[RFC PATCH v2] ceph: ceph: fix out-of-bound array access when doing a file read

["Luis Henriques (SUSE)" <luis.henriques@linux.dev>]

__ceph_sync_read() does not correctly handle reads when the inode size is
zero.  It is easy to hit a NULL pointer dereference by continuously reading
a file while, on another client, we keep truncating and writing new data
into it.

The NULL pointer dereference happens when the inode size is zero but the
read op returns some data (ceph_osdc_wait_request()).  This will lead to
'left' being set to a huge value due to the overflow in:

	left = i_size - off;

and, in the loop that follows, the pages[] array being accessed beyond
num_pages.

This patch fixes the issue simply by checking the inode size and returning
if it is zero, even if there was data from the read op.

Link: https://tracker.ceph.com/issues/67524
Fixes: 1065da21e5df ("ceph: stop copying to iter at EOF on sync reads")
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
---
 fs/ceph/file.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 4b8d59ebda00..41d4eac128bb 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -1066,7 +1066,7 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 	if (ceph_inode_is_shutdown(inode))
 		return -EIO;
 
-	if (!len)
+	if (!len || !i_size)
 		return 0;
 	/*
 	 * flush any page cache pages in this range.  this
@@ -1154,6 +1154,9 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 		doutc(cl, "%llu~%llu got %zd i_size %llu%s\n", off, len,
 		      ret, i_size, (more ? " MORE" : ""));
 
+		if (i_size == 0)
+			ret = 0;
+
 		/* Fix it to go to end of extent map */
 		if (sparse && ret >= 0)
 			ret = ceph_sparse_ext_map_end(op);