Re: [PATCH v2] cifs: fix strcat buffer overflow and reduce raciness in smb21_set_oplock_level()

[Kai-Heng Feng]

at 02:42, Pavel Shilovsky <piastryyy@xxxxxxxxx> wrote:

> ÑÑ, 8 ÐÐÑ 2019 Ð. Ð 01:23, Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>:
> > at 02:28, Pavel Shilovsky <piastryyy@xxxxxxxxx> wrote:
> >
> > > ÐÑ, 7 ÐÐÑ 2019 Ð. Ð 09:13, Steve French via samba-technical
> > > <samba-technical@xxxxxxxxxxxxxxx>:
> > > > merged into cifs-2.6.git for-next
> > > >
> > > > On Tue, May 7, 2019 at 10:17 AM Christoph Probst via samba-technical
> > > > <samba-technical@xxxxxxxxxxxxxxx> wrote:
> > > > > Change strcat to strncpy in the "None" case to fix a buffer overflow
> > > > > when cinode->oplock is reset to 0 by another thread accessing the same
> > > > > cinode. It is never valid to append "None" to any other message.
> > > > >
> > > > > Consolidate multiple writes to cinode->oplock to reduce raciness.
> > > > >
> > > > > Signed-off-by: Christoph Probst <kernel@xxxxxxxxx>
> > > > > ---
> > > > >  fs/cifs/smb2ops.c | 14 ++++++++------
> > > > >  1 file changed, 8 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
> > > > > index c36ff0d..aa61dcf 100644
> > > > > --- a/fs/cifs/smb2ops.c
> > > > > +++ b/fs/cifs/smb2ops.c
> > > > > @@ -2917,26 +2917,28 @@ smb21_set_oplock_level(struct cifsInodeInfo
> > > > > *cinode, __u32 oplock,
> > > > >                        unsigned int epoch, bool *purge_cache)
> > > > >  {
> > > > >         char message[5] = {0};
> > > > > +       unsigned int new_oplock = 0;
> > > > >
> > > > >         oplock &= 0xFF;
> > > > >         if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
> > > > >                 return;
> > > > >
> > > > > -       cinode->oplock = 0;
> > > > >         if (oplock & SMB2_LEASE_READ_CACHING_HE) {
> > > > > -               cinode->oplock |= CIFS_CACHE_READ_FLG;
> > > > > +               new_oplock |= CIFS_CACHE_READ_FLG;
> > > > >                 strcat(message, "R");
> > > > >         }
> > > > >         if (oplock & SMB2_LEASE_HANDLE_CACHING_HE) {
> > > > > -               cinode->oplock |= CIFS_CACHE_HANDLE_FLG;
> > > > > +               new_oplock |= CIFS_CACHE_HANDLE_FLG;
> > > > >                 strcat(message, "H");
> > > > >         }
> > > > >         if (oplock & SMB2_LEASE_WRITE_CACHING_HE) {
> > > > > -               cinode->oplock |= CIFS_CACHE_WRITE_FLG;
> > > > > +               new_oplock |= CIFS_CACHE_WRITE_FLG;
> > > > >                 strcat(message, "W");
> > > > >         }
> > > > > -       if (!cinode->oplock)
> > > > > -               strcat(message, "None");
> > > > > +       if (!new_oplock)
> > > > > +               strncpy(message, "None", sizeof(message));
> > > > > +
> > > > > +       cinode->oplock = new_oplock;
> > > > >         cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
> > > > >                  &cinode->vfs_inode);
> > > > >  }
> > > > > --
> > > > > 2.1.4
> >
> > Doesnât the race still happen, but implicitly here?
> > cinode->oplock = new_oplock;
> >
> > Is it possible to just introduce a lock to force its proper ordering?
> > e.g.
> >
> > diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
> > index bf5b8264e119..a3c3c6156d17 100644
> > --- a/fs/cifs/cifsfs.c
> > +++ b/fs/cifs/cifsfs.c
> > @@ -267,6 +267,7 @@ cifs_alloc_inode(struct super_block *sb)
> >           * server, can not assume caching of file data or metadata.
> >           */
> >          cifs_set_oplock_level(cifs_inode, 0);
> > +       mutex_init(&cifs_inode->oplock_mutex);
> >          cifs_inode->flags = 0;
> >          spin_lock_init(&cifs_inode->writers_lock);
> >          cifs_inode->writers = 0;
> > diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
> > index 37b5ddf27ff1..6dfd4ab16c4f 100644
> > --- a/fs/cifs/cifsglob.h
> > +++ b/fs/cifs/cifsglob.h
> > @@ -1214,6 +1214,7 @@ struct cifsInodeInfo {
> >          struct list_head openFileList;
> >          __u32 cifsAttrs; /* e.g. DOS archive bit, sparse, compressed, system */
> >          unsigned int oplock;            /* oplock/lease level we have */
> > +       struct mutex oplock_mutex;
> >          unsigned int epoch;             /* used to track lease state changes */
> >   #define CIFS_INODE_PENDING_OPLOCK_BREAK   (0) /* oplock break in progress */
> >   #define CIFS_INODE_PENDING_WRITERS       (1) /* Writes in progress */
> > diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
> > index b20063cf774f..796b23712e71 100644
> > --- a/fs/cifs/smb2ops.c
> > +++ b/fs/cifs/smb2ops.c
> > @@ -1901,6 +1901,7 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode,
> > __u32 oplock,
> >          if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
> >                  return;
> >
> > +       mutex_lock(&cinode->oplock_mutex);
> >          cinode->oplock = 0;
> >          if (oplock & SMB2_LEASE_READ_CACHING_HE) {
> >                  cinode->oplock |= CIFS_CACHE_READ_FLG;
> > @@ -1916,6 +1917,8 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode,
> > __u32 oplock,
> >          }
> >          if (!cinode->oplock)
> >                  strcat(message, "None");
> > +       mutex_unlock(&cinode->oplock_mutex);
> > +
> >          cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
> >                   &cinode->vfs_inode);
> >   }
> >
> > Kai-Heng
>
> Unless you calculations on the oplock value or accessing it multiple
> times with some logic involved I don't think locking will help much.
> If two threads are assigning the same variable, you can end up with
> two possible outcomes regardless of whether locking is used or not.

Yes you are right, didnât think of this case.

> Locking will be needed once we start to make proper decisions based on
> previous and new values of the oplock to purge a page cache or flush
> buffered data. This still needs to be done and is out of the scope of
> this patch which aims to fix the buffer overflow error.

Thanks for your explanation.

Kai-Heng

> --
> Best regards,
> Pavel Shilovsky