linux-kernel - Re: [PATCH v3] fs/sysv: Null check to prevent null-ptr-deref bug

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Date:   Tue, 30 May 2023 07:59:16 -0700
From:   Prince Kumar Maurya <princekumarmaurya06@...il.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     skhan@...uxfoundation.org, viro@...iv.linux.org.uk,
        chenzhongjin@...wei.com, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzkaller-bugs@...glegroups.com, linux-fsdevel@...r.kernel.org,
        syzbot+aad58150cbc64ba41bdc@...kaller.appspotmail.com
Subject: Re: [PATCH v3] fs/sysv: Null check to prevent null-ptr-deref bug

On Tue, May 30, 2023 at 1:26 AM Christian Brauner <brauner@...nel.org> wrote:
>
> On Sun, May 28, 2023 at 11:44:22AM -0700, Prince Kumar Maurya wrote:
> > sb_getblk(inode->i_sb, parent) return a null ptr and taking lock on
> > that leads to the null-ptr-deref bug.
> >
> > Reported-by: syzbot+aad58150cbc64ba41bdc@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=aad58150cbc64ba41bdc
> > Signed-off-by: Prince Kumar Maurya <princekumarmaurya06@...il.com>
> > ---
> > Change since v2: Updated subject and added Reported-by and closes tags.
> >
> >  fs/sysv/itree.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/fs/sysv/itree.c b/fs/sysv/itree.c
> > index b22764fe669c..3a6b66e719fd 100644
> > --- a/fs/sysv/itree.c
> > +++ b/fs/sysv/itree.c
> > @@ -145,6 +145,8 @@ static int alloc_branch(struct inode *inode,
> >                */
> >               parent = block_to_cpu(SYSV_SB(inode->i_sb), branch[n-1].key);
> >               bh = sb_getblk(inode->i_sb, parent);
> > +             if (!bh)
> > +                     break;
>
> When you break here you'll hit:
>
> /* Allocation failed, free what we already allocated */
> for (i = 1; i < n; i++)
>         bforget(branch[i].bh);
> for (i = 0; i < n; i++)
>         sysv_free_block(inode->i_sb, branch[i].key);
>
> below. The cleanup paths were coded in the assumption that sb_getblk()
> can't fail. So bforget() can assume that branch[i].bh has been allocated
> and set up. So that bforget(branch[i].bh) is your next pending NULL
> deref afaict.


I doubt that would happen. There is a break above as well, before we do
sb_getblk().

/* Allocate the next block */
branch[n].key = sysv_new_block(inode->i_sb);
if (!branch[n].key)
   break;

The clean up code path runs till i is less than n not equal to n which
would have caused the problem.