linux-kernel - Re: [PATCH] fs/xfs: replace strncpy with strscpy

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Message-ID: <6grkelqkotgz6hnwce5n7gzdixgzlaqkwzo243f7vyqjas3c4g@viyu57vmxltz>
Date: Mon, 30 Jun 2025 20:38:08 +0200
From: Carlos Maiolino <cem@...nel.org>
To: Pranav Tyagi <pranav.tyagi03@...il.com>
Cc: skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev, 
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/xfs: replace strncpy with strscpy

On Mon, Jun 30, 2025 at 02:36:01PM +0530, Pranav Tyagi wrote:
> On Mon, Jun 30, 2025 at 2:09 PM Carlos Maiolino <cem@...nel.org> wrote:
> >
> > On Tue, Jun 17, 2025 at 06:15:46PM +0530, Pranav Tyagi wrote:
> > > Replace the deprecated strncpy() with strscpy() as the destination
> > > buffer should be NUL-terminated and does not require any trailing
> > > NUL-padding. Also, since NUL-termination is guaranteed,
> >
> > NUL-termination is only guaranteed if you copy into the buffer one less
> > byte than the label requires, i.e XFSLABEL_MAX.
> >
> > > use sizeof(label) in place of XFSLABEL_MAX as the size
> > > parameter.
> >
> > This is wrong, see below why.
> >
> > >
> > > Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.com>
> > > ---
> > >  fs/xfs/xfs_ioctl.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > > index d250f7f74e3b..9f4d68c5b5ab 100644
> > > --- a/fs/xfs/xfs_ioctl.c
> > > +++ b/fs/xfs/xfs_ioctl.c
> > > @@ -992,7 +992,7 @@ xfs_ioc_getlabel(
> > >       /* 1 larger than sb_fname, so this ensures a trailing NUL char */
> > >       memset(label, 0, sizeof(label));
> > >       spin_lock(&mp->m_sb_lock);
> > > -     strncpy(label, sbp->sb_fname, XFSLABEL_MAX);
> > > +     strscpy(label, sbp->sb_fname, sizeof(label));
> >
> > This is broken and you created a buffer overrun here.
> >
> > XFSLABEL_MAX is set to 12 bytes. The current label size is 13 bytes:
> >
> > char                    label[XFSLABEL_MAX + 1];
> >
> > This ensures the label will always have a null termination character as
> > long as you copy XFSLABEL_MAX bytes into the label.
> >
> > - strncpy(label, sbp->sb_fname, XFSLABEL_MAX);
> >
> > Copies 12 bytes from sb_fname into label. This ensures we always have a
> > trailing \0 at the last byte.
> >
> > Your version:
> >
> > strscpy(label, sbp->sb_fname, sizeof(label));
> >
> > Copies 13 bytes from sb_fname into the label buffer.
> >
> > This not only could have copied a non-null byte to the last byte in the
> > label buffer, but also But sbp->sb_fname size is XFSLABEL_MAX, so you
> > are reading beyond the source buffer size, causing a buffer overrun as you
> > can see on the kernel test robot report.
> >
> > Carlos
> >
> > >       spin_unlock(&mp->m_sb_lock);
> > >
> > >       if (copy_to_user(user_label, label, sizeof(label)))
> > > --
> > > 2.49.0
> > >
> 
> Hi,
> 
> Thank you for the feedback. I understand that my patch is incorrect and
> it causes a buffer overrun. The destination buffer is indeed, already, null
> terminated. Would you like me to send a corrected patch which uses
> strscpy() (as strncpy() is deprecated)?

Sure, do so.

Carlos

> 
> Regret the inconvenience.
> 
> Regards
> Pranav Tyagi