Merge tag 'for-linus-2021-04-08' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

Pull close_range() fix from Christian Brauner:
 "Syzbot reported a bug in close_range.

  Debugging this showed we didn't recalculate the current maximum fd
  number for CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC after we unshared
  the file descriptors table. As a result, max_fd could exceed the
  current fdtable maximum causing us to set excessive bits.

  As a concrete example, let's say the user requested everything from fd
  4 to ~0UL to be closed and their current fdtable size is 256 with
  their highest open fd being 4. With CLOSE_RANGE_UNSHARE the caller
  will end up with a new fdtable which has room for 64 file descriptors
  since that is the lowest fdtable size we accept. But now max_fd will
  still point to 255 and needs to be adjusted. Fix this by retrieving
  the correct maximum fd value in __range_cloexec().

  I've carried this fix for a little while but since there was no
  linux-next release over easter I waited until now.

  With this change close_range() can be further simplified but imho we
  are in no hurry to do that and so I'll defer this for the 5.13 merge
  window"

* tag 'for-linus-2021-04-08' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  file: fix close_range() for unshare+cloexec

1 files changed, 17 insertions, 4 deletions

diff --git a/fs/file.c b/fs/file.c
index f3a4bac2cbe9..f633348029a5 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -629,17 +629,30 @@ int close_fd(unsigned fd)
 }
 EXPORT_SYMBOL(close_fd); /* for ksys_close() */
 
+/**
+ * last_fd - return last valid index into fd table
+ * @cur_fds: files struct
+ *
+ * Context: Either rcu read lock or files_lock must be held.
+ *
+ * Returns: Last valid index into fdtable.
+ */
+static inline unsigned last_fd(struct fdtable *fdt)
+{
+	return fdt->max_fds - 1;
+}
+
 static inline void __range_cloexec(struct files_struct *cur_fds,
 				   unsigned int fd, unsigned int max_fd)
 {
 	struct fdtable *fdt;
 
-	if (fd > max_fd)
-		return;
-
+	/* make sure we're using the correct maximum value */
 	spin_lock(&cur_fds->file_lock);
 	fdt = files_fdtable(cur_fds);
-	bitmap_set(fdt->close_on_exec, fd, max_fd - fd + 1);
+	max_fd = min(last_fd(fdt), max_fd);
+	if (fd <= max_fd)
+		bitmap_set(fdt->close_on_exec, fd, max_fd - fd + 1);
 	spin_unlock(&cur_fds->file_lock);
 }