Patch "btrfs: Fix possible off-by-one in btrfs_search_path_in_tree" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

btrfs: Fix possible off-by-one in btrfs_search_path_in_tree

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-fix-possible-off-by-one-in-btrfs_search_path_in_tree.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Nikolay Borisov <[email protected]>
Date: Fri, 1 Dec 2017 11:19:42 +0200
Subject: btrfs: Fix possible off-by-one in btrfs_search_path_in_tree

From: Nikolay Borisov <[email protected]>

[ Upstream commit c8bcbfbd239ed60a6562964b58034ac8a25f4c31 ]

The name char array passed to btrfs_search_path_in_tree is of size BTRFS_INO_LOOKUP_PATH_MAX (4080). So the actual accessible char indexes are in the range of [0, 4079]. Currently the code uses the define but this represents an off-by-one.

Implications:

Size of btrfs_ioctl_ino_lookup_args is 4096, so the new byte will be written to extra space, not some padding that could be provided by the allocator.

btrfs-progs store the arguments on stack, but kernel does own copy of the ioctl buffer and the off-by-one overwrite does not affect userspace, but the ending 0 might be lost.

Kernel ioctl buffer is allocated dynamically so we're overwriting somebody else's memory, and the ioctl is privileged if args.objectid is not 256. Which is in most cases, but resolving a subvolume stored in another directory will trigger that path.

Before this patch the buffer was one byte larger, but then the -1 was not added.

---
 fs/btrfs/ioctl.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -2231,7 +2231,7 @@ static noinline int btrfs_search_path_in
 	if (!path)
 		return -ENOMEM;
 
-	ptr = &name[BTRFS_INO_LOOKUP_PATH_MAX];
+	ptr = &name[BTRFS_INO_LOOKUP_PATH_MAX - 1];
 
 	key.objectid = tree_id;
 	key.type = BTRFS_ROOT_ITEM_KEY;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/btrfs-fix-possible-off-by-one-in-btrfs_search_path_in_tree.patch