[PATCH] file: fix close_range() for unshare+cloexec

[Christian Brauner]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
---
 fs/file.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index f3a4bac2cbe9..5ef62377d924 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -632,6 +632,7 @@ EXPORT_SYMBOL(close_fd); /* for ksys_close() */
 static inline void __range_cloexec(struct files_struct *cur_fds,
 				   unsigned int fd, unsigned int max_fd)
 {
+	unsigned int cur_max;
 	struct fdtable *fdt;
 
 	if (fd > max_fd)
@@ -639,7 +640,12 @@ static inline void __range_cloexec(struct files_struct *cur_fds,
 
 	spin_lock(&cur_fds->file_lock);
 	fdt = files_fdtable(cur_fds);
-	bitmap_set(fdt->close_on_exec, fd, max_fd - fd + 1);
+	/* make very sure we're using the correct maximum value */
+	cur_max = fdt->max_fds;
+	cur_max--;
+	cur_max = min(max_fd, cur_max);
+	if (fd <= cur_max)
+		bitmap_set(fdt->close_on_exec, fd, cur_max - fd + 1);
 	spin_unlock(&cur_fds->file_lock);
 }
 
-- 
2.27.0