linux-kernel - Re: [PATCH] cifs: fix strcat buffer overflow in smb21_set_oplock

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Message-ID: <CAKywueR6DcfkzGcZUgydV4n6F4MKDEOvtCaM-gQSonX02tA9tg@mail.gmail.com>
Date:   Mon, 6 May 2019 12:03:45 -0700
From:   Pavel Shilovsky <pavel.shilovsky@...il.com>
To:     Steve French <smfrench@...il.com>
Cc:     Jeremy Allison <jra@...ba.org>, Steve French <sfrench@...ba.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        samba-technical <samba-technical@...ts.samba.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Christoph Probst <kernel@...bst.it>
Subject: Re: [PATCH] cifs: fix strcat buffer overflow in smb21_set_oplock_level()

The patch itself is fine but I think we have a bigger problem here:

3052 >-------cinode->oplock = 0;

here we reset oplock level of the inode, so forcing all the IO go to the server.

3053 >-------if (oplock & SMB2_LEASE_READ_CACHING_HE) {
3054 >------->-------cinode->oplock |= CIFS_CACHE_READ_FLG;
3055 >------->-------strcat(message, "R");
3056 >-------}
3057 >-------if (oplock & SMB2_LEASE_HANDLE_CACHING_HE) {
3058 >------->-------cinode->oplock |= CIFS_CACHE_HANDLE_FLG;
3059 >------->-------strcat(message, "H");
3060 >-------}
3061 >-------if (oplock & SMB2_LEASE_WRITE_CACHING_HE) {
3062 >------->-------cinode->oplock |= CIFS_CACHE_WRITE_FLG;

this and 3 above cases are all racy with other threads opening the
same file and the oplock break thread.

3063 >------->-------strcat(message, "W");
3064 >-------}
3065 >-------if (!cinode->oplock)
3066 >------->-------strcat(message, "None");
3067 >-------cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
3068 >------->------- &cinode->vfs_inode);

Besides the fix in the patch I would temporarily suggest to not touch
cinode->oplock more than once in this function - have a local variable
cifs_oplock which accumulates flags and set cinode->oplock at the very
end. It reduce raciness a little bit but this code requires much more
thinking about proper locking I guess.

Best regards,
Pavel Shilovskiy

пн, 6 мая 2019 г. в 10:02, Steve French via samba-technical
<samba-technical@...ts.samba.org>:
>
> We could always switch it to strncpy :)
>
> In any case - he is correct, it is better than what was there because
> we should not strcat unless the array were locked across the whole
> function
>
> On Mon, May 6, 2019 at 11:57 AM Jeremy Allison <jra@...ba.org> wrote:
> >
> > On Mon, May 06, 2019 at 11:53:44AM -0500, Steve French via samba-technical wrote:
> > > I think strcpy is clearer - but I don't think it can overflow since if
> > > R, W or W were written to "message" then cinode->oplock would be
> > > non-zero so we would never strcap "None"
> >
> > Ahem. In Samba we have :
> >
> > lib/util/safe_string.h:#define strcpy(dest,src) __ERROR__XX__NEVER_USE_STRCPY___;
> >
> > Maybe you should do likewise :-).
> >
> > > On Mon, May 6, 2019 at 10:26 AM Christoph Probst <kernel@...bst.it> wrote:
> > > >
> > > > Change strcat to strcpy in the "None" case as it is never valid to append
> > > > "None" to any other message. It may also overflow char message[5], in a
> > > > race condition on cinode if cinode->oplock is unset by another thread
> > > > after "RHW" or "RH" had been written to message.
> > > >
> > > > Signed-off-by: Christoph Probst <kernel@...bst.it>
> > > > ---
> > > >  fs/cifs/smb2ops.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
> > > > index c36ff0d..5fd5567 100644
> > > > --- a/fs/cifs/smb2ops.c
> > > > +++ b/fs/cifs/smb2ops.c
> > > > @@ -2936,7 +2936,7 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode, __u32 oplock,
> > > >                 strcat(message, "W");
> > > >         }
> > > >         if (!cinode->oplock)
> > > > -               strcat(message, "None");
> > > > +               strcpy(message, "None");
> > > >         cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
> > > >                  &cinode->vfs_inode);
> > > >  }
> > > > --
> > > > 2.1.4
> > > >
> > >
> > >
> > > --
> > > Thanks,
> > >
> > > Steve
> > >
>
>
>
> --
> Thanks,
>
> Steve
>