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