort (types)
>>> +
>>>      # Now, the enum names
>>>      print "\ntypedef enum"
>>>      print "{"
>>> -  for (tnm in types) {
>>> -    split (tnm, indices, SUBSEP);
>>> -    t = indices[1];
>>> -    n = indices[2];
>>> -    m = indices[3];
>>> -    printf ("  TUNABLE_ENUM_NAME(%s, %s, %s),\n", t, n, m);
>>> -  }
>>> +  # Make the tunable with environment variables aliases first, their index
>>> +  # will be used in the tunable parsing.
>>> +  print_tunable_id_t(1)
>>> +  print_tunable_id_t(0)
>>>      print "} tunable_id_t;\n"
>>>        print "\n#ifdef TUNABLES_INTERNAL"
>>>      # Internal definitions.
>>>      print "# define TUNABLE_NAME_MAX " (max_name_len + 1)
>>>      print "# define TUNABLE_ALIAS_MAX " (max_alias_len + 1)
>>> +  print "# define TUNABLE_NUM_ENV_ALIAS " (num_env_alias)
>>>      print "# include \"dl-tunable-types.h\""
>>>      # Finally, the tunable list.
>>> -  print "static tunable_t tunable_list[] attribute_relro = {"
>>> -  for (tnm in types) {
>>> -    split (tnm, indices, SUBSEP);
>>> -    t = indices[1];
>>> -    n = indices[2];
>>> -    m = indices[3];
>>> -    printf ("  {TUNABLE_NAME_S(%s, %s, %s)", t, n, m)
>>> -    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, {%s}, false, %s},\n",
>>> -        types[t,n,m], minvals[t,n,m], maxvals[t,n,m], default_val[t,n,m],
>>> -        default_val[t,n,m], env_alias[t,n,m]);
>>> -  }
>>> +  print "static tunable_t tunable_list[] attribute_relro __attribute_used__ = {"
>>> +  print_tunable_entry(1)
>>> +  print_tunable_entry(0)
>>>      print "};"
>>>      print "#endif"
>>>    }
>