[PATCH v2] fs: Set file_handle::handle_bytes before referencing file_handle::f_handle

[Kees Cook]

Since __counted_by(handle_bytes) was added to struct file_handle, we need
to explicitly set it in the one place it wasn't yet happening prior to
accessing the flex array "f_handle". For robustness also check for a
negative value for handle_bytes, which is possible for an "int", but
nothing appears to set.

Fixes: 1b43c4629756 ("fs: Annotate struct file_handle with __counted_by() and use struct_size()")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: Jan Kara <jack@xxxxxxx>
Cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
Cc: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-nfs@xxxxxxxxxxxxxxx
Cc: linux-hardening@xxxxxxxxxxxxxxx
 v2: more bounds checking, add comments, dropped reviews since logic changed
 v1: https://lore.kernel.org/all/20240403215358.work.365-kees@xxxxxxxxxx/
---
 fs/fhandle.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 8a7f86c2139a..854f866eaad2 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -40,6 +40,11 @@ static long do_sys_name_to_handle(const struct path *path,
 			 GFP_KERNEL);
 	if (!handle)
 		return -ENOMEM;
+	/*
+	 * Since handle->f_handle is about to be written, make sure the
+	 * associated __counted_by(handle_bytes) variable is correct.
+	 */
+	handle->handle_bytes = f_handle.handle_bytes;
 
 	/* convert handle size to multiple of sizeof(u32) */
 	handle_dwords = f_handle.handle_bytes >> 2;
@@ -51,8 +56,8 @@ static long do_sys_name_to_handle(const struct path *path,
 	handle->handle_type = retval;
 	/* convert handle size to bytes */
 	handle_bytes = handle_dwords * sizeof(u32);
-	handle->handle_bytes = handle_bytes;
-	if ((handle->handle_bytes > f_handle.handle_bytes) ||
+	/* check if handle_bytes would have exceeded the allocation */
+	if ((handle_bytes < 0) || (handle_bytes > f_handle.handle_bytes) ||
 	    (retval == FILEID_INVALID) || (retval < 0)) {
 		/* As per old exportfs_encode_fh documentation
 		 * we could return ENOSPC to indicate overflow
@@ -68,6 +73,8 @@ static long do_sys_name_to_handle(const struct path *path,
 		handle_bytes = 0;
 	} else
 		retval = 0;
+	/* the "valid" number of bytes may fewer than originally allocated */
+	handle->handle_bytes = handle_bytes;
 	/* copy the mount id */
 	if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
 	    copy_to_user(ufh, handle,
-- 
2.34.1