linux-kernel - Re: [PATCH] fs: add check for symlink corrupted

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Message-ID: <20241116013950.1563199-1-lizhi.xu@windriver.com>
Date: Sat, 16 Nov 2024 09:39:50 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <viro@...iv.linux.org.uk>
CC: <almaz.alexandrovich@...agon-software.com>, <brauner@...nel.org>,
        <jack@...e.cz>, <linux-fsdevel@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <lizhi.xu@...driver.com>,
        <ntfs3@...ts.linux.dev>,
        <syzbot+73d8fc29ec7cba8286fa@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH] fs: add check for symlink corrupted

On Fri, 15 Nov 2024 13:24:55 +0000, Al Viro wrote:
> 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
Yes, ntfs3 does not handle the relevant code of i_link.
> with aftermath of memory corruption, so it's definitely papering over
> the actual bug here.
I see that finding out how the value of i_link becomes 2 is the key.