Commit 7d83fb14 authored Apr 16, 2018 by Darrick J. Wong

xfs: prevent creating negative-sized file via INSERT_RANGE




During the "insert range" fallocate operation, i_size grows by the
specified 'len' bytes.  XFS verifies that i_size + len < s_maxbytes, as
it should.  But this comparison is done using the signed 'loff_t', and
'i_size + len' can wrap around to a negative value, causing the check to
incorrectly pass, resulting in an inode with "negative" i_size.  This is
possible on 64-bit platforms, where XFS sets s_maxbytes = LLONG_MAX.
ext4 and f2fs don't run into this because they set a smaller s_maxbytes.

Fix it by using subtraction instead.

Reproducer:
    xfs_io -f file -c "truncate $(((1<<63)-1))" -c "finsert 0 4096"

Fixes: a904b1ca ("xfs: Add support FALLOC_FL_INSERT_RANGE for fallocate")
Cc: <stable@vger.kernel.org> # v4.1+
Originally-From: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: fix signed integer addition overflow too]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

parent 2c4306f7

fs/xfs/xfs_file.c

+9 −5

Original line number	Diff line number	Diff line
		@@ -779,21 +779,25 @@ xfs_file_fallocate(
		goto out_unlock;
		} else if (mode & FALLOC_FL_INSERT_RANGE) {
		unsigned int blksize_mask = i_blocksize(inode) - 1;
		loff_t isize = i_size_read(inode);

		new_size = i_size_read(inode) + len;
		if (offset & blksize_mask \|\| len & blksize_mask) {
		error = -EINVAL;
		goto out_unlock;
		}

		/* check the new inode size does not wrap through zero */
		if (new_size > inode->i_sb->s_maxbytes) {
		/*
		* New inode size must not exceed ->s_maxbytes, accounting for
		* possible signed overflow.
		*/
		if (inode->i_sb->s_maxbytes - isize < len) {
		error = -EFBIG;
		goto out_unlock;
		}
		new_size = isize + len;

		/* Offset should be less than i_size */
		if (offset >= i_size_read(inode)) {
		if (offset >= isize) {
		error = -EINVAL;
		goto out_unlock;
		}