ceph: improve error handling and short/overflow-read logic in __ceph_sync_read()

This patch refines the read logic in __ceph_sync_read() to ensure more
predictable and efficient behavior in various edge cases.

- Return early if the requested read length is zero or if the file size
  (`i_size`) is zero.
- Initialize the index variable (`idx`) where needed and reorder some
  code to ensure it is always set before use.
- Improve error handling by checking for negative return values earlier.
- Remove redundant encrypted file checks after failures. Only attempt
  filesystem-level decryption if the read succeeded.
- Simplify leftover calculations to correctly handle cases where the
  read extends beyond the end of the file or stops short.  This can be
  hit by continuously reading a file while, on another client, we keep
  truncating and writing new data into it.
- This resolves multiple issues caused by integer and consequent buffer
  overflow (`pages` array being accessed beyond `num_pages`):
  - https://tracker.ceph.com/issues/67524
  - https://tracker.ceph.com/issues/68980
  - https://tracker.ceph.com/issues/68981

Cc: stable@vger.kernel.org
Fixes: 1065da21e5 ("ceph: stop copying to iter at EOF on sync reads")
Reported-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
Signed-off-by: Alex Markuze <amarkuze@redhat.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

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
@@ -1086,7 +1086,7 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 		int num_pages;
 		size_t page_off;
 		bool more;
-		int idx;
+		int idx = 0;
 		size_t left;
 		struct ceph_osd_req_op *op;
 		u64 read_off = off;
@@ -1160,7 +1160,14 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 		else if (ret == -ENOENT)
 			ret = 0;
 
-		if (ret > 0 && IS_ENCRYPTED(inode)) {
+		if (ret < 0) {
+			ceph_osdc_put_request(req);
+			if (ret == -EBLOCKLISTED)
+				fsc->blocklisted = true;
+			break;
+		}
+
+		if (IS_ENCRYPTED(inode)) {
 			int fret;
 
 			fret = ceph_fscrypt_decrypt_extents(inode, pages,
@@ -1187,7 +1194,7 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 		}
 
 		/* Short read but not EOF? Zero out the remainder. */
-		if (ret >= 0 && ret < len && (off + ret < i_size)) {
+		if (ret < len && (off + ret < i_size)) {
 			int zlen = min(len - ret, i_size - off - ret);
 			int zoff = page_off + ret;
 
@@ -1197,13 +1204,11 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 			ret += zlen;
 		}
 
-		idx = 0;
+		if (off + ret > i_size)
-		if (ret <= 0)
+			left = (i_size > off) ? i_size - off : 0;
-			left = 0;
-		else if (off + ret > i_size)
-			left = i_size - off;
 		else
 			left = ret;
+
 		while (left > 0) {
 			size_t plen, copied;
 
@@ -1222,12 +1227,6 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
 
 		ceph_osdc_put_request(req);
 
-		if (ret < 0) {
-			if (ret == -EBLOCKLISTED)
-				fsc->blocklisted = true;
-			break;
-		}
-
 		if (off >= i_size || !more)
 			break;
 	}