login: pututxline could fail to overwrite existing entries [BZ #24902]
Commit Message
The internal_getut_r function updates the file_offset variable and
therefore must always update last_entry as well.
Previously, if pututxline could not upgrade the read lock to a
write lock, internal_getut_r would update file_offset only,
without updating last_entry, and a subsequent call would not
overwrite the existing utmpx entry at file_offset, instead
creating a new entry. This has been observed to cause unbounded
file growth in high-load situations.
This commit removes the buffer argument to internal_getut_r and
updates the last_entry variable directly, along with file_offset.
Initially reported and fixed by Ondřej Lysoněk.
2019-08-21 Florian Weimer <fweimer@redhat.com>
[BZ #24902]
* login/Makefile (tests): Add tst-pututxline-lockfail.
(tst-pututxline-lockfail): Link with -lpthread.
* login/utmp_file.c (internal_getut_r): Remove buffer argument.
(__libc_getutid_r): Adjust.
(__libc_pututline): Likewise. Check for file_offset == -1.
* login/tst-pututxline-lockfail.c: New file.
Comments
On Wed, Aug 21 2019, Florian Weimer wrote:
>
> Previously, if pututxline could not upgrade the read lock to a
> write lock, internal_getut_r would update file_offset only,
> without updating last_entry, and a subsequent call would not
> overwrite the existing utmpx entry at file_offset, instead
> creating a new entry. This has been observed to cause unbounded
> file growth in high-load situations.
Looks good to me with the typo below fixed. Thanks.
Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>
> +/* Create the initial entry in a subprocess, so that the utmp
> + subsystem in the original rocess is not disturbed. */
~~~~~~
Typo: process?
> +__attribute__ ((noreturn)) static void
> +subprocess_lock_file (void)
> +{
> + int fd = xopen (path, O_RDONLY, 0);
> +
> + struct flock64 fl =
> + {
> + .l_type = F_RDLCK,
> + fl.l_whence = SEEK_SET,
> + };
> + TEST_COMPARE (fcntl64 (fd, F_SETLKW, &fl), 0);
> +
> + /* Signal to the main process that the lock has been acquired. */
> + xpthread_barrier_wait (barrier);
> +
> + /* Wait for the unlock request from the main process. */
> + xpthread_barrier_wait (barrier);
> +
> + /* Implicitly unlock the file. */
> + xclose (fd);
> +
> + /* Overwrite the existing entry. */
> + TEST_COMPARE (utmpname (path), 0);
> + errno = 0;
> + setutxent ();
> + TEST_COMPARE (errno, 0);
> + TEST_VERIFY (write_entry (102) != NULL);
OK. This will replace the only entry in the utmp file.
(that's my first contact with the user management database file and
functions, so I'll not 100% sure I got it right).
> +static int
> +do_test (void)
> +{
> + xclose (create_temp_file ("tst-pututxline-lockfail-", &path));
> +
> + {
> + pthread_barrierattr_t attr;
> + xpthread_barrierattr_init (&attr);
> + xpthread_barrierattr_setpshared (&attr, PTHREAD_SCOPE_PROCESS);
> + barrier = support_shared_allocate (sizeof (*barrier));
> + xpthread_barrier_init (barrier, &attr, 2);
> + xpthread_barrierattr_destroy (&attr);
> + }
> +
> + /* Write the initial entry. */
> + support_isolate_in_subprocess (subprocess_create_entry, NULL);
> +
> + pid_t locker_pid = xfork ();
> + if (locker_pid == 0)
> + subprocess_lock_file ();
> +
> + /* Wait for the file locking to complete. */
> + xpthread_barrier_wait (barrier);
> +
> + /* Try to add another entry. This attempt will fail, with EINTR or
> + EAGAIN. */
> + TEST_COMPARE (utmpname (path), 0);
> + TEST_VERIFY (write_entry (102) == NULL);
> + if (errno != EINTR)
> + TEST_COMPARE (errno, EAGAIN);
OK. This will try to update the current entry, leaving file_offset and
last_entry in an inconsistent state (the bug this patch fixes).
> + /* Signal the subprocess to overwrite the entry. */
> + xpthread_barrier_wait (barrier);
> +
> + /* Wait for write and unlock to complete. */
> + {
> + int status;
> + xwaitpid (locker_pid, &status, 0);
> + TEST_COMPARE (status, 0);
> + }
> +
> + /* The file is no longer locked, so this operation will succeed. */
> + TEST_VERIFY (write_entry (103) != NULL);
> + errno = 0;
> + endutxent ();
> + TEST_COMPARE (errno, 0);
> +
> + /* Check that there is just one entry with the expected contents.
> + If pututxline becomes desynchronized internally, the entry is not
> + overwritten (bug 24902). */
> + errno = 0;
> + setutxent ();
> + TEST_COMPARE (errno, 0);
> + struct utmpx *ut = getutxent ();
> + TEST_VERIFY_EXIT (ut != NULL);
> + TEST_COMPARE (ut->ut_type, LOGIN_PROCESS);
> + TEST_COMPARE_STRING (ut->ut_id, "1");
> + TEST_COMPARE_STRING (ut->ut_user, "root");
> + TEST_COMPARE (ut->ut_pid, 103);
Without the fix, this assert fails, because getutxent finds the
not-overwritten entry with pid == 102. After the fix, it works
correctly (note: I tested the patch with and without the actual fix).
> + TEST_COMPARE_STRING (ut->ut_line, "entry");
> + TEST_COMPARE_STRING (ut->ut_host, "localhost");
> + TEST_VERIFY (getutxent () == NULL);
Without the fix, this assert fails, because there is an extra entry in
the database file.
@@ -43,7 +43,8 @@ endif
subdir-dirs = programs
vpath %.c programs
-tests := tst-utmp tst-utmpx tst-grantpt tst-ptsname tst-getlogin tst-updwtmpx
+tests := tst-utmp tst-utmpx tst-grantpt tst-ptsname tst-getlogin tst-updwtmpx \
+ tst-pututxline-lockfail
# Build the -lutil library with these extra functions.
extra-libs := libutil
@@ -71,3 +72,5 @@ endif
$(inst_libexecdir)/pt_chown: $(objpfx)pt_chown $(+force)
$(make-target-directory)
-$(INSTALL_PROGRAM) -m 4755 -o root $< $@
+
+$(objpfx)tst-pututxline-lockfail: $(shared-thread-library)
new file mode 100644
@@ -0,0 +1,176 @@
+/* Test the lock upgrade path in tst-pututxline.
+ Copyright (C) 2019 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public License as
+ published by the Free Software Foundation; either version 2.1 of the
+ License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; see the file COPYING.LIB. If
+ not, see <http://www.gnu.org/licenses/>. */
+
+/* pututxline upgrades the read lock on the file to a write lock.
+ This test verifies that if the lock upgrade fails, the utmp
+ subsystem remains in a consistent state, so that pututxline can be
+ called again. */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/xthread.h>
+#include <support/xunistd.h>
+#include <unistd.h>
+#include <utmp.h>
+#include <utmpx.h>
+
+/* Path to the temporary utmp file. */
+static char *path;
+
+/* Used to synchronize the subprocesses. The barrier itself is
+ allocated in shared memory. */
+static pthread_barrier_t *barrier;
+
+/* Use pututxline to write an entry for PID. */
+static struct utmpx *
+write_entry (pid_t pid)
+{
+ struct utmpx ut =
+ {
+ .ut_type = LOGIN_PROCESS,
+ .ut_id = "1",
+ .ut_user = "root",
+ .ut_pid = pid,
+ .ut_line = "entry",
+ .ut_host = "localhost",
+ };
+ return pututxline (&ut);
+}
+
+/* Create the initial entry in a subprocess, so that the utmp
+ subsystem in the original rocess is not disturbed. */
+static void
+subprocess_create_entry (void *closure)
+{
+ TEST_COMPARE (utmpname (path), 0);
+ TEST_VERIFY (write_entry (101) != NULL);
+}
+
+/* Acquire an advisory read lock on PATH. */
+__attribute__ ((noreturn)) static void
+subprocess_lock_file (void)
+{
+ int fd = xopen (path, O_RDONLY, 0);
+
+ struct flock64 fl =
+ {
+ .l_type = F_RDLCK,
+ fl.l_whence = SEEK_SET,
+ };
+ TEST_COMPARE (fcntl64 (fd, F_SETLKW, &fl), 0);
+
+ /* Signal to the main process that the lock has been acquired. */
+ xpthread_barrier_wait (barrier);
+
+ /* Wait for the unlock request from the main process. */
+ xpthread_barrier_wait (barrier);
+
+ /* Implicitly unlock the file. */
+ xclose (fd);
+
+ /* Overwrite the existing entry. */
+ TEST_COMPARE (utmpname (path), 0);
+ errno = 0;
+ setutxent ();
+ TEST_COMPARE (errno, 0);
+ TEST_VERIFY (write_entry (102) != NULL);
+ errno = 0;
+ endutxent ();
+ TEST_COMPARE (errno, 0);
+
+ _exit (0);
+}
+
+static int
+do_test (void)
+{
+ xclose (create_temp_file ("tst-pututxline-lockfail-", &path));
+
+ {
+ pthread_barrierattr_t attr;
+ xpthread_barrierattr_init (&attr);
+ xpthread_barrierattr_setpshared (&attr, PTHREAD_SCOPE_PROCESS);
+ barrier = support_shared_allocate (sizeof (*barrier));
+ xpthread_barrier_init (barrier, &attr, 2);
+ xpthread_barrierattr_destroy (&attr);
+ }
+
+ /* Write the initial entry. */
+ support_isolate_in_subprocess (subprocess_create_entry, NULL);
+
+ pid_t locker_pid = xfork ();
+ if (locker_pid == 0)
+ subprocess_lock_file ();
+
+ /* Wait for the file locking to complete. */
+ xpthread_barrier_wait (barrier);
+
+ /* Try to add another entry. This attempt will fail, with EINTR or
+ EAGAIN. */
+ TEST_COMPARE (utmpname (path), 0);
+ TEST_VERIFY (write_entry (102) == NULL);
+ if (errno != EINTR)
+ TEST_COMPARE (errno, EAGAIN);
+
+ /* Signal the subprocess to overwrite the entry. */
+ xpthread_barrier_wait (barrier);
+
+ /* Wait for write and unlock to complete. */
+ {
+ int status;
+ xwaitpid (locker_pid, &status, 0);
+ TEST_COMPARE (status, 0);
+ }
+
+ /* The file is no longer locked, so this operation will succeed. */
+ TEST_VERIFY (write_entry (103) != NULL);
+ errno = 0;
+ endutxent ();
+ TEST_COMPARE (errno, 0);
+
+ /* Check that there is just one entry with the expected contents.
+ If pututxline becomes desynchronized internally, the entry is not
+ overwritten (bug 24902). */
+ errno = 0;
+ setutxent ();
+ TEST_COMPARE (errno, 0);
+ struct utmpx *ut = getutxent ();
+ TEST_VERIFY_EXIT (ut != NULL);
+ TEST_COMPARE (ut->ut_type, LOGIN_PROCESS);
+ TEST_COMPARE_STRING (ut->ut_id, "1");
+ TEST_COMPARE_STRING (ut->ut_user, "root");
+ TEST_COMPARE (ut->ut_pid, 103);
+ TEST_COMPARE_STRING (ut->ut_line, "entry");
+ TEST_COMPARE_STRING (ut->ut_host, "localhost");
+ TEST_VERIFY (getutxent () == NULL);
+ errno = 0;
+ endutxent ();
+ TEST_COMPARE (errno, 0);
+
+ xpthread_barrier_destroy (barrier);
+ support_shared_free (barrier);
+ free (path);
+ return 0;
+}
+
+#include <support/test-driver.c>
@@ -185,9 +185,11 @@ __libc_getutent_r (struct utmp *buffer, struct utmp **result)
}
+/* Search for *ID, updating last_entry and file_offset. Return 0 on
+ success and -1 on failure. If the locking operation failed, write
+ true to *LOCK_FAILED. */
static int
-internal_getut_r (const struct utmp *id, struct utmp *buffer,
- bool *lock_failed)
+internal_getut_r (const struct utmp *id, bool *lock_failed)
{
int result = -1;
@@ -206,7 +208,7 @@ internal_getut_r (const struct utmp *id, struct utmp *buffer,
while (1)
{
/* Read the next entry. */
- if (__read_nocancel (file_fd, buffer, sizeof (struct utmp))
+ if (__read_nocancel (file_fd, &last_entry, sizeof (struct utmp))
!= sizeof (struct utmp))
{
__set_errno (ESRCH);
@@ -215,7 +217,7 @@ internal_getut_r (const struct utmp *id, struct utmp *buffer,
}
file_offset += sizeof (struct utmp);
- if (id->ut_type == buffer->ut_type)
+ if (id->ut_type == last_entry.ut_type)
break;
}
}
@@ -227,7 +229,7 @@ internal_getut_r (const struct utmp *id, struct utmp *buffer,
while (1)
{
/* Read the next entry. */
- if (__read_nocancel (file_fd, buffer, sizeof (struct utmp))
+ if (__read_nocancel (file_fd, &last_entry, sizeof (struct utmp))
!= sizeof (struct utmp))
{
__set_errno (ESRCH);
@@ -236,7 +238,7 @@ internal_getut_r (const struct utmp *id, struct utmp *buffer,
}
file_offset += sizeof (struct utmp);
- if (__utmp_equal (buffer, id))
+ if (__utmp_equal (&last_entry, id))
break;
}
}
@@ -265,7 +267,7 @@ __libc_getutid_r (const struct utmp *id, struct utmp *buffer,
/* We don't have to distinguish whether we can lock the file or
whether there is no entry. */
bool lock_failed = false;
- if (internal_getut_r (id, &last_entry, &lock_failed) < 0)
+ if (internal_getut_r (id, &lock_failed) < 0)
{
*result = NULL;
return -1;
@@ -330,10 +332,9 @@ unlock_return:
struct utmp *
__libc_pututline (const struct utmp *data)
{
- if (!maybe_setutent ())
+ if (!maybe_setutent () || file_offset == -1l)
return NULL;
- struct utmp buffer;
struct utmp *pbuf;
int found;
@@ -369,7 +370,7 @@ __libc_pututline (const struct utmp *data)
else
{
bool lock_failed = false;
- found = internal_getut_r (data, &buffer, &lock_failed);
+ found = internal_getut_r (data, &lock_failed);
if (__builtin_expect (lock_failed, false))
{