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

[Al Viro]

On Fri, Nov 15, 2024 at 01:06:15PM +0000, Al Viro wrote:
> On Fri, Nov 15, 2024 at 05:49:08PM +0800, Lizhi Xu wrote:
> > syzbot reported a null-ptr-deref in pick_link. [1]
> > When symlink's inode is corrupted, the value of the i_link is 2 in this case,
> > it will trigger null pointer deref when accessing *res in pick_link(). 
> > 
> > To avoid this issue, add a check for inode mode, return -EINVAL when it's
> > not symlink.
> 
> NAK.  Don't paper over filesystem bugs at pathwalk time - it's the wrong
> place for that.  Fix it at in-core inode creation time.

BTW, seeing that ntfs doesn't even touch ->i_link, you are dealing
with aftermath of memory corruption, so it's definitely papering over
the actual bug here.