From 93e60d3df358c0ae6f3dba79e1c9684657683d89 Mon Sep 17 00:00:00 2001
From: Till Kamppeter <till.kamppeter@gmail.com>
Date: Wed, 17 May 2023 11:11:29 +0200
Subject: [PATCH] beh backend: Use execv() instead of system() - CVE-2023-24805
With execv() command line arguments are passed as separate strings and
not the full command line in a single string. This prevents arbitrary
command execution by escaping the quoting of the arguments in a job
with a forged job title.
In addition, done the following fixes and improvements:
- Do not allow '/' in the scheme of the URI (= backend executable
name), to assure that only backends inside /usr/lib/cups/backend/
are used.
- URI must have ':', to split off scheme, otherwise error out.
- Check return value of snprintf() to create call path for backend, to
error out on truncation of a too long scheme or on complete failure
due to a completely odd scheme.
- Use strncat() instead of strncpy() for getting scheme from URI, the latter
does not require setting terminating zero byte in case of truncation.
- Also exclude "." or ".." as scheme, as directories are not valid CUPS
backends.
- Do not use fprintf() in sigterm_handler(), to not interfere with a
fprintf() which could be running in the main process when
sigterm_handler() is triggered.
- Use "static volatile int" for global variable job_canceled.
---
backend/beh.c | 107 +++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 84 insertions(+), 23 deletions(-)
diff --git a/backend/beh.c b/backend/beh.c
index 225fd27d5..8d51235b1 100644
--- a/backend/beh.c
+++ b/backend/beh.c
@@ -22,12 +22,13 @@
#include "backend-private.h"
#include <cups/array.h>
#include <ctype.h>
+#include <sys/wait.h>
/*
* Local globals...
*/
-static int job_canceled = 0; /* Set to 1 on SIGTERM */
+static volatile int job_canceled = 0; /* Set to 1 on SIGTERM */
/*
* Local functions...
@@ -213,21 +214,40 @@ call_backend(char *uri, /* I - URI of final destination */
char **argv, /* I - Command-line arguments */
char *filename) { /* I - File name of input data */
const char *cups_serverbin; /* Location of programs */
+ char *backend_argv[8]; /* Arguments for backend */
char scheme[1024], /* Scheme from URI */
*ptr, /* Pointer into scheme */
- cmdline[65536]; /* Backend command line */
- int retval;
+ backend_path[2048]; /* Backend path */
+ int pid = 0, /* Process ID of backend */
+ wait_pid, /* Process ID from wait() */
+ wait_status, /* Status from child */
+ retval = 0;
+ int bytes;
/*
* Build the backend command line...
*/
- strncpy(scheme, uri, sizeof(scheme) - 1);
- if (strlen(uri) > 1023)
- scheme[1023] = '\0';
+ scheme[0] = '\0';
+ strncat(scheme, uri, sizeof(scheme) - 1);
if ((ptr = strchr(scheme, ':')) != NULL)
*ptr = '\0';
-
+ else {
+ fprintf(stderr,
+ "ERROR: beh: Invalid URI, no colon (':') to mark end of scheme part.\n");
+ exit (CUPS_BACKEND_FAILED);
+ }
+ if (strchr(scheme, '/')) {
+ fprintf(stderr,
+ "ERROR: beh: Invalid URI, scheme contains a slash ('/').\n");
+ exit (CUPS_BACKEND_FAILED);
+ }
+ if (!strcmp(scheme, ".") || !strcmp(scheme, "..")) {
+ fprintf(stderr,
+ "ERROR: beh: Invalid URI, scheme (\"%s\") is a directory.\n",
+ scheme);
+ exit (CUPS_BACKEND_FAILED);
+ }
if ((cups_serverbin = getenv("CUPS_SERVERBIN")) == NULL)
cups_serverbin = CUPS_SERVERBIN;
@@ -235,16 +255,29 @@ call_backend(char *uri, /* I - URI of final destination */
fprintf(stderr,
"ERROR: beh: Direct output into a file not supported.\n");
exit (CUPS_BACKEND_FAILED);
- } else
- snprintf(cmdline, sizeof(cmdline),
- "%s/backend/%s '%s' '%s' '%s' '%s' '%s' %s",
- cups_serverbin, scheme, argv[1], argv[2], argv[3],
- /* Apply number of copies only if beh was called with a
- file name and not with the print data in stdin, as
- backends should handle copies only if they are called
- with a file name */
- (argc == 6 ? "1" : argv[4]),
- argv[5], filename);
+ }
+
+ backend_argv[0] = uri;
+ backend_argv[1] = argv[1];
+ backend_argv[2] = argv[2];
+ backend_argv[3] = argv[3];
+ /* Apply number of copies only if beh was called with a file name
+ and not with the print data in stdin, as backends should handle
+ copies only if they are called with a file name */
+ backend_argv[4] = (argc == 6 ? "1" : argv[4]);
+ backend_argv[5] = argv[5];
+ backend_argv[6] = filename;
+ backend_argv[7] = NULL;
+
+ bytes = snprintf(backend_path, sizeof(backend_path),
+ "%s/backend/%s", cups_serverbin, scheme);
+ if (bytes < 0 || bytes >= sizeof(backend_path))
+ {
+ fprintf(stderr,
+ "ERROR: beh: Invalid scheme (\"%s\"), could not determing backend path.\n",
+ scheme);
+ return (CUPS_BACKEND_FAILED);
+ }
/*
* Overwrite the device URI and run the actual backend...
@@ -253,18 +286,44 @@ call_backend(char *uri, /* I - URI of final destination */
setenv("DEVICE_URI", uri, 1);
fprintf(stderr,
- "DEBUG: beh: Executing backend command line \"%s\"...\n",
- cmdline);
+ "DEBUG: beh: Executing backend command line \"%s '%s' '%s' '%s' '%s' '%s' %s\"...\n",
+ backend_path, backend_argv[1], backend_argv[2], backend_argv[3],
+ backend_argv[4], backend_argv[5], backend_argv[6]);
fprintf(stderr,
"DEBUG: beh: Using device URI: %s\n",
uri);
- retval = system(cmdline) >> 8;
+ if ((pid = fork()) == 0) {
+ /*
+ * Child comes here...
+ */
+
+ /* Run the backend */
+ execv(backend_path, backend_argv);
- if (retval == -1)
fprintf(stderr, "ERROR: Unable to execute backend command line: %s\n",
strerror(errno));
+ exit(1);
+ } else if (pid < 0) {
+ /*
+ * Unable to fork!
+ */
+
+ return (CUPS_BACKEND_FAILED);
+ }
+
+ while ((wait_pid = wait(&wait_status)) < 0 && errno == EINTR);
+
+ if (wait_pid >= 0 && wait_status) {
+ if (WIFEXITED(wait_status))
+ retval = WEXITSTATUS(wait_status);
+ else if (WTERMSIG(wait_status) != SIGTERM)
+ retval = WTERMSIG(wait_status);
+ else
+ retval = 0;
+ }
+
return (retval);
}
@@ -277,8 +336,10 @@ static void
sigterm_handler(int sig) { /* I - Signal number (unused) */
(void)sig;
- fprintf(stderr,
- "DEBUG: beh: Job canceled.\n");
+ const char * const msg = "DEBUG: beh: Job canceled.\n";
+ /* The if() is to eliminate the return value and silence the warning
+ about an unused return value. */
+ if (write(2, msg, strlen(msg)));
if (job_canceled)
_exit(CUPS_BACKEND_OK);
--
2.40.1