linux-kernel - Re: [PATCH] fat: Read buffer overflow

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Date:	Sat, 8 Aug 2009 09:56:21 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Roel Kluin <roel.kluin@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fat: Read buffer overflow

Hi Hirofumi-san,

Roel Kluin <roel.kluin@...il.com> writes:
>> If len is less or equal to 0, this results in a read of s[-1].
>>
>> Signed-off-by: Roel Kluin <roel.kluin@...il.com>
>> ---
>> diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
>> index 6f27853..114ff6d 100644
>> --- a/fs/fat/namei_vfat.c
>> +++ b/fs/fat/namei_vfat.c
>> @@ -202,6 +202,9 @@ static inline int vfat_is_used_badchars(const wchar_t *s, int len)
>>  {
>>       int i;
>>
>> +     if (len <= 0)
>> +             return -EINVAL;
>> +
>>       for (i = 0; i < len; i++)
>>               if (vfat_bad_char(s[i]))
>>                       return -EINVAL;

On Sat, Aug 8, 2009 at 2:06 AM, OGAWA
Hirofumi<hirofumi@...l.parknet.co.jp> wrote:
> Um..., what case is this possible? vfat_add_entry() checks (len == 0).
> And if xlate_to_uni() couldn't convert name, it should return the error.

Yes, but we pass "ulen" to vfat_is_used_badchars(). The value of
"ulen" is a returned in the "longlen" argument of xlate_to_uni() which
in turn is calculated as follows for the UTF-8 case:

  int name_len = strlen(name);
  *outlen = utf8s_to_utf16s(name, PATH_MAX, (wchar_t *) outname);
  *outlen -= (name_len - len);
  *longlen = *outlen;

Maybe "*outlen" can never be negative because of some invariants that
I don't see but it's so non-obvious to me that I'd like to see the
explicit check in vfat_is_used_badchars().

                        Pekka
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