[PATCH] nilfs2: fix potential out-of-bounds memory access in nilfs_find_entry()

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

Syzbot reported that when searching for records in a directory where
the inode's i_size is corrupted and has a large value, memory access
outside the folio/page range may occur, or a use-after-free bug may be
detected if KASAN is enabled.

This is because nilfs_last_byte(), which is called by
nilfs_find_entry() and others to calculate the number of valid bytes of
directory data in a page from i_size and the page index, loses the
upper 32 bits of the 64-bit size information due to an inappropriate
type of local variable to which the i_size value is assigned.

This caused a large byte offset value due to underflow in the end
address calculation in the calling nilfs_find_entry(), resulting in
memory access that exceeds the folio/page size.

Fix this issue by changing the type of the local variable causing the
bit loss from "unsigned int" to "u64".  The return value of
nilfs_last_byte() is also of type "unsigned int", but it is truncated
so as not to exceed PAGE_SIZE and no bit loss occurs, so no change is
required.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+96d5d14c47d97015c624@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=96d5d14c47d97015c624
Tested-by: syzbot+96d5d14c47d97015c624@syzkaller.appspotmail.com
Fixes: 2ba466d74ed7 ("nilfs2: directory entry operations")
Cc: stable@vger.kernel.org
---
Andrew, please apply this as a bug fix.

This fixes a potential out-of-page memory access bug recently
reported by syzbot.

Thanks,
Ryusuke Konishi

 fs/nilfs2/dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nilfs2/dir.c b/fs/nilfs2/dir.c
index a8602729586a..f61c58fbf117 100644
--- a/fs/nilfs2/dir.c
+++ b/fs/nilfs2/dir.c
@@ -70,7 +70,7 @@ static inline unsigned int nilfs_chunk_size(struct inode *inode)
  */
 static unsigned int nilfs_last_byte(struct inode *inode, unsigned long page_nr)
 {
-	unsigned int last_byte = inode->i_size;
+	u64 last_byte = inode->i_size;
 
 	last_byte -= page_nr << PAGE_SHIFT;
 	if (last_byte > PAGE_SIZE)
-- 
2.43.0