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Message-ID: <437ec6fa34af1ccfadee2d62770e52d99ebf75c3.camel@redhat.com>
Subject: Re: [PATCH] fs/nilfs2: copy userspace-array safely
From:   Philipp Stanner <pstanner@redhat.com>
To:     Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc:     linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        Dave Airlie <airlied@redhat.com>
Date:   Fri, 03 Nov 2023 18:56:49 +0100
In-Reply-To: <CAKFNMok9WTV5uX3BBfr8eYiuJXvUY6TuYW=tQYvBzdCUYsxcwA@mail.gmail.com>
References: <20231102183751.47413-1-pstanner@redhat.com>
         <CAKFNMok9WTV5uX3BBfr8eYiuJXvUY6TuYW=tQYvBzdCUYsxcwA@mail.gmail.com>
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On Sat, 2023-11-04 at 02:44 +0900, Ryusuke Konishi wrote:
> On Fri, Nov 3, 2023 at 3:38=E2=80=AFAM Philipp Stanner wrote:
> >=20
> > ioctl.c utilizes memdup_user() to copy a userspace array. This is
> > done
> > without an overflow-check.
> >=20
> > Use the new wrapper memdup_array_user() to copy the array more
> > safely.
> >=20
> > Suggested-by: Dave Airlie <airlied@redhat.com>
> > Signed-off-by: Philipp Stanner <pstanner@redhat.com>
> > ---
> > Linus recently merged this new wrapper for Kernel v6.7
>=20
> The following overflow check is performed just before the usage of
> memdup_user():
>=20
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (nsegs > UINT_MAX / sizeof(=
__u64))
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 goto out;
>=20
> This was introduced by commit 1ecd3c7ea76488 ("nilfs2: avoid
> overflowing segment numbers in nilfs_ioctl_clean_segments()") to
> avoid
> overflowing nsegs * sizeof(__u64) in the subsequent call to
> memdup_user().
>=20
> I learned about memdup_array_user() this time, and it seems to check
> for overflow when multiplying two size_t arguments (i.e. the number
> of
> elements and size of the array to be copied).
>=20
> Since size_t is 32-bit or 64-bit depending on the architecture, I
> think the overflow check that memdup_array_user() does
> is included in the above upper limit check by UINT_MAX.
>=20
> So, for security reasons, I don't think this change is necessary.=C2=A0
> (Am
> I missing something?)

No, I think you are right. My commit message was very generic =E2=80=93 it'=
s
more about unifying array-duplication.
I should rephrase it.

>=20
> In terms of cleanup, I think the clarification this patch brings is
> good, but in that case, I'm concerned about the duplication of
> overflow checks.

Alright, so would you prefer a patch that uses memdup_array_user() and,
consequently, removes the preceding check?

Regards,
P.

>=20
> Thanks,
> Ryusuke Konishi
>=20
> > ---
> > =C2=A0fs/nilfs2/ioctl.c | 8 ++++----
> > =C2=A01 file changed, 4 insertions(+), 4 deletions(-)
> >=20
> > diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
> > index 40ffade49f38..6a9dceebb18d 100644
> > --- a/fs/nilfs2/ioctl.c
> > +++ b/fs/nilfs2/ioctl.c
> > @@ -877,11 +877,11 @@ static int nilfs_ioctl_clean_segments(struct
> > inode *inode, struct file *filp,
> >=20
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /*
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * argv[4] points to se=
gment numbers this ioctl cleans.=C2=A0 We
> > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * use kmalloc() for its buf=
fer because memory used for the
> > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * segment numbers is enough=
 small.
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * use kmalloc() for its buf=
fer because the memory used for
> > the
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * segment numbers is small =
enough.
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */
> > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 kbufs[4] =3D memdup_user((void __=
user *)(unsigned
> > long)argv[4].v_base,
> > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 nsegs * sizeof(__u64));
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 kbufs[4] =3D memdup_array_user((v=
oid __user *)(unsigned
> > long)argv[4].v_base,
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 nsegs, s=
izeof(__u64));
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (IS_ERR(kbufs[4])) {
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 ret =3D PTR_ERR(kbufs[4]);
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 goto out;
> > --
> > 2.41.0
> >=20
>=20