Patch "fsverity: explicitly check for buffer overflow in build_merkle_tree()" has been added to the 6.3-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

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

    fsverity: explicitly check for buffer overflow in build_merkle_tree()

to the 6.3-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:
     fsverity-explicitly-check-for-buffer-overflow-in-build_merkle_tree.patch
and it can be found in the queue-6.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 39049b69ec9fc125fa1f314165dcc86f72cb72ec Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@xxxxxxxxxx>
Date: Mon, 27 Mar 2023 21:15:05 -0700
Subject: fsverity: explicitly check for buffer overflow in build_merkle_tree()

From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit 39049b69ec9fc125fa1f314165dcc86f72cb72ec upstream.

The new Merkle tree construction algorithm is a bit fragile in that it
may overflow the 'root_hash' array if the tree actually generated does
not match the calculated tree parameters.

This should never happen unless there is a filesystem bug that allows
the file size to change despite deny_write_access(), or a bug in the
Merkle tree logic itself.  Regardless, it's fairly easy to check for
buffer overflow here, so let's do so.

This is a robustness improvement only; this case is not currently known
to be reachable.  I've added a Fixes tag anyway, since I recommend that
this be included in kernels that have the mentioned commit.

Fixes: 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE")
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20230328041505.110162-1-ebiggers@xxxxxxxxxx
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/verity/enable.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -13,6 +13,7 @@
 
 struct block_buffer {
 	u32 filled;
+	bool is_root_hash;
 	u8 *data;
 };
 
@@ -24,6 +25,14 @@ static int hash_one_block(struct inode *
 	struct block_buffer *next = cur + 1;
 	int err;
 
+	/*
+	 * Safety check to prevent a buffer overflow in case of a filesystem bug
+	 * that allows the file size to change despite deny_write_access(), or a
+	 * bug in the Merkle tree logic itself
+	 */
+	if (WARN_ON_ONCE(next->is_root_hash && next->filled != 0))
+		return -EINVAL;
+
 	/* Zero-pad the block if it's shorter than the block size. */
 	memset(&cur->data[cur->filled], 0, params->block_size - cur->filled);
 
@@ -97,6 +106,7 @@ static int build_merkle_tree(struct file
 		}
 	}
 	buffers[num_levels].data = root_hash;
+	buffers[num_levels].is_root_hash = true;
 
 	BUILD_BUG_ON(sizeof(level_offset) != sizeof(params->level_start));
 	memcpy(level_offset, params->level_start, sizeof(level_offset));


Patches currently in stable-queue which might be from ebiggers@xxxxxxxxxx are

queue-6.3/fsverity-explicitly-check-for-buffer-overflow-in-build_merkle_tree.patch
queue-6.3/fsverity-reject-fs_ioc_enable_verity-on-mode-3-fds.patch