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: a904b1ca5751 ("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>

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 299aee4b7b0b35b14df223cc7d92efecdd026d1f..eed073cc47783411b09d9efcc8e23f6d272bb666 100644 (file)
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -778,22 +778,26 @@ xfs_file_fallocate(
                if (error)
                        goto out_unlock;
        } else if (mode & FALLOC_FL_INSERT_RANGE) {
-               unsigned int blksize_mask = i_blocksize(inode) - 1;
+               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;
                }