Unverified Commit 9b5b8722 authored Apr 02, 2021 by Christian Brauner

file: fix close_range() for unshare+cloexec



syzbot reported a bug when putting the last reference to a tasks file
descriptor table. 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. So 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().

Reported-by:  <syzbot+283ce5a46486d6acdbaf@syzkaller.appspotmail.com>
Fixes: 582f1fb6 ("fs, close_range: add flag CLOSE_RANGE_CLOEXEC")
Fixes: fec8a6a6 ("close_range: unshare all fds for CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC")
Cc: Christoph Hellwig <hch@lst.de>
Cc: Giuseppe Scrivano <gscrivan@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

parent 0d02ec6b

fs/file.c

+17 −4

Original line number	Diff line number	Diff line
		@@ -629,16 +629,29 @@ 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);
		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);
		}