From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from goalie.tycho.ncsc.mil (goalie [144.51.242.250]) by tarius.tycho.ncsc.mil (8.14.4/8.14.4) with ESMTP id v3QIDrnL024520 for ; Wed, 26 Apr 2017 14:13:54 -0400 Message-ID: <1493230416.17296.3.camel@trentalancia.net> Subject: Re: [PATCH v3] libsemanage: remove lock files From: Guido Trentalancia To: selinux@tycho.nsa.gov Date: Wed, 26 Apr 2017 20:13:36 +0200 In-Reply-To: <1493211369.32540.4.camel@tycho.nsa.gov> References: <58517705.198270.1492699110308@pim.register.it> <100DD2AB-228E-47B5-8058-C9F030AEA665@trentalancia.net> <201704251630.03844.russell@coker.com.au> <1493150773.12050.2.camel@trentalancia.net> <1493152517.17316.0.camel@trentalancia.net> <20170426120308.GA8528@meriadoc.perfinion.com> <1493211369.32540.4.camel@tycho.nsa.gov> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 List-Id: "Security-Enhanced Linux \(SELinux\) mailing list" List-Post: List-Help: Hello. On Wed, 26/04/2017 at 08.56 -0400, Stephen Smalley wrote: > On Wed, 2017-04-26 at 20:03 +0800, Jason Zaman wrote: > > On Tue, Apr 25, 2017 at 10:35:17PM +0200, Guido Trentalancia wrote: > > > Do not use flock() for file locking, but instead use generic text > > > files > > > that keep track of the process ID (PID) of the locking process. > > > > > > Remove semanage read and transaction lock files upon releasing > > > them. > > > > > > This third version fixes a bug in the previous version and also > > > applies > > > cleanly to the latest git tree. > > > > > > Signed-off-by: Guido Trentalancia > > > --- > > >  src/Makefile         |    2 > > >  src/semanage_store.c |  214 > > > +++++++++++++++++++++++++++++++++++++- > > > ------------- > > >  2 files changed, 160 insertions(+), 56 deletions(-) > > > > > > --- a/src/Makefile 2017-04-25 22:27:38.105555427 +0200 > > > +++ b/src/Makefile 2017-04-25 22:28:58.512555098 +0200 > > > @@ -91,7 +91,7 @@ $(LIBA): $(OBJS) > > >   $(RANLIB) $@ > > >   > > >  $(LIBSO): $(LOBJS) > > > - $(CC) $(CFLAGS) $(LDFLAGS) -shared -o $@ $^ -lsepol > > > -laudit -lselinux -lbz2 -Wl,-soname,$(LIBSO),--version- > > > script=libsemanage.map,-z,defs > > > + $(CC) $(CFLAGS) $(LDFLAGS) -shared -o $@ $^ -lm -lsepol > > > -laudit -lselinux -lbz2 -Wl,-soname,$(LIBSO),--version- > > > script=libsemanage.map,-z,defs > > >   ln -sf $@ $(TARGET) > > >   > > >  $(LIBPC): $(LIBPC).in ../VERSION > > > --- a/src/semanage_store.c 2017-04-20 16:30:21.218209972 > > > +0200 > > > +++ b/src/semanage_store.c 2017-04-25 22:24:35.883556172 > > > +0200 > > > @@ -45,6 +45,8 @@ typedef struct dbase_policydb dbase_t; > > >  #include > > >  #include > > >  #include > > > +#include > > > +#include > > >  #include > > >  #include > > >  #include > > > @@ -52,11 +54,24 @@ typedef struct dbase_policydb dbase_t; > > >  #include > > >  #include > > >  #include > > > +#include > > >  #include > > >  #include > > >  #include > > >  #include > > >   > > > +#include > > > + > > > +#ifndef CONFIG_BASE_SMALL > > > +#define CONFIG_BASE_SMALL       0 > > > +#endif > > > + > > > +#include > > > + > > > +#ifndef PID_MAX_DEFAULT > > > +#define PID_MAX_DEFAULT 32768 > > > +#endif > > > + > > >  #include "debug.h" > > >  #include "utilities.h" > > >   > > > @@ -76,6 +91,8 @@ enum semanage_file_defs { > > >  static char > > > *semanage_paths[SEMANAGE_NUM_STORES][SEMANAGE_STORE_NUM_PATHS]; > > >  static char *semanage_files[SEMANAGE_NUM_FILES] = { NULL }; > > >  static int semanage_paths_initialized = 0; > > > +static int pid_max; > > > +static ssize_t pid_max_length; > > >   > > >  /* These are paths relative to the bottom of the module store */ > > >  static const char *semanage_relative_files[SEMANAGE_NUM_FILES] = > > > { > > > @@ -427,8 +442,23 @@ cleanup: > > >  int semanage_check_init(semanage_handle_t *sh, const char > > > *prefix) > > >  { > > >   int rc; > > > + int fd; > > > + char root[PATH_MAX]; > > > + ssize_t amount_read; > > > + > > >   if (semanage_paths_initialized == 0) { > > > - char root[PATH_MAX]; > > > + pid_max = PID_MAX_DEFAULT; > > > + pid_max_length = ceil(log10(PID_MAX_DEFAULT + > > > 1)); > > > + > > > + fd = open("/proc/sys/kernel/pid_max", O_RDONLY); > > > + if (fd > 0) { > > > + char sysctlstring[pid_max_length]; > > > + amount_read = read(fd, sysctlstring, > > > pid_max_length); > > > + if (amount_read > 0) { > > > + pid_max = atoi(sysctlstring); > > > + pid_max_length = > > > ceil(log10(pid_max + 1)); > > > + } > > > + } > > >   > > >   rc = snprintf(root, > > >         sizeof(root), > > > @@ -528,16 +558,23 @@ char *semanage_conf_path(void) > > >   > > >  /**************** functions that create module store > > > ***************/ > > >   > > > -/* Check that the semanage store exists.  If 'create' is non- > > > zero > > > then > > > - * create the directories.  Returns 0 if module store exists > > > (either > > > - * already or just created), -1 if does not exist or could not > > > be > > > - * read, or -2 if it could not create the store. */ > > > +/* Check that the semanage store exists and that the read lock > > > can > > > be > > > + * taken.  If 'create' is non-zero then it creates the > > > directories > > > + * and the lock file.  Returns 0 if the module store exists > > > (either > > > + * already or just created) and the read lock can be taken, -1 > > > if > > > it > > > + * does not exist or it is not possible to read from it, or -2 > > > if > > > it > > > + * could not create the store or it could not take the lock > > > file. > > > */ > > >  int semanage_create_store(semanage_handle_t * sh, int create) > > >  { > > >   struct stat sb; > > >   int mode_mask = R_OK | W_OK | X_OK; > > >   const char *path = semanage_files[SEMANAGE_ROOT]; > > >   int fd; > > > + pid_t pid, lock_pid; > > > + char *pid_string, *lock_pid_string; > > > + size_t pid_length; > > > + ssize_t pid_bytes; > > > + int invalid_lock = 0; > > >   > > >   if (stat(path, &sb) == -1) { > > >   if (errno == ENOENT && create) { > > > @@ -607,24 +644,81 @@ int semanage_create_store(semanage_handl > > >   return -1; > > >   } > > >   } > > > + pid = getpid(); > > > + pid_string = malloc(pid_max_length * sizeof(char)); > > > + sprintf(pid_string, "%d", pid); > > > + pid_length = strlen(pid_string); > > >   path = semanage_files[SEMANAGE_READ_LOCK]; > > >   if (stat(path, &sb) == -1) { > > >   if (errno == ENOENT && create) { > > >   if ((fd = creat(path, S_IRUSR | > > > S_IWUSR)) > > > == -1) { > > >   ERR(sh, "Could not create lock > > > file at %s.", > > >       path); > > > + free(pid_string); > > >   return -2; > > >   } > > > + pid_bytes = write(fd, pid_string, > > > pid_length); > > > > I'm pretty sure there is a race here. and another one between > > stat() > > and > > creat(). you can have two processes create and truncate the file > > then > > one write on top of the other. leaving lock files around is > > definitely > > the safest option. and are 0 bytes so its not like you're wasting > > any > > disk space. > > > > Overall I disagree with changing this just because a random file is > > left > > around. NFS may be a legit reason to change but as russel said, I > > think > > locks work somewhat over NFS too. Also, NFS mounting /home or /usr > > is > > common but /var is pretty commonly not NFS mounted. > > I agree; absent a demonstration of an actual problem with the current > libsemanage locking scheme, let's leave it alone.  Keeping around two > empty files is not a problem and this is a fairly well-established > paradigm for locking on Linux.  flock(2) works over NFS since Linux > 2.6.12. If I am not wrong, I found the information on the following web page: https://linux.die.net/man/2/flock It looks like a manual page, but I don't know if that information is obsolete or not (there's no date, that's the bad side of some of the web). There are other reports online, I have not tested so I cannot tell. As for the problem pointed out by Jason, I have improved it by removing the code that leads to the other race condition: if you need a revised version (v4) of the patch, please let me know, otherwise I take there isn't enough interest in the change or the information is wrong/outdated. Regards, Guido