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.

Cc: <stable@vger.kernel.org>
Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
Fixes: 370e812b3ec1 ("exfat: add nls operations")
Signed-off-by: Jeongjun Park <aha310510@gmail.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';
--

Hello!

On Monday 06 October 2025 20:45:07 Jeongjun Park wrote:
> After the loop that converts characters to ucs2 ends, the variable i 
> may be greater than or equal to len.

It is really possible to have "i" greater than len? Because I do not see
from the code how such thing could happen.

I see only a case when i is equal to len (which is also overflow).

My understanding:
while-loop condition ensures that i cannot be greater than len and i is
increased by exfat_convert_char_to_ucs2() function which has upper bound
of "len-i". So value of i can be increased maximally by (len-i) which
could lead to maximal value of i to be just "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.
> 
> Cc: <stable@vger.kernel.org>
> Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> Fixes: 370e812b3ec1 ("exfat: add nls operations")
> Signed-off-by: Jeongjun Park <aha310510@gmail.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')

What about "if (i < len)" condition instead?

The p_cstring is the nul term string and my understanding is that the
"p_cstring[i] != '\0'" is checking that i is at position of strlen()+1.
So should not be "if (i < len)" the same check without need to
dereference the p_cstring?

>  		lossy |= NLS_NAME_OVERLEN;
>  
>  	*uniname = '\0';
> --

Hi Pali

Pali Rohár <pali@kernel.org> wrote:
>
> Hello!
>
> On Monday 06 October 2025 20:45:07 Jeongjun Park wrote:
> > After the loop that converts characters to ucs2 ends, the variable i
> > may be greater than or equal to len.
>
> It is really possible to have "i" greater than len? Because I do not see
> from the code how such thing could happen.
>
> I see only a case when i is equal to len (which is also overflow).
>
> My understanding:
> while-loop condition ensures that i cannot be greater than len and i is
> increased by exfat_convert_char_to_ucs2() function which has upper bound
> of "len-i". So value of i can be increased maximally by (len-i) which
> could lead to maximal value of i to be just "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.
> >
> > Cc: <stable@vger.kernel.org>
> > Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> > Fixes: 370e812b3ec1 ("exfat: add nls operations")
> > Signed-off-by: Jeongjun Park <aha310510@gmail.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')
>
> What about "if (i < len)" condition instead?
>
> The p_cstring is the nul term string and my understanding is that the
> "p_cstring[i] != '\0'" is checking that i is at position of strlen()+1.
> So should not be "if (i < len)" the same check without need to
> dereference the p_cstring?
>

Thank you for the detailed explanation! I misunderstood.

In summary, since the variable i can never be greater than len, we don't
need to consider this case. Therefore, if i is less than len, we can
determine that an nls loss has occurred.

I think that under normal nls conditions, i should be equal to len
immediately after the while loop terminates, so changing the condition
here to "if (i != len)" would be a better way to make this clear.

This way, we can check for an nls loss without dereferencing p_cstring,
and we can clearly indicate that i should be equal to len when the while
loop terminates. What do you think?

Regards,
Jeongjun Park

On Thursday 09 October 2025 18:05:26 Jeongjun Park wrote:
> Hi Pali
> 
> Pali Rohár <pali@kernel.org> wrote:
> >
> > Hello!
> >
> > On Monday 06 October 2025 20:45:07 Jeongjun Park wrote:
> > > After the loop that converts characters to ucs2 ends, the variable i
> > > may be greater than or equal to len.
> >
> > It is really possible to have "i" greater than len? Because I do not see
> > from the code how such thing could happen.
> >
> > I see only a case when i is equal to len (which is also overflow).
> >
> > My understanding:
> > while-loop condition ensures that i cannot be greater than len and i is
> > increased by exfat_convert_char_to_ucs2() function which has upper bound
> > of "len-i". So value of i can be increased maximally by (len-i) which
> > could lead to maximal value of i to be just "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.
> > >
> > > Cc: <stable@vger.kernel.org>
> > > Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> > > Fixes: 370e812b3ec1 ("exfat: add nls operations")
> > > Signed-off-by: Jeongjun Park <aha310510@gmail.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')
> >
> > What about "if (i < len)" condition instead?
> >
> > The p_cstring is the nul term string and my understanding is that the
> > "p_cstring[i] != '\0'" is checking that i is at position of strlen()+1.
> > So should not be "if (i < len)" the same check without need to
> > dereference the p_cstring?
> >
> 
> Thank you for the detailed explanation! I misunderstood.
> 
> In summary, since the variable i can never be greater than len, we don't
> need to consider this case. Therefore, if i is less than len, we can
> determine that an nls loss has occurred.
> 
> I think that under normal nls conditions, i should be equal to len
> immediately after the while loop terminates, so changing the condition
> here to "if (i != len)" would be a better way to make this clear.
> 
> This way, we can check for an nls loss without dereferencing p_cstring,
> and we can clearly indicate that i should be equal to len when the while
> loop terminates. What do you think?
> 
> Regards,
> Jeongjun Park

Hello, yes, this is how I understood what is the code doing and how to
simple fix this reported problem.

On Mon, Oct 6, 2025 at 8:45 PM Jeongjun Park <aha310510@gmail.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?
Thanks.
>
> Cc: <stable@vger.kernel.org>
> Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> Fixes: 370e812b3ec1 ("exfat: add nls operations")
> Signed-off-by: Jeongjun Park <aha310510@gmail.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';
> --

Hi Namjae,

Namjae Jeon <linkinjeon@kernel.org> wrote:
>
> On Mon, Oct 6, 2025 at 8:45 PM Jeongjun Park <aha310510@gmail.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@vger.kernel.org>
> > Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> > Fixes: 370e812b3ec1 ("exfat: add nls operations")
> > Signed-off-by: Jeongjun Park <aha310510@gmail.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