[PATCH] xfs: prevent creating negative-sized file via INSERT_RANGE

[Eric Biggers <ebiggers3@gmail.com>]

From: Eric Biggers <ebiggers@google.com>

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 doing an unsigned comparison 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+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/xfs/xfs_file.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 299aee4b7b0b..56a820efeb2a 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -786,8 +786,11 @@ xfs_file_fallocate(
 			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 ((u64)new_size > inode->i_sb->s_maxbytes) {
 			error = -EFBIG;
 			goto out_unlock;
 		}
-- 
2.17.0.484.g0c8726318c-goog