btrfs: send: limit number of clones and allocated memory size

The arg->clone_sources_count is u64 and can trigger a warning when a
huge value is passed from user space and a huge array is allocated.
Limit the allocated memory to 8MiB (can be increased if needed), which
in turn limits the number of clone sources to 8M / sizeof(struct
clone_root) = 8M / 40 = 209715.  Real world number of clones is from
tens to hundreds, so this is future proof.

Reported-by: syzbot+4376a9a073770c173269@syzkaller.appspotmail.com
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 67f7c69..26dace7 100644 (file)
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -8072,10 +8072,10 @@ long btrfs_ioctl_send(struct inode *inode, struct btrfs_ioctl_send_args *arg)
        /*
         * Check that we don't overflow at later allocations, we request
         * clone_sources_count + 1 items, and compare to unsigned long inside
-        * access_ok.
+        * access_ok. Also set an upper limit for allocation size so this can't
+        * easily exhaust memory. Max number of clone sources is about 200K.
         */
-       if (arg->clone_sources_count >
-           ULONG_MAX / sizeof(struct clone_root) - 1) {
+       if (arg->clone_sources_count > SZ_8M / sizeof(struct clone_root)) {
                ret = -EINVAL;
                goto out;
        }