Re: [committed 2/2] tunables: Terminate if end of input is reached (CVE-2023-4911)

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 03/10/23 14:08, Siddhesh Poyarekar wrote:
> The string parsing routine may end up writing beyond bounds of tunestr
> if the input tunable string is malformed, of the form name=name=val.
> This gets processed twice, first as name=name=val and next as name=val,
> resulting in tunestr being name=name=val:name=val, thus overflowing
> tunestr.
> 
> Terminate the parsing loop at the first instance itself so that tunestr
> does not overflow.
> 
> This also fixes up tst-env-setuid-tunables to actually handle failures
> correct and add new tests to validate the fix for this CVE.
> 
> Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
> Reviewed-by: Carlos O'Donell <carlos@redhat.com>
> ---
>  NEWS                          |  5 +++++
>  elf/dl-tunables.c             | 17 +++++++++-------
>  elf/tst-env-setuid-tunables.c | 37 +++++++++++++++++++++++++++--------
>  3 files changed, 44 insertions(+), 15 deletions(-)
> 
> diff --git a/NEWS b/NEWS
> index a94650da64..cc4b81f0ac 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -64,6 +64,11 @@ Security related changes:
>    an application calls getaddrinfo for AF_INET6 with AI_CANONNAME,
>    AI_ALL and AI_V4MAPPED flags set.
>  
> +  CVE-2023-4911: If a tunable of the form NAME=NAME=VAL is passed in the
> +  environment of a setuid program and NAME is valid, it may result in a
> +  buffer overflow, which could be exploited to achieve escalated
> +  privileges.  This flaw was introduced in glibc 2.34.
> +
>  The following bugs are resolved with this release:
>  
>    [The release manager will add the list generated by
> diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
> index 62b7332d95..cae67efa0a 100644
> --- a/elf/dl-tunables.c
> +++ b/elf/dl-tunables.c
> @@ -180,11 +180,7 @@ parse_tunables (char *tunestr, char *valstring)
>        /* If we reach the end of the string before getting a valid name-value
>  	 pair, bail out.  */
>        if (p[len] == '\0')
> -	{
> -	  if (__libc_enable_secure)
> -	    tunestr[off] = '\0';
> -	  return;
> -	}
> +	break;
>  
>        /* We did not find a valid name-value pair before encountering the
>  	 colon.  */
> @@ -244,9 +240,16 @@ parse_tunables (char *tunestr, char *valstring)
>  	    }
>  	}
>  
> -      if (p[len] != '\0')
> -	p += len + 1;
> +      /* We reached the end while processing the tunable string.  */
> +      if (p[len] == '\0')
> +	break;
> +
> +      p += len + 1;
>      }
> +
> +  /* Terminate tunestr before we leave.  */
> +  if (__libc_enable_secure)
> +    tunestr[off] = '\0';
>  }
>  

So how should we handle what might be inconsistent invalid inputs like:

  glibc.malloc.mmap_threshold=4096=4096

Since glibc does not provide any TUNABLE_SECLEVEL_NONE, this tunables
will be ignored.  But for TUNABLE_SECLEVEL_NONE one, the value is
still parsed by _dl_strtoul or stored in the tunable.

>  /* Enable the glibc.malloc.check tunable in SETUID/SETGID programs only when
> diff --git a/elf/tst-env-setuid-tunables.c b/elf/tst-env-setuid-tunables.c
> index 7dfb0e073a..f0b92c97e7 100644
> --- a/elf/tst-env-setuid-tunables.c
> +++ b/elf/tst-env-setuid-tunables.c
> @@ -50,6 +50,8 @@ const char *teststrings[] =
>    "glibc.malloc.perturb=0x800:not_valid.malloc.check=2:glibc.malloc.mmap_threshold=4096",
>    "glibc.not_valid.check=2:glibc.malloc.mmap_threshold=4096",
>    "not_valid.malloc.check=2:glibc.malloc.mmap_threshold=4096",
> +  "glibc.malloc.mmap_threshold=glibc.malloc.mmap_threshold=4096",
> +  "glibc.malloc.check=2",
>    "glibc.malloc.garbage=2:glibc.maoc.mmap_threshold=4096:glibc.malloc.check=2",
>    "glibc.malloc.check=4:glibc.malloc.garbage=2:glibc.maoc.mmap_threshold=4096",
>    ":glibc.malloc.garbage=2:glibc.malloc.check=1",
> @@ -68,6 +70,8 @@ const char *resultstrings[] =
>    "glibc.malloc.perturb=0x800:glibc.malloc.mmap_threshold=4096",
>    "glibc.malloc.mmap_threshold=4096",
>    "glibc.malloc.mmap_threshold=4096",
> +  "glibc.malloc.mmap_threshold=glibc.malloc.mmap_threshold=4096",
> +  "",
>    "",
>    "",
>    "",
> @@ -81,11 +85,18 @@ test_child (int off)
>  {
>    const char *val = getenv ("GLIBC_TUNABLES");
>  
> +  printf ("    [%d] GLIBC_TUNABLES is %s\n", off, val);
> +  fflush (stdout);
>    if (val != NULL && strcmp (val, resultstrings[off]) == 0)
>      return 0;
>  
>    if (val != NULL)
> -    printf ("[%d] Unexpected GLIBC_TUNABLES VALUE %s\n", off, val);
> +    printf ("    [%d] Unexpected GLIBC_TUNABLES VALUE %s, expected %s\n",
> +	    off, val, resultstrings[off]);
> +  else
> +    printf ("    [%d] GLIBC_TUNABLES environment variable absent\n", off);
> +
> +  fflush (stdout);
>  
>    return 1;
>  }
> @@ -106,21 +117,26 @@ do_test (int argc, char **argv)
>        if (ret != 0)
>  	exit (1);
>  
> -      exit (EXIT_SUCCESS);
> +      /* Special return code to make sure that the child executed all the way
> +	 through.  */
> +      exit (42);
>      }
>    else
>      {
> -      int ret = 0;
> -
>        /* Spawn tests.  */
>        for (int i = 0; i < array_length (teststrings); i++)
>  	{
>  	  char buf[INT_BUFSIZE_BOUND (int)];
>  
> -	  printf ("Spawned test for %s (%d)\n", teststrings[i], i);
> +	  printf ("[%d] Spawned test for %s\n", i, teststrings[i]);
>  	  snprintf (buf, sizeof (buf), "%d\n", i);
> +	  fflush (stdout);
>  	  if (setenv ("GLIBC_TUNABLES", teststrings[i], 1) != 0)
> -	    exit (1);
> +	    {
> +	      printf ("    [%d] Failed to set GLIBC_TUNABLES: %m", i);
> +	      support_record_failure ();
> +	      continue;
> +	    }
>  
>  	  int status = support_capture_subprogram_self_sgid (buf);
>  
> @@ -128,9 +144,14 @@ do_test (int argc, char **argv)
>  	  if (WEXITSTATUS (status) == EXIT_UNSUPPORTED)
>  	    return EXIT_UNSUPPORTED;
>  
> -	  ret |= status;
> +	  if (WEXITSTATUS (status) != 42)
> +	    {
> +	      printf ("    [%d] child failed with status %d\n", i,
> +		      WEXITSTATUS (status));
> +	      support_record_failure ();
> +	    }
>  	}
> -      return ret;
> +      return 0;
>      }
>  }
>