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

[Kai-Heng Feng]

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

> Thanks for cleaning it up!
>
> Reviewed-by: Pavel Shilovsky <pshilov@xxxxxxxxxxxxx>
>
> --
> Best regards,
> Pavel Shilovsky