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

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Message-ID: <CAH2r5mvBH8wO9rS2UwCpgjbhcLp89-RLbr1wxK76Pj=OprKwOA@mail.gmail.com>
Date:   Tue, 12 Apr 2022 17:32:03 -0500
From:   Steve French <smfrench@...il.com>
To:     Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        Steve French <sfrench@...ba.org>,
        Stefan Metzmacher <metze@...ba.org>,
        samba-technical <samba-technical@...ts.samba.org>,
        LKML <linux-kernel@...r.kernel.org>,
        ronnie sahlberg <ronniesahlberg@...il.com>
Subject: Re: [PATCH] cifs: potential buffer overflow in handling symlinks

Wouldn't it be easier and clearer to do the compare vs the maximum len ie

if (link_len > CIFS_MF_SYMLINK_LINK_MAXLEN)

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@...cle.com> 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@...cle.com>
> ---
>  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
>


-- 
Thanks,

Steve