[PATCH v2 1/6] smb/client: validate new EOF for insert range

[Huiwen He <huiwen.he@linux.dev>]

From: Huiwen He <hehuiwen@kylinos.cn>

smb3_insert_range() does not check if the new file size
(i_size + len) is valid. This allows FALLOC_FL_INSERT_RANGE to bypass
RLIMIT_FSIZE, exceed s_maxbytes, or produce a size outside the loff_t
range.

Use check_add_overflow() to calculate the new EOF. Validate it with
inode_newsize_ok() before modifying the file.

Reproducer, using a file on a CIFS mount:

	bash -c '
		FILE=/mnt/cifs/repro

		trap "" SIGXFSZ
		ulimit -f 3072		# RLIMIT_FSIZE = 3 MiB

		# A regular write is stopped at 3 MiB.
		dd if=/dev/zero of="$FILE" bs=1M count=4 status=none
		stat -c "size after write: %s" "$FILE"

		# Insert 2 MiB into a 2 MiB file.
		truncate -s 2M "$FILE"
		fallocate -i -o 0 -l 2M "$FILE"
		stat -c "size after insert: %s" "$FILE"
	'

Before this change, the regular write stops at the 3 MiB limit, but
insert range grows the file to 4 MiB:

	dd: error writing '/mnt/cifs/repro': File too large
	size after write: 3145728
	size after insert: 4194304

After this change, insert range also fails at the limit and leaves the
2 MiB file unchanged:

	dd: error writing '/mnt/cifs/repro': File too large
	size after write: 3145728
	fallocate: fallocate failed: File too large
	size after insert: 2097152

Fixes: 7fe6fe95b936 ("cifs: add FALLOC_FL_INSERT_RANGE support")
Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
---
 fs/smb/client/smb2ops.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/smb2ops.c b/fs/smb/client/smb2ops.c
index 192649fec25d..bc7dda6d825a 100644
--- a/fs/smb/client/smb2ops.c
+++ b/fs/smb/client/smb2ops.c
@@ -3985,7 +3985,8 @@ static long smb3_insert_range(struct file *file, struct cifs_tcon *tcon,
 	struct cifsFileInfo *cfile = file->private_data;
 	struct inode *inode = file_inode(file);
 	struct cifsInodeInfo *cifsi = CIFS_I(inode);
-	__u64 count, old_eof, new_eof;
+	u64 count;
+	loff_t old_eof, new_eof;
 
 	xid = get_xid();
 
@@ -3995,8 +3996,15 @@ static long smb3_insert_range(struct file *file, struct cifs_tcon *tcon,
 		goto out;
 	}
 
+	if (check_add_overflow(old_eof, len, &new_eof)) {
+		rc = -EFBIG;
+		goto out;
+	}
+	rc = inode_newsize_ok(inode, new_eof);
+	if (rc)
+		goto out;
+
 	count = old_eof - off;
-	new_eof = old_eof + len;
 
 	filemap_invalidate_lock(inode->i_mapping);
 	rc = filemap_write_and_wait_range(inode->i_mapping, off, new_eof - 1);
-- 
2.43.0