linux-kernel - Re: [RFC PATCH v2] ceph: ceph: fix out-of-bound array access when doing a file read

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Message-ID: <CAO8a2SgMLurHP=o_ENbvOFMci8bcX0TP_18rbjrYJSbmV9CrMA@mail.gmail.com>
Date: Thu, 28 Nov 2024 21:09:42 +0200
From: Alex Markuze <amarkuze@...hat.com>
To: Luis Henriques <luis.henriques@...ux.dev>
Cc: Goldwyn Rodrigues <rgoldwyn@...e.de>, Xiubo Li <xiubli@...hat.com>, 
	Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2] ceph: ceph: fix out-of-bound array access when
 doing a file read

Good catch, I'm reworking the ergonomics of this function, this ret
error code is checked and carried through the loop and checked every
other line.

On Thu, Nov 28, 2024 at 8:53 PM Luis Henriques <luis.henriques@...ux.dev> wrote:
>
> Hi!
>
> On Thu, Nov 28 2024, Alex Markuze wrote:
> > On Thu, Nov 28, 2024 at 7:43 PM Luis Henriques <luis.henriques@...ux.dev> wrote:
> >>
> >> Hi Alex,
> >>
> >> [ Thank you for looking into this. ]
> >>
> >> On Wed, Nov 27 2024, Alex Markuze wrote:
> >>
> >> > Hi, Folks.
> >> > AFAIK there is no side effect that can affect MDS with this fix.
> >> > This crash happens following this patch
> >> > "1065da21e5df9d843d2c5165d5d576be000142a6" "ceph: stop copying to iter
> >> > at EOF on sync reads".
> >> >
> >> > Per your fix Luis, it seems to address only the cases when i_size goes
> >> > to zero but can happen anytime the `i_size` goes below  `off`.
> >> > I propose fixing it this way:
> >>
> >> Hmm... you're probably right.  I didn't see this happening, but I guess it
> >> could indeed happen.
> >>
> >> > diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> >> > index 4b8d59ebda00..19b084212fee 100644
> >> > --- a/fs/ceph/file.c
> >> > +++ b/fs/ceph/file.c
> >> > @@ -1066,7 +1066,7 @@ ssize_t __ceph_sync_read(struct inode *inode,
> >> > loff_t *ki_pos,
> >> >         if (ceph_inode_is_shutdown(inode))
> >> >                 return -EIO;
> >> >
> >> > -       if (!len)
> >> > +       if (!len || !i_size)
> >> >                 return 0;
> >> >         /*
> >> >          * flush any page cache pages in this range.  this
> >> > @@ -1200,12 +1200,11 @@ ssize_t __ceph_sync_read(struct inode *inode,
> >> > loff_t *ki_pos,
> >> >                 }
> >> >
> >> >                 idx = 0;
> >> > -               if (ret <= 0)
> >> > -                       left = 0;
> >>
> >> Right now I don't have any means for testing this patch.  However, I don't
> >> think this is completely correct.  By removing the above condition you're
> >> discarding cases where an error has occurred (i.e. where ret is negative).
> >
> > I didn't discard it though :).
> > I folded it into the `if` statement. I find the if else construct
> > overly verbose and cumbersome.
> >
> > +                       left = (ret > 0) ? ret : 0;
> >
>
> Right, but with your patch, if 'ret < 0', we could still hit the first
> branch instead of that one:
>
>                 if (off + ret > i_size)
>                         left = (i_size > off) ? i_size - off : 0;
>                 else
>                         left = (ret > 0) ? ret : 0;
>
> Cheers,
> --
> Luís
>