[PATCH v2] copy_file_range: limit size if in compat mode

[Miklos Szeredi <mszeredi@redhat.com>]

If the process runs in 32-bit compat mode, copy_file_range results can be
in the in-band error range.  In this case limit copy length to MAX_RW_COUNT
to prevent a signed overflow.

Reported-by: Florian Weimer <fweimer@redhat.com>
Closes: https://lore.kernel.org/all/lhuh5ynl8z5.fsf@oldenburg.str.redhat.com/
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
v2:
 - simplified logic (Amir)

 fs/read_write.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index c5b6265d984b..833bae068770 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1576,6 +1576,13 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	if (len == 0)
 		return 0;
 
+	/*
+	 * Make sure return value doesn't overflow in 32bit compat mode.  Also
+	 * limit the size for all cases except when calling ->copy_file_range().
+	 */
+	if (splice || !file_out->f_op->copy_file_range || in_compat_syscall())
+		len = min_t(size_t, MAX_RW_COUNT, len);
+
 	file_start_write(file_out);
 
 	/*
@@ -1589,9 +1596,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 						      len, flags);
 	} else if (!splice && file_in->f_op->remap_file_range && samesb) {
 		ret = file_in->f_op->remap_file_range(file_in, pos_in,
-				file_out, pos_out,
-				min_t(loff_t, MAX_RW_COUNT, len),
-				REMAP_FILE_CAN_SHORTEN);
+				file_out, pos_out, len, REMAP_FILE_CAN_SHORTEN);
 		/* fallback to splice */
 		if (ret <= 0)
 			splice = true;
@@ -1624,8 +1629,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	 * to splicing from input file, while file_start_write() is held on
 	 * the output file on a different sb.
 	 */
-	ret = do_splice_direct(file_in, &pos_in, file_out, &pos_out,
-			       min_t(size_t, len, MAX_RW_COUNT), 0);
+	ret = do_splice_direct(file_in, &pos_in, file_out, &pos_out, len, 0);
 done:
 	if (ret > 0) {
 		fsnotify_access(file_in);
-- 
2.49.0