Bug 2176844

@mdomonko @ffesti @pmatilai can you look at this?

Fapolicyd is using librpm. It is iterating over all the files in installed packages. 
We don't want to write anything but there are some new files created by librpm under fapolicyd user and group.

https://github.com/linux-application-whitelisting/fapolicyd/blob/main/src/library/rpm-backend.c

How can we prevent such a behavior?

These files are BerkeleyDB "shared regions" [1].  A quick look reveals that the use of rpmtsInitIterator() in rpm-backend.c leads to rpmtsOpenDB() in librpm which may explain where those files are being created.

This likely isn't fapolicyd specific, the same could happen with any librpm application.  It seems like an implementation detail of librpm (or BDB for that matter), not something that the calling application should need to care about so it remains to be seen whether it's just the librpm API lacking here or a straight up bug.

Florian, Panu, any thoughts?

[1] https://docs.oracle.com/cd/E17275_01/html/programmer_reference/env_region.html

And by "bug", I mean the fact that there can be files in the /var/lib/rpm directory that are created as a result of an librpm API call and that are owned by the user/group running the process, not root.

That's supposed to be a "can't happen" because /var/lib/rpm is supposed to be owned by root:root and nobody else can write there.

What does 'ls -ld /var/lib/rpm/' say? If there's something loosening up the rpmdb directory permissions then that's pretty severe.

Top directory permissions are correct:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
[root@vm-stig8 ~]# ls -ld /var/lib/rpm
drwxr-xr-x. 2 root root 4096 Mar 10 12:49 /var/lib/rpm
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Through stracing (in Permissive or else ...), I can see that capabilities are kept before switching user:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
1738  12:55:13.504793 prctl(PR_SET_KEEPCAPS, 1) = 0 <0.000010>
1738  12:55:13.504830 capset({version=_LINUX_CAPABILITY_VERSION_3, pid=1738}, {effective=1<<CAP_DAC_OVERRIDE|1<<CAP_SETGID|1<<CAP_SETUID|1<<CAP_SYS_PTRACE|1<<CAP_SYS_ADMIN|1<<CAP_SYS_NICE|1<<CAP_SYS_RESOURCE|1<<CAP_AUDIT_WRITE, permitted=1<<CAP_DAC_OVERRIDE|1<<CAP_SETGID|1<<CAP_SETUID|1<<CAP_SYS_PTRACE|1<<CAP_SYS_ADMIN|1<<CAP_SYS_NICE|1<<CAP_SYS_RESOURCE|1<<CAP_AUDIT_WRITE, inheritable=0}) = 0 <0.000008>
1738  12:55:13.504864 setresgid(991, 991, 991) = 0 <0.000007>
1738  12:55:13.504892 setgroups(0, NULL) = 0 <0.000010>
1738  12:55:13.504921 setresuid(995, 995, 995) = 0 <0.000010>
1738  12:55:13.504949 prctl(PR_SET_KEEPCAPS, 0) = 0 <0.000006>
1738  12:55:13.504974 capset({version=_LINUX_CAPABILITY_VERSION_3, pid=1738}, {effective=1<<CAP_DAC_OVERRIDE|1<<CAP_SYS_PTRACE|1<<CAP_SYS_ADMIN|1<<CAP_SYS_NICE|1<<CAP_SYS_RESOURCE|1<<CAP_AUDIT_WRITE, permitted=1<<CAP_DAC_OVERRIDE|1<<CAP_SYS_PTRACE|1<<CAP_SYS_ADMIN|1<<CAP_SYS_NICE|1<<CAP_SYS_RESOURCE|1<<CAP_AUDIT_WRITE, inheritable=0}) = 0 <0.000006>
 :
1738  12:55:13.517812 openat(AT_FDCWD</>, "/var/lib/rpm/__db.001", O_RDWR|O_CREAT|O_EXCL, 0644 <unfinished ...>
1738  12:55:13.517898 <... openat resumed>) = 8</var/lib/rpm/__db.001> <0.000069>
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Using systemtap, I could find that capset() is called by:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
(gdb) bt
#0  0x00007f73e16510bb in capset () from /lib64/libc.so.6
#1  0x00007f73e2839dbb in capng_apply () from /lib64/libcap-ng.so.0
#2  0x00007f73e2839ee5 in capng_change_id () from /lib64/libcap-ng.so.0
#3  0x000055d2d240a5e8 in main (argc=<optimized out>, argv=<optimized out>) at daemon/fapolicyd.c:493
(gdb) f 3
#3  0x000055d2d240a5e8 in main (argc=<optimized out>, argv=<optimized out>) at daemon/fapolicyd.c:493
493			if (capng_change_id(config.uid, config.gid,
(gdb) list
488		if (config.uid != 0) {
489			capng_clear(CAPNG_SELECT_BOTH);
490			capng_updatev(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED,
491				CAP_DAC_OVERRIDE, CAP_SYS_ADMIN, CAP_SYS_PTRACE,
492				CAP_SYS_NICE, CAP_SYS_RESOURCE, CAP_AUDIT_WRITE, -1);
493			if (capng_change_id(config.uid, config.gid,
494								CAPNG_DROP_SUPP_GRP)) {
495				msg(LOG_ERR, "Cannot change to uid %d", config.uid);
496				exit(1);
497			} else
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

This DAC_OVERRIDE makes sense to be able to check files from everybody, but at the time RPMDB is read, it should not have the capability enabled.

(In reply to Panu Matilainen from comment #4)
> That's supposed to be a "can't happen" because /var/lib/rpm is supposed to
> be owned by root:root and nobody else can write there.
> 
> What does 'ls -ld /var/lib/rpm/' say? If there's something loosening up the
> rpmdb directory permissions then that's pretty severe.

Fapolicyd has and needs CAP_DAC_OVERRIDE capability that's why files are created there... 

Would it make sense to implement a possibility to pass a flag to librpm that we don't want to create anything?

Unfortunately not. BDB needs to create those __db.* environment files for shared locking, even if for read-only access. Regular user queries are forced to run in private locking mode because they can't create or write to those files, and regularly produce garbage because of that. Which is something you don't want in fapolicyd.
So it's a damned if you do, damned if you don't situation.

Do you think it's possible to reproduce also in RHEL9? I mean whether BDB can be used there?

No, RHEL9 only has minimal readonly bdb-support which doesn't create such files.

I've created fapolicyd downstream workaround for rhel8. It corrects those files with chown. What do you think?

diff -up ./src/daemon/fapolicyd.c.librpm-workaround ./src/daemon/fapolicyd.c
--- ./src/daemon/fapolicyd.c.librpm-workaround	2023-07-10 11:19:19.507044648 +0200
+++ ./src/daemon/fapolicyd.c	2023-07-10 11:19:19.509044621 +0200
@@ -572,7 +572,7 @@ int main(int argc, const char *argv[])
 		capng_clear(CAPNG_SELECT_BOTH);
 		capng_updatev(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED,
 			CAP_DAC_OVERRIDE, CAP_SYS_ADMIN, CAP_SYS_PTRACE,
-			CAP_SYS_NICE, CAP_SYS_RESOURCE, CAP_AUDIT_WRITE, -1);
+			CAP_SYS_NICE, CAP_SYS_RESOURCE, CAP_AUDIT_WRITE, CAP_CHOWN, -1);
 		if (capng_change_id(config.uid, config.gid,
 							CAPNG_DROP_SUPP_GRP)) {
 			msg(LOG_ERR, "Cannot change to uid %d", config.uid);
diff -up ./src/library/rpm-backend.c.librpm-workaround ./src/library/rpm-backend.c
--- ./src/library/rpm-backend.c.librpm-workaround	2023-06-15 16:45:14.000000000 +0200
+++ ./src/library/rpm-backend.c	2023-07-10 11:22:07.066794595 +0200
@@ -32,7 +32,12 @@
 #include <rpm/rpmdb.h>
 #include <rpm/rpmpgp.h>
 #include <fnmatch.h>
+#include <glob.h>
+#include <pwd.h>
+#include <grp.h>
+#include <fcntl.h>
 
+#include <unistd.h>
 #include <uthash.h>
 
 #include "message.h"
@@ -59,6 +64,50 @@ backend rpm_backend =
 static rpmts ts = NULL;
 static rpmdbMatchIterator mi = NULL;
 
+static void fix_files(void)
+{
+	glob_t glob_result;
+	const char *pattern = "/var/lib/rpm/__*";
+
+	struct passwd * usr = getpwnam("fapolicyd");
+	if (usr == NULL) {
+		return;
+	}
+
+	struct group * grp = getgrnam("fapolicyd");
+	if (grp == NULL) {
+		return;
+	}
+
+	int return_value = glob(pattern, 0, NULL, &glob_result);
+	if (return_value != 0) {
+		return;
+	}
+
+	for (int i = 0; i < glob_result.gl_pathc; ++i) {
+
+		int fd = open(glob_result.gl_pathv[i], O_NOFOLLOW);
+
+		if (fd == -1)
+			continue;
+
+		struct stat file_stat;
+		if (fstat(fd, &file_stat) != 0) {
+			continue;
+		}
+
+		if (file_stat.st_uid == usr->pw_uid &&
+			file_stat.st_gid == grp->gr_gid) {
+
+			fchown(fd, 0, 0);
+		}
+
+		close(fd);
+	}
+
+	globfree(&glob_result);
+}
+
 static int init_rpm(void)
 {
 	return rpmReadConfigFiles ((const char *)NULL, (const char *)NULL);
@@ -201,8 +250,13 @@ static int rpm_load_list(const conf_t *c
 		return rc;
 	}
 
+	int fixed = 0;
 	// Loop across the rpm database
 	while (get_next_package_rpm()) {
+		if (!fixed) {
+			fixed = 1;
+			fix_files();
+		}
 		// Loop across the packages
 		while (get_next_file_rpm()) {
 			// We do not want directories or symlinks in the

This bug is going to be migrated.

Contact point for migration questions or issues: rsroka
Guidance for Bugzilla users to test their Jira account or create one if needed:

https://redhat.service-now.com/help?id=kb_article_view&sysparm_article=KB0016394
https://redhat.service-now.com/help?id=kb_article_view&sysparm_article=KB0016694
https://redhat.service-now.com/help?id=kb_article_view&sysparm_article=KB0016774