Received-SPF: pass (google.com: domain of linux-kernel+bounces-115125-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Precedence: bulk
MIME-Version: 1.0
References: <20240320105436.4781-1-puranjay12@gmail.com> <CAADnVQJ3o6DsURi=N_KXx+mbW9r7__3LrwYLyYwuoMOsqFHPkw@mail.gmail.com>
 <1686adb401b34adaa3b703ec1a8ffe49@AcuMS.aculab.com> <CAADnVQJzfnK0Mv6HVKZ38VDuAemzbmSMeYscf77YoEy0SgWw+A@mail.gmail.com>
 <c73258ae8425470f90ad31c424cebe3a@AcuMS.aculab.com>
In-Reply-To: <c73258ae8425470f90ad31c424cebe3a@AcuMS.aculab.com>
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Sun, 24 Mar 2024 16:48:55 -0700
Message-ID: <CAADnVQ+d3pumHKBA1tTwy-8Vm21xu5Xckqv+4y-27FidBVetvQ@mail.gmail.com>
Subject: Re: [PATCH bpf] bpf: verifier: prevent userspace memory access
To: David Laight <David.Laight@aculab.com>
Cc: Puranjay Mohan <puranjay12@gmail.com>, Ilya Leoshkevich <iii@linux.ibm.com>, 
	"David S. Miller" <davem@davemloft.net>, David Ahern <dsahern@kernel.org>, 
	Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, 
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, X86 ML <x86@kernel.org>, 
	"H. Peter Anvin" <hpa@zytor.com>, Jean-Philippe Brucker <jean-philippe@linaro.org>, 
	Network Development <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>, 
	LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Sun, Mar 24, 2024 at 3:30=E2=80=AFPM David Laight <David.Laight@aculab.c=
om> wrote:
>
> From: Alexei Starovoitov
> > Sent: 24 March 2024 20:43
> >
> > On Sun, Mar 24, 2024 at 1:05=E2=80=AFPM David Laight <David.Laight@acul=
ab.com> wrote:
> > >
> > > From: Alexei Starovoitov
> > > > Sent: 21 March 2024 06:08
> > > >
> > > > On Wed, Mar 20, 2024 at 3:55=E2=80=AFAM Puranjay Mohan <puranjay12@=
gmail.com> wrote:
> > > > >
> > > > > The JITs need to implement bpf_arch_uaddress_limit() to define wh=
ere
> > > > > the userspace addresses end for that architecture or TASK_SIZE is=
 taken
> > > > > as default.
> > > > >
> > > > > The implementation is as follows:
> > > > >
> > > > > REG_AX =3D  SRC_REG
> > > > > if(offset)
> > > > >         REG_AX +=3D offset;
> > > > > REG_AX >>=3D 32;
> > > > > if (REG_AX <=3D (uaddress_limit >> 32))
> > > > >         DST_REG =3D 0;
> > > > > else
> > > > >         DST_REG =3D *(size *)(SRC_REG + offset);
> > > >
> > > > The patch looks good, but it seems to be causing s390 CI failures.
> > >
> > > I'm confused by the need for this check (and, IIRC, some other bpf
> > > code that does kernel copies that can fault - and return an error).
> > >
> > > I though that the entire point of bpf was that is sanitised and
> > > verified everything to limit what the 'program' could do in order
> > > to stop it overwriting (or even reading) kernel structures that
> > > is wasn't supposed to access.
> > >
> > > So it just shouldn't have a address that might be (in any way)
> > > invalid.
> >
> > bpf tracing progs can call bpf_probe_read_kernel() which
> > can read any kernel memory.
> > This is nothing but an inlined version of it.
>
> It was the getsockopt() code were I saw the copy_nocheck() calls.
> Those have to be broken.

No. If you mean csum_partial_copy_nocheck() then they're fine.

> Although the way some of the options use the ptr:len supplied by
> the application you stand no chance of do an in-kernel call
> without a proper buffer descriptor argument (with separate optlen
> and bufferlen fields.)
>
> >
> > > The only possible address verify is access_ok() to ensure that
> > > a uses address really is a user address.
> >
> > access_ok() considerations don't apply.
> > We're not dealing with user memory access.
>
> If you do need a check for 'not a user address' don't you want to just
> require access_ok() fail?
> That would be architecture independent.

No. access_ok() can only be used on the user addr.
access_ok() =3D=3D false on the kernel addr doesn't mean that
it's a kernel addr.