Change getlocalbase() to not allocate any heap memory

After the commit of the current version, Scott Long pointed out, that an
attacker might be able to cause a use-after-free access if this function
returned the value of the sysctl variable "user.localbase" by freeing
the allocated memory without the cached address being cleared in the
library function.

To resolve this issue, I have proposed the originally suggested version
with a statically allocated buffer in a review (D27370). There was no
feedback on this review and after waiting for more than 2 weeks, the
potential security issue is fixed by this commit. (There was no security
risk in practice, since none of the programs converted to use this
function attempted to free the buffer. The address could only have
pointed into the heap if user.localbase was set to a non-default value,
into r/o data or the environment, else.)

This version uses a static buffer of size LOCALBASE_CTL_LEN, which
defaults to MAXPATHLEN. This does not increase the memory footprint
of the library at this time, since its data segment grows from less
than 7 KB to less than 8 KB, i.e. it will get two 4 KB pages on typical
architectures, anyway.

Compiling with LOCALBASE_CTL_LEN defined as 0 will remove the code
that accesses the sysctl variable, values between 1 and MAXPATHLEN-1
will limit the maximum size of the prefix. When built with such a
value and if too large a value has been configured in user.localbase,
the value defined as ILLEGAL_PREFIX will be returned to cause any
file operations on that result to fail. (Default value is "/dev/null/",
the review contained "/\177", but I assume that "/dev/null" exists and
can not be accessed as a directory. Any other string that can be assumed
not be a valid path prefix could be used.)

I do suggest to use LOCALBASE_CTL_LEN to size the in-kernel buffer for
the user.localbase variable, too. Doing this would guarantee that the
result always fit into the buffer in this library function (unless run
on a kernel built with a different buffer size.)

The function always returns a valid string, and only in case it is built
with a small static buffer and run on a system with too large a value in
user.localbase, the ILLEGAL_PREFIX will be returned, effectively causing
the created path to be non-existent.

Differential Revision:	https://reviews.freebsd.org/D27370

lib/libutil/getlocalbase.3

@@ -27,7 +27,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 18, 2020
+.Dd November 25, 2020
 .Dt GETLOCALBASE 3
 .Os
 .Sh NAME
@@ -59,7 +59,7 @@ If that is undefined then the default of
 .Pa /usr/local
 is used.
 .Pp
-The value returned by the
+The contents of the string returned by the
 .Fn getlocalbase
 function shall not be modified.
 .Sh IMPLEMENTATION NOTES
@@ -67,13 +67,34 @@ Calls to
 .Fn getlocalbase
 will perform a setugid check on the running binary before checking the
 environment.
+.Pp
+The address returned by
+.Fn getlocalbase
+will point into the executing processes environment if it is the result of
+.Fn getenv "LOCALBASE" ,
+to a static buffer if it is the result of
+.Fn sysctl "user.localbase" ,
+and to a constant string if the compiled in default value is returned.
+.Pp
+The same value will be returned on successive calls during the run-time
+of the program, ignoring any changes to the environment variable or the
+sysctl value that might have been made.
+.Pp
+The
+.Fn getlocalbase
+function can be compiled with a non-default value of LOCALBASE_CTL_LEN.
+A value of 0 will disable fetching of the sysctl value, a value less than
+MAXPATHLEN will put a limit on the maximum string length supported for
+this sysctl value.
+If built with a non-default value of LOCALBASE_CTL_LEN, a value of the
+user.localbase sysctl variable longer than this value will make
+.Fn getlocalbase
+return a valid string that is not a valid path prefix in any filesystem.
 .Sh RETURN VALUES
 The
 .Fn getlocalbase
-function always succeeds and returns a pointer to a string, whose length
+function returns a pointer to a string, whose length may exceed MAXPATHLEN,
-may exceed MAXPATHLEN if it has been derived from the environment variable
+if it has been obtained from the environment.
-LOCALBASE.
-No length checks are performed on the result.
 .Sh ENVIRONMENT
 The
 .Fn getlocalbase
@@ -83,7 +104,7 @@ environment variable.
 .Sh ERRORS
 The
 .Fn getlocalbase
-function always succeeds.
+function always succeeds and returns a valid pointer to a string.
 .Sh SEE ALSO
 .Xr env 1 ,
 .Xr src.conf 5 ,

lib/libutil/getlocalbase.c

@@ -31,6 +31,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/param.h>
 #include <sys/sysctl.h>
 #include <sys/limits.h>
+#include <errno.h>
 #include <stdlib.h>
 #include <paths.h>
 #include <libutil.h>
@@ -40,35 +41,50 @@ __FBSDID("$FreeBSD$");
 #define _PATH_LOCALBASE "/usr/local"
 #endif
 
+#ifndef LOCALBASE_CTL_LEN
+#define LOCALBASE_CTL_LEN MAXPATHLEN
+#endif
+
+/* Any prefix guaranteed to not be the start of a valid path name */
+#define ILLEGAL_PREFIX "/dev/null/"
+
 const char *
 getlocalbase(void)
 {
-	static const int localbase_oid[2] = {CTL_USER, USER_LOCALBASE};
+#if LOCALBASE_CTL_LEN > 0 
+	int localbase_oid[2] = {CTL_USER, USER_LOCALBASE};
+	static char localpath[LOCALBASE_CTL_LEN];
+	size_t localpathlen = LOCALBASE_CTL_LEN;
+#endif
 	char *tmppath;
-	size_t tmplen;
 	static const char *localbase = NULL;
 
+	if (localbase != NULL)
+		return (localbase);
+
 	if (issetugid() == 0) {
 		tmppath = getenv("LOCALBASE");
-		if (tmppath != NULL && tmppath[0] != '\0')
+		if (tmppath != NULL && tmppath[0] != '\0') {
-			return (tmppath);
-	}
-	if (sysctl(localbase_oid, 2, NULL, &tmplen, NULL, 0) == 0 &&
-	    (tmppath = malloc(tmplen)) != NULL && 
-	    sysctl(localbase_oid, 2, tmppath, &tmplen, NULL, 0) == 0) {
-		/*
-		 * Check for some other thread already having 
-		 * set localbase - this should use atomic ops.
-		 * The amount of memory allocated above may leak,
-		 * if a parallel update in another thread is not
-		 * detected and the non-NULL pointer is overwritten.
-		 */
-		if (tmppath[0] != '\0' &&
-		    (volatile const char*)localbase == NULL)
 			localbase = tmppath;
-		else
+			return (localbase);
-			free((void*)tmppath);
+		}
-		return (localbase);
 	}
-	return (_PATH_LOCALBASE);
+
+#if LOCALBASE_CTL_LEN > 0
+	if (sysctl(localbase_oid, 2, localpath, &localpathlen, NULL, 0) != 0) {
+		if (errno != ENOMEM)
+			localbase = _PATH_LOCALBASE;
+		else
+			localbase = ILLEGAL_PREFIX;
+	} else {
+		if (localpath[0] != '\0')
+			localbase = localpath;
+		else
+			localbase = _PATH_LOCALBASE;
+	}
+#else
+	localbase = _PATH_LOCALBASE;
+#endif
+
+	return (localbase);
 }