Fix SXID_ERASE behavior in setuid programs (BZ #27471)

When parse_tunables tries to erase a tunable marked as SXID_ERASE for
setuid programs, it ends up setting the envvar string iterator
incorrectly, because of which it may parse the next tunable
incorrectly.  Given that currently the implementation allows malformed
and unrecognized tunables pass through, it may even allow SXID_ERASE
tunables to go through.

This change revamps the SXID_ERASE implementation so that:

- Only valid tunables are written back to the tunestr string, because
  of which children of SXID programs will only inherit a clean list of
  identified tunables that are not SXID_ERASE.

- Unrecognized tunables get scrubbed off from the environment and
  subsequently from the child environment.

- This has the side-effect that a tunable that is not identified by
  the setxid binary, will not be passed on to a non-setxid child even
  if the child could have identified that tunable.  This may break
  applications that expect this behaviour but expecting such tunables
  to cross the SXID boundary is wrong.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

(cherry picked from commit 2ed18c5b534d9e92fc006202a5af0df6b72e7aca)

diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index 4c9d36e3980758b9..bbc3679e3564a766 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -178,6 +178,7 @@ parse_tunables (char *tunestr, char *valstring)
     return;
 
   char *p = tunestr;
+  size_t off = 0;
 
   while (true)
     {
@@ -191,7 +192,11 @@ 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')
-	return;
+	{
+	  if (__libc_enable_secure)
+	    tunestr[off] = '\0';
+	  return;
+	}
 
       /* We did not find a valid name-value pair before encountering the
 	 colon.  */
@@ -217,35 +222,28 @@ parse_tunables (char *tunestr, char *valstring)
 
 	  if (tunable_is_name (cur->name, name))
 	    {
-	      /* If we are in a secure context (AT_SECURE) then ignore the tunable
-		 unless it is explicitly marked as secure.  Tunable values take
-		 precendence over their envvar aliases.  */
+	      /* If we are in a secure context (AT_SECURE) then ignore the
+		 tunable unless it is explicitly marked as secure.  Tunable
+		 values take precedence over their envvar aliases.  We write
+		 the tunables that are not SXID_ERASE back to TUNESTR, thus
+		 dropping all SXID_ERASE tunables and any invalid or
+		 unrecognized tunables.  */
 	      if (__libc_enable_secure)
 		{
-		  if (cur->security_level == TUNABLE_SECLEVEL_SXID_ERASE)
+		  if (cur->security_level != TUNABLE_SECLEVEL_SXID_ERASE)
 		    {
-		      if (p[len] == '\0')
-			{
-			  /* Last tunable in the valstring.  Null-terminate and
-			     return.  */
-			  *name = '\0';
-			  return;
-			}
-		      else
-			{
-			  /* Remove the current tunable from the string.  We do
-			     this by overwriting the string starting from NAME
-			     (which is where the current tunable begins) with
-			     the remainder of the string.  We then have P point
-			     to NAME so that we continue in the correct
-			     position in the valstring.  */
-			  char *q = &p[len + 1];
-			  p = name;
-			  while (*q != '\0')
-			    *name++ = *q++;
-			  name[0] = '\0';
-			  len = 0;
-			}
+		      if (off > 0)
+			tunestr[off++] = ':';
+
+		      const char *n = cur->name;
+
+		      while (*n != '\0')
+			tunestr[off++] = *n++;
+
+		      tunestr[off++] = '=';
+
+		      for (size_t j = 0; j < len; j++)
+			tunestr[off++] = value[j];
 		    }
 
 		  if (cur->security_level != TUNABLE_SECLEVEL_NONE)
@@ -258,9 +256,7 @@ parse_tunables (char *tunestr, char *valstring)
 	    }
 	}
 
-      if (p[len] == '\0')
-	return;
-      else
+      if (p[len] != '\0')
 	p += len + 1;
     }
 }
diff --git a/elf/tst-env-setuid-tunables.c b/elf/tst-env-setuid-tunables.c
index a48281b175af6920..0b9b075c40598c6f 100644
--- a/elf/tst-env-setuid-tunables.c
+++ b/elf/tst-env-setuid-tunables.c
@@ -45,11 +45,37 @@
 const char *teststrings[] =
 {
   "glibc.malloc.check=2:glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.check=2:glibc.malloc.check=2:glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.check=2:glibc.malloc.mmap_threshold=4096:glibc.malloc.check=2",
+  "glibc.malloc.perturb=0x800",
+  "glibc.malloc.perturb=0x800:glibc.malloc.mmap_threshold=4096",
+  "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.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",
+  "glibc.malloc.check=1:glibc.malloc.check=2",
+  "not_valid.malloc.check=2",
+  "glibc.not_valid.check=2",
 };
 
 const char *resultstrings[] =
 {
   "glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.perturb=0x800",
+  "glibc.malloc.perturb=0x800:glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.perturb=0x800:glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.mmap_threshold=4096",
+  "glibc.malloc.mmap_threshold=4096",
+  "",
+  "",
+  "",
+  "",
+  "",
+  "",
 };
 
 static int