tunables: Terminate if end of input is reached (CVE-2023-4911)

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>

3 files changed, 44 insertions, 15 deletions

diff --git a/NEWS b/NEWS
index a94650d..cc4b81f 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 62b7332..cae67ef 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';
 }
 
 /* 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 7dfb0e0..f0b92c9 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;
     }
 }