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

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Date:   Wed, 13 Apr 2022 14:05:47 +0530
From:   Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
To:     Steve French <smfrench@...il.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

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@...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
>>
> 
>