cache: close race window when unlocking slots

We use POSIX advisory record locks to control access to cache slots, but
these have an unhelpful behaviour in that they are released when any
file descriptor referencing the file is closed by this process.

Mostly this is okay, since we know we won't be opening the lock file
anywhere else, but there is one place that it does matter: when we
restore stdout we dup2() over a file descriptor referring to the file,
thus closing that descriptor.

Since we restore stdout before unlocking the slot, this creates a window
during which the slot content can be overwritten.  The fix is reasonably
straightforward: simply restore stdout after unlocking the slot, but the
diff is a bit bigger because this requires us to move the temporary
stdout FD into struct cache_slot.

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>

1 files changed, 14 insertions, 23 deletions

diff --git a/cache.c b/cache.c
index 0901e6e..2c70be7 100644
--- a/cache.c
+++ b/cache.c
@@ -29,6 +29,7 @@ struct cache_slot {
         cache_fill_fn fn;
         int cache_fd;
         int lock_fd;
+        int stdout_fd;
         const char *cache_name;
         const char *lock_name;
         int match;
@@ -197,6 +198,13 @@ static int unlock_slot(struct cache_slot *slot, int replace_old_slot)
         else
                 err = unlink(slot->lock_name);
 
+        /* Restore stdout and close the temporary FD. */
+        if (slot->stdout_fd >= 0) {
+                dup2(slot->stdout_fd, STDOUT_FILENO);
+                close(slot->stdout_fd);
+                slot->stdout_fd = -1;
+        }
+
         if (err)
                 return errno;
 
@@ -208,42 +216,24 @@ static int unlock_slot(struct cache_slot *slot, int replace_old_slot)
  */
 static int fill_slot(struct cache_slot *slot)
 {
-        int tmp;
-
         /* Preserve stdout */
-        tmp = dup(STDOUT_FILENO);
-        if (tmp == -1)
+        slot->stdout_fd = dup(STDOUT_FILENO);
+        if (slot->stdout_fd == -1)
                 return errno;
 
         /* Redirect stdout to lockfile */
-        if (dup2(slot->lock_fd, STDOUT_FILENO) == -1) {
-                close(tmp);
+        if (dup2(slot->lock_fd, STDOUT_FILENO) == -1)
                 return errno;
-        }
 
         /* Generate cache content */
         slot->fn();
 
         /* Make sure any buffered data is flushed to the file */
-        if (fflush(stdout)) {
-                close(tmp);
+        if (fflush(stdout))
                 return errno;
-        }
 
         /* update stat info */
-        if (fstat(slot->lock_fd, &slot->cache_st)) {
-                close(tmp);
-                return errno;
-        }
-
-        /* Restore stdout */
-        if (dup2(tmp, STDOUT_FILENO) == -1) {
-                close(tmp);
-                return errno;
-        }
-
-        /* Close the temporary filedescriptor */
-        if (close(tmp))
+        if (fstat(slot->lock_fd, &slot->cache_st))
                 return errno;
 
         return 0;
@@ -393,6 +383,7 @@ int cache_process(int size, const char *path, const char *key, int ttl,
         strbuf_addstr(&lockname, ".lock");
         slot.fn = fn;
         slot.ttl = ttl;
+        slot.stdout_fd = -1;
         slot.cache_name = filename.buf;
         slot.lock_name = lockname.buf;
         slot.key = key;