pids: fix a race in pid generation that causes pids to be reused immediately

A program that repeatedly forks and waits is susceptible to having the
same pid repeated, especially when it competes with another instance of
the same program.  This is really bad for bash implementation.
Furthermore, many shell scripts assume that pid numbers will not be used
for some length of time.

Race Description:

A                                    B

// pid == offset == n                // pid == offset == n + 1
test_and_set_bit(offset, map->page)
                                     test_and_set_bit(offset, map->page);
                                     pid_ns->last_pid = pid;
pid_ns->last_pid = pid;
                                     // pid == n + 1 is freed (wait())

                                     // Next fork()...
                                     last = pid_ns->last_pid; // == n
                                     pid = last + 1;

Code to reproduce it (Running multiple instances is more effective):

#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

// The distance mod 32768 between two pids, where the first pid is expected
// to be smaller than the second.
int PidDistance(pid_t first, pid_t second) {
  return (second + 32768 - first) % 32768;
}

int main(int argc, char* argv[]) {
  int failed = 0;
  pid_t last_pid = 0;
  int i;
  printf("%d\n", sizeof(pid_t));
  for (i = 0; i < 10000000; ++i) {
    if (i % 32786 == 0)
      printf("Iter: %d\n", i/32768);
    int child_exit_code = i % 256;
    pid_t pid = fork();
    if (pid == -1) {
      fprintf(stderr, "fork failed, iteration %d, errno=%d", i, errno);
      exit(1);
    }
    if (pid == 0) {
      // Child
      exit(child_exit_code);
    } else {
      // Parent
      if (i > 0) {
        int distance = PidDistance(last_pid, pid);
        if (distance == 0 || distance > 30000) {
          fprintf(stderr,
                  "Unexpected pid sequence: previous fork: pid=%d, "
                  "current fork: pid=%d for iteration=%d.\n",
                  last_pid, pid, i);
          failed = 1;
        }
      }
      last_pid = pid;
      int status;
      int reaped = wait(&status);
      if (reaped != pid) {
        fprintf(stderr,
                "Wait return value: expected pid=%d, "
                "got %d, iteration %d\n",
                pid, reaped, i);
        failed = 1;
      } else if (WEXITSTATUS(status) != child_exit_code) {
        fprintf(stderr,
                "Unexpected exit status %x, iteration %d\n",
                WEXITSTATUS(status), i);
        failed = 1;
      }
    }
  }
  exit(failed);
}

Thanks to Ted Tso for the key ideas of this implementation.
Signed-off-by: Salman Qazi <sqazi@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/pid.c

@@ -122,6 +122,43 @@ static void free_pidmap(struct upid *upid)
 	atomic_inc(&map->nr_free);
 }
 
+/*
+ * If we started walking pids at 'base', is 'a' seen before 'b'?
+ */
+static int pid_before(int base, int a, int b)
+{
+	/*
+	 * This is the same as saying
+	 *
+	 * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
+	 * and that mapping orders 'a' and 'b' with respect to 'base'.
+	 */
+	return (unsigned)(a - base) < (unsigned)(b - base);
+}
+
+/*
+ * We might be racing with someone else trying to set pid_ns->last_pid.
+ * We want the winner to have the "later" value, because if the
+ * "earlier" value prevails, then a pid may get reused immediately.
+ *
+ * Since pids rollover, it is not sufficient to just pick the bigger
+ * value.  We have to consider where we started counting from.
+ *
+ * 'base' is the value of pid_ns->last_pid that we observed when
+ * we started looking for a pid.
+ *
+ * 'pid' is the pid that we eventually found.
+ */
+static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
+{
+	int prev;
+	int last_write = base;
+	do {
+		prev = last_write;
+		last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
+	} while ((prev != last_write) && (pid_before(base, last_write, pid)));
+}
+
 static int alloc_pidmap(struct pid_namespace *pid_ns)
 {
 	int i, offset, max_scan, pid, last = pid_ns->last_pid;
@@ -154,7 +191,7 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
-					pid_ns->last_pid = pid;
+					set_last_pid(pid_ns, last, pid);
 					return pid;
 				}
 				offset = find_next_offset(map, offset);