[PATCH] smb: client: fix fiemap wild read on empty truncated response

[Abdifatah Suruur]

smb3_fiemap() treats -E2BIG from SMB2_ioctl() as a truncated range blob
and loops for the next chunk. If the server reports
STATUS_BUFFER_OVERFLOW with OutputCount == 0, SMB2_ioctl() returns
-E2BIG with out_data == NULL and out_data_len == 0, and the existing
length checks accept the empty result. The loop then evaluates

	next = out_data[num - 1].file_offset + ...

with num == 0, reading 16 bytes before NULL.

Reject a truncated range blob that contains no range, so the resume
offset is never computed from an empty response.

Reproduced on 6.6.87.2 with a malicious SMB2 server that answers
FSCTL_QUERY_ALLOCATED_RANGES with STATUS_BUFFER_OVERFLOW and
OutputCount == 0; an FS_IOC_FIEMAP ioctl on the mount oopses in
cifs_fiemap() with CR2 = fffffffffffffff8. The code path is unchanged
on mainline.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Abdifatah Suruur <suruurism@xxxxxxxxx>
Reviewed-by: Namjae Jeon <linkinjeon@xxxxxxxxxx>
---
 fs/smb/client/smb2ops.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/smb/client/smb2ops.c b/fs/smb/client/smb2ops.c
index 20624f5df..7eeb33640 100644
--- a/fs/smb/client/smb2ops.c
+++ b/fs/smb/client/smb2ops.c
@@ -4360,6 +4360,15 @@ static int smb3_fiemap(struct cifs_tcon *tcon,
 		goto out;
 	}
 
+	/*
+	 * A truncated range blob must contain at least one range, otherwise
+	 * there is no offset to resume from and out_data is NULL.
+	 */
+	if (!last_blob && out_data_len == 0) {
+		rc = -EINVAL;
+		goto out;
+	}
+
 	num = out_data_len / sizeof(struct file_allocated_range_buffer);
 	for (i = 0; i < num; i++) {
 		flags = 0;
-- 
2.53.0