linux-kernel - Re: [PATCH] exfat: fix out-of-bounds in exfat_nls_to

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Message-ID: <CAO9qdTHx-EYBeo1mfgVzcwQT5M6iwtVsBZTjAEVQugcfTsVtjA@mail.gmail.com>
Date: Wed, 8 Oct 2025 17:52:47 +0900
From: Jeongjun Park <aha310510@...il.com>
To: Namjae Jeon <linkinjeon@...nel.org>
Cc: sj1557.seo@...sung.com, yuezhang.mo@...y.com, viro@...iv.linux.org.uk, 
	pali@...nel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org, syzbot+98cc76a76de46b3714d4@...kaller.appspotmail.com
Subject: Re: [PATCH] exfat: fix out-of-bounds in exfat_nls_to_ucs2()

Hi Namjae,

Namjae Jeon <linkinjeon@...nel.org> wrote:
>
> On Mon, Oct 6, 2025 at 8:45 PM Jeongjun Park <aha310510@...il.com> wrote:
> >
> Hi Jeongjun,
> > After the loop that converts characters to ucs2 ends, the variable i
> > may be greater than or equal to len. However, when checking whether the
> > last byte of p_cstring is NULL, the variable i is used as is, resulting
> > in an out-of-bounds read if i >= len.
> >
> > Therefore, to prevent this, we need to modify the function to check
> > whether i is less than len, and if i is greater than or equal to len,
> > to check p_cstring[len - 1] byte.
> I think we need to pass FSLABEL_MAX - 1 to exfat_nls_to_utf16, not FSLABEL_MAX.
> Can you check it and update the patch?

If the only reason to change len to FSLABEL_MAX - 1 is to prevent
out-of-bounds, this isn't a very appropriate solution.

Because the return value of exfat_convert_char_to_ucs2() can be greater
than 1, even if len is set to FSLABEL_MAX - 1, i may still be FSLABEL_MAX
when the loop ends. Therefore, checking the last byte of p_cstring with
the min() function is essential to ensure out-of-bounds prevention.

> Thanks.
> >
> > Cc: <stable@...r.kernel.org>
> > Reported-by: syzbot+98cc76a76de46b3714d4@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> > Fixes: 370e812b3ec1 ("exfat: add nls operations")
> > Signed-off-by: Jeongjun Park <aha310510@...il.com>
> > ---
> >  fs/exfat/nls.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/exfat/nls.c b/fs/exfat/nls.c
> > index 8243d94ceaf4..a52f3494eb20 100644
> > --- a/fs/exfat/nls.c
> > +++ b/fs/exfat/nls.c
> > @@ -616,7 +616,7 @@ static int exfat_nls_to_ucs2(struct super_block *sb,
> >                 unilen++;
> >         }
> >
> > -       if (p_cstring[i] != '\0')
> > +       if (p_cstring[min(i, len - 1)] != '\0')
> >                 lossy |= NLS_NAME_OVERLEN;
> >
> >         *uniname = '\0';
> > --

Regards,
Jeongjun Park