[PATCH v3] ext4: limit length to bitmap_maxbytes - blocksize in punch_hole

[Tadeusz Struk <tadeusz.struk@linaro.org>]

Syzbot found an issue [1] in ext4_fallocate().
The C reproducer [2] calls fallocate(), passing size 0xffeffeff000ul,
and offset 0x1000000ul, which, when added together exceed the
bitmap_maxbytes for the inode. This triggers a BUG in
ext4_ind_remove_space(). According to the comments in this function
the 'end' parameter needs to be one block after the last block to be
removed. In the case when the BUG is triggered it points to the last
block. Modify the ext4_punch_hole() function and add constraint that
caps the length to satisfy the one before laster block requirement.

LINK: [1] https://syzkaller.appspot.com/bug?id=b80bd9cf348aac724a4f4dff251800106d721331
LINK: [2] https://syzkaller.appspot.com/text?tag=ReproC&x=14ba0238700000

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Ritesh Harjani <riteshh@linux.ibm.com>
Cc: <linux-ext4@vger.kernel.org>
Cc: <stable@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>

Fixes: a4bb6b64e39a ("ext4: enable "punch hole" functionality")
Reported-by: syzbot+7a806094edd5d07ba029@syzkaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
--
v3: Modify the length instead of returning an error.
v2: Change sbi->s_blocksize to inode->i_sb->s_blocksize in maxlength
 computation.
---
 fs/ext4/inode.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 1ce13f69fbec..60bf31765d07 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3958,7 +3958,8 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
 	struct super_block *sb = inode->i_sb;
 	ext4_lblk_t first_block, stop_block;
 	struct address_space *mapping = inode->i_mapping;
-	loff_t first_block_offset, last_block_offset;
+	loff_t first_block_offset, last_block_offset, max_length;
+	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 	handle_t *handle;
 	unsigned int credits;
 	int ret = 0, ret2 = 0;
@@ -4001,6 +4002,14 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
 		   offset;
 	}
 
+	/*
+	 * For punch hole the length + offset needs to be within one block
+	 * before last range. Adjust the length if it goes beyond that limit.
+	 */
+	max_length = sbi->s_bitmap_maxbytes - inode->i_sb->s_blocksize;
+	if (offset + length > max_length)
+		length = max_length - offset;
+
 	if (offset & (sb->s_blocksize - 1) ||
 	    (offset + length) & (sb->s_blocksize - 1)) {
 		/*
-- 
2.35.1