[PATCH] fs: reject inodes whose size the superblock cannot address

[Sascha Hauer <s.hauer@pengutronix.de>]

f_size is an alias for the inode's i_size, and filesystems read that
size straight from untrusted media. ext4's ext4_isize() builds it as
((loff_t)size_high << 32) | size, so bit 31 of size_high lands in the
sign bit; squashfs takes an unchecked le64 for LREG inodes. A negative
i_size then feeds the offset arithmetic in __read(), __write() and
friends, where it either wraps a size_t count to something huge or --
depending on whether size_t is 32 or 64 bit -- turns the comparison
unsigned and skips the EOF clamp altogether.

Catch this the way Linux does: give the superblock a ceiling and refuse
sizes beyond it, instead of hardening every arithmetic site. Default
s_maxbytes to MAX_LFS_FILESIZE in init_super(), which runs before the
driver probe, so the filesystems that already lower it (jffs2, ubifs,
squashfs, 9p) keep doing so and everyone else stops sitting at zero.

The check goes into do_dentry_open() rather than into iget: ten drivers
only learn the size in their ->open() callback and write it to the
inode through file->f_size, so a lookup time check would miss them.
Casting to u64 makes a negative size exceed any sane s_maxbytes, so the
sign is covered too; FILE_SIZE_STREAM is negative on purpose and has to
be excluded.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
---
 fs/fs.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/fs.c b/fs/fs.c
index ce41f23f88..9518f28539 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -986,6 +986,7 @@ int fsdev_open_cdev(struct fs_device *fsdev)
 static void init_super(struct super_block *sb)
 {
 	INIT_LIST_HEAD(&sb->s_inodes);
+	sb->s_maxbytes = MAX_LFS_FILESIZE;
 }
 
 static int fsdev_umount(struct fs_device *fsdev)
@@ -2613,6 +2614,14 @@ static int rmdirat(int dirfd, const char *pathname)
 	return errno_set(error);
 }
 
+static bool i_size_valid(struct inode *inode)
+{
+	if (inode->i_size == FILE_SIZE_STREAM)
+		return true;
+
+	return (u64)inode->i_size <= inode->i_sb->s_maxbytes;
+}
+
 static int do_dentry_open(struct file *f)
 {
 	int error;
@@ -2628,6 +2637,12 @@ static int do_dentry_open(struct file *f)
 			return error;
 	}
 
+	if (!i_size_valid(f->f_inode)) {
+		dev_warn(&f->fsdev->dev, "%s: bad i_size value: %lld\n",
+			 f->path, f->f_size);
+		return -EUCLEAN;
+	}
+
 	if (f->f_flags & O_TRUNC) {
 		error = fsdev_truncate(f, 0);
 		f->f_size = 0;
-- 
2.47.3