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package/cups-filters: fix CVE-2023-24805
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Fix CVE-2023-24805: cups-filters contains backends, filters, and other
software required to get the cups printing service working on operating
systems other than macos. If you use the Backend Error Handler (beh) to
create an accessible network printer, this security vulnerability can
cause remote code execution. `beh.c` contains the line `retval =
system(cmdline) >> 8;` which calls the `system` command with the operand
`cmdline`. `cmdline` contains multiple user controlled, unsanitized
values. As a result an attacker with network access to the hosted print
server can exploit this vulnerability to inject system commands which
are executed in the context of the running server. This issue has been
addressed in commit `8f2740357` and is expected to be bundled in the
next release. Users are advised to upgrade when possible and to restrict
access to network printers in the meantime.

Signed-off-by: Fabrice Fontaine <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>
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@ffontaine @tpetazzoni
ffontaine authored and tpetazzoni committed Nov 4, 2023
1 parent a13c6c7 commit 738029b
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208 changes: 208 additions & 0 deletions package/cups-filters/0001-beh-backend-Use-execv-instead-of-system-CVE-2023-24805.patch
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Original file line number Diff line number Diff line change
@@ -0,0 +1,208 @@
From 93e60d3df358c0ae6f3dba79e1c9684657683d89 Mon Sep 17 00:00:00 2001
From: Till Kamppeter <[email protected]>
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.

Upstream: https://github.com/OpenPrinting/cups-filters/commit/93e60d3df358c0ae6f3dba79e1c9684657683d89
Signed-off-by: Fabrice Fontaine <[email protected]>
---
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);
3 changes: 3 additions & 0 deletions package/cups-filters/cups-filters.mk
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,9 @@ CUPS_FILTERS_LICENSE = GPL-2.0, GPL-2.0+, GPL-3.0, GPL-3.0+, LGPL-2, LGPL-2.1+,
CUPS_FILTERS_LICENSE_FILES = COPYING
CUPS_FILTERS_CPE_ID_VENDOR = linuxfoundation

# 0001-beh-backend-Use-execv-instead-of-system-CVE-2023-24805.patch
CUPS_FILTERS_IGNORE_CVES += CVE-2023-24805

CUPS_FILTERS_DEPENDENCIES = cups libglib2 lcms2 qpdf fontconfig freetype jpeg

CUPS_FILTERS_CONF_OPTS = \
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