Re: [PATCH] cifs: potential buffer overflow in handling symlinks

[Harshit Mogalapalli]

Hi Steve,
On 13/04/22 4:02 am, Steve French wrote:
> Wouldn't it be easier and clearer to do the compare vs the maximum len ie
>
> if (link_len > CIFS_MF_SYMLINK_LINK_MAXLEN)
Yes, thats cleaner. Will send a v2 patch.

Thanks,
Harshit
> instead of
>
> if (link_len > buf_len - CIFS_MF_SYMLINK_LINK_OFFSET)
>
> since buf_len is  CIFS_MF_SYMLINK_FILE_SIZE and looking at link.c line
> 26 and 27 this means we can use CIFS_MF_SYMLINK_LINK_OFFSET for the
> comparison:
>
> #define CIFS_MF_SYMLINK_LINK_MAXLEN (1024)
> #define CIFS_MF_SYMLINK_FILE_SIZE \
>          (CIFS_MF_SYMLINK_LINK_OFFSET + CIFS_MF_SYMLINK_LINK_MAXLEN)
>
> On Tue, Apr 12, 2022 at 1:01 AM Harshit Mogalapalli
> <harshit.m.mogalapalli@xxxxxxxxxx> wrote:
> > Smatch printed a warning:
> >          arch/x86/crypto/poly1305_glue.c:198 poly1305_update_arch() error:
> >          __memcpy() 'dctx->buf' too small (16 vs u32max)
> >
> > It's caused because Smatch marks 'link_len' as untrusted since it comes
> > from sscanf(). Add a check to ensure that 'link_len' is not larger than
> > the size of the 'link_str' buffer.
> >
> > Fixes: c69c1b6eaea1 ("cifs: implement CIFSParseMFSymlink()")
> > Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
> > ---
> >   fs/cifs/link.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/cifs/link.c b/fs/cifs/link.c
> > index 852e54e..ebfedae 100644
> > --- a/fs/cifs/link.c
> > +++ b/fs/cifs/link.c
> > @@ -85,6 +85,9 @@
> >          if (rc != 1)
> >                  return -EINVAL;
> >
> > +       if (link_len > buf_len - CIFS_MF_SYMLINK_LINK_OFFSET)
> > +               return -EINVAL;
> > +
> >          rc = symlink_hash(link_len, link_str, md5_hash);
> >          if (rc) {
> >                  cifs_dbg(FYI, "%s: MD5 hash failure: %d\n", __func__, rc);
> > --
> > 1.8.3.1