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: <CAO8a2SjWXbVxDy4kcKF6JSesB=_QEfb=ZfPbwXpiY_GUuwA8zQ@mail.gmail.com>
Date: Wed, 27 Nov 2024 15:47:02 +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

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:

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;
-               else if (off + ret > i_size)
-                       left = i_size - off;
+               if (off + ret > i_size)
+                       left = (i_size > off) ? i_size - off : 0;
                else
-                       left = ret;
+                       left = (ret > 0) ? ret : 0;
+
                while (left > 0) {
                        size_t plen, copied;


On Thu, Nov 7, 2024 at 1:09 PM Luis Henriques <luis.henriques@...ux.dev> wrote:
>
> (CC'ing Alex)
>
> On Wed, Nov 06 2024, Goldwyn Rodrigues wrote:
>
> > Hi Xiubo,
> >
> >> BTW, so in the following code:
> >>
> >> 1202                 idx = 0;
> >> 1203                 if (ret <= 0)
> >> 1204                         left = 0;
> >> 1205                 else if (off + ret > i_size)
> >> 1206                         left = i_size - off;
> >> 1207                 else
> >> 1208                         left = ret;
> >>
> >> The 'ret' should be larger than '0', right ?
> >>
> >> If so we do not check anf fix it in the 'else if' branch instead?
> >>
> >> Because currently the read path code won't exit directly and keep
> >> retrying to read if it found that the real content length is longer than
> >> the local 'i_size'.
> >>
> >> Again I am afraid your current fix will break the MIX filelock semantic ?
> >
> > Do you think changing left to ssize_t instead of size_t will
> > fix the problem?
> >
> > diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> > index 4b8d59ebda00..f8955773bdd7 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
> > @@ -1087,7 +1087,7 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
> >               size_t page_off;
> >               bool more;
> >               int idx;
> > -             size_t left;
> > +             ssize_t left;
> >               struct ceph_osd_req_op *op;
> >               u64 read_off = off;
> >               u64 read_len = len;
> >
>
> I *think* (although I haven't tested it) that you're patch should work as
> well.  But I also think it's a bit more hacky: the overflow will still be
> there:
>
>                 if (ret <= 0)
>                         left = 0;
>                 else if (off + ret > i_size)
>                         left = i_size - off;
>                 else
>                         left = ret;
>                 while (left > 0) {
>                         // ...
>                 }
>
> If 'i_size' is '0', 'left' (which is now signed) will now have a negative
> value in the 'else if' branch and the loop that follows will not be
> executed.  My version will simply set 'ret' to '0' before this 'if'
> construct.
>
> So, in my opinion, what needs to be figured out is whether this will cause
> problems on the MDS side or not.  Because on the kernel client, it should
> be safe to ignore reads to an inode that has size set to '0', even if
> there's already data available to be read.  Eventually, the inode metadata
> will get updated and by then we can retry the read.
>
> Unfortunately, the MDS continues to be a huge black box for me and the
> locking code in particular is very tricky.  I'd rather defer this for
> anyone that is familiar with the code.
>
> Cheers,
> --
> Luís
>