Re: [PATCH] fs: add check for symlink corrupted

[Al Viro]

On Sat, Nov 16, 2024 at 09:02:07AM +0800, Lizhi Xu wrote:

> Our idea is the same. Because d_is_symlink() has confirmed the mode of
> symlink in step_into(), I will confirm whether the mode of symlink's inode
> has changed when the value of i_link is 2 in pick_link().
> > 	do something and return
> > 
> > so we are checking whether the inode is a symlink before calling
> > pick_link(). And yes, the d_is_symlink() is using cached type in
> > dentry->d_flags so they could mismatch. But inode is not supposed to change
> > its type during its lifetime so if there is a mismatch that is the problem
> > that needs to be fixed.
> I think syzbot executed the following two syscalls when triggering this problem:
> 
> link(&(0x7f0000000200)='./file0\x00', &(0x7f0000000240)='./bus\x00')
> mount$overlay(0x0, &(0x7f00000000c0)='./bus\x00', 0x0, 0x0, 0x0)
> 
> Obviously, this is to mount a link. Whether the mount operation itself will
> change or corrupt the i_link value and mode value of the symlink is not
> clear to me yet.

Odds are, it's not a valid struct inode instance in the first place.
It's not inode->i_link that is a problem (*nothing* should ever store
that value in there and ntfs doesn't even try that - grep and you'll see);
it's inode itself.

Have you tried KASAN-enabled build?  Might be interesting to see if
it catches anything...