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From: Masami Hiramatsu <mhiramat@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Christoph Hellwig <hch@lst.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-parisc@vger.kernel.org,
	linux-um <linux-um@lists.infradead.org>,
	Netdev <netdev@vger.kernel.org>,
	bpf@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 11/18] maccess: remove strncpy_from_unsafe
Date: Thu, 14 May 2020 08:20:54 +0900	[thread overview]
Message-ID: <20200514082054.f817721ce196f134e6820644@kernel.org> (raw)
In-Reply-To: <0c1a7066-b269-9695-b94a-bb5f4f20ebd8@iogearbox.net>

On Thu, 14 May 2020 00:36:28 +0200
Daniel Borkmann <daniel@iogearbox.net> wrote:

> On 5/13/20 9:28 PM, Christoph Hellwig wrote:
> > On Wed, May 13, 2020 at 12:11:27PM -0700, Linus Torvalds wrote:
> >> On Wed, May 13, 2020 at 9:01 AM Christoph Hellwig <hch@lst.de> wrote:
> >>>
> >>> +static void bpf_strncpy(char *buf, long unsafe_addr)
> >>> +{
> >>> +       buf[0] = 0;
> >>> +       if (strncpy_from_kernel_nofault(buf, (void *)unsafe_addr,
> >>> +                       BPF_STRNCPY_LEN))
> >>> +               strncpy_from_user_nofault(buf, (void __user *)unsafe_addr,
> >>> +                               BPF_STRNCPY_LEN);
> >>> +}
> >>
> >> This seems buggy when I look at it.
> >>
> >> It seems to think that strncpy_from_kernel_nofault() returns an error code.
> >>
> >> Not so, unless I missed where you changed the rules.
> > 
> > I didn't change the rules, so yes, this is wrong.
> > 
> >> Also, I do wonder if we shouldn't gate this on TASK_SIZE, and do the
> >> user trial first. On architectures where this thing is valid in the
> >> first place (ie kernel and user addresses are separate), the test for
> >> address size would allow us to avoid a pointless fault due to an
> >> invalid kernel access to user space.
> >>
> >> So I think this function should look something like
> >>
> >>    static void bpf_strncpy(char *buf, long unsafe_addr)
> >>    {
> >>            /* Try user address */
> >>            if (unsafe_addr < TASK_SIZE) {
> >>                    void __user *ptr = (void __user *)unsafe_addr;
> >>                    if (strncpy_from_user_nofault(buf, ptr, BPF_STRNCPY_LEN) >= 0)
> >>                            return;
> >>            }
> >>
> >>            /* .. fall back on trying kernel access */
> >>            buf[0] = 0;
> >>            strncpy_from_kernel_nofault(buf, (void *)unsafe_addr,
> >> BPF_STRNCPY_LEN);
> >>    }
> >>
> >> or similar. No?
> > 
> > So on say s390 TASK_SIZE_USUALLy is (-PAGE_SIZE), which means we'd alway
> > try the user copy first, which seems odd.
> > 
> > I'd really like to here from the bpf folks what the expected use case
> > is here, and if the typical argument is kernel or user memory.
> 
> It's used for both. Given this is enabled on pretty much all program types, my
> assumption would be that usage is still more often on kernel memory than user one.

For trace_kprobe.c current order (kernel -> user fallback) is preferred
because it has another function dedicated for user memory.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>

  parent reply	other threads:[~2020-05-13 23:21 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-13 16:00 clean up and streamline probe_kernel_* and friends v2 Christoph Hellwig
2020-05-13 16:00 ` [PATCH 01/18] maccess: unexport probe_kernel_write and probe_user_write Christoph Hellwig
2020-05-13 16:00 ` [PATCH 02/18] maccess: remove various unused weak aliases Christoph Hellwig
2020-05-13 16:00 ` [PATCH 03/18] maccess: remove duplicate kerneldoc comments Christoph Hellwig
2020-05-13 16:00 ` [PATCH 04/18] maccess: clarify " Christoph Hellwig
2020-05-13 16:00 ` [PATCH 05/18] maccess: update the top of file comment Christoph Hellwig
2020-05-13 16:00 ` [PATCH 06/18] maccess: rename strncpy_from_unsafe_user to strncpy_from_user_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 07/18] maccess: rename strncpy_from_unsafe_strict to strncpy_from_kernel_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 08/18] maccess: rename strnlen_unsafe_user to strnlen_user_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 09/18] maccess: remove probe_read_common and probe_write_common Christoph Hellwig
2020-05-13 16:00 ` [PATCH 10/18] maccess: unify the probe kernel arch hooks Christoph Hellwig
2020-05-14  1:13   ` Masami Hiramatsu
2020-05-19  5:46     ` Christoph Hellwig
2020-05-13 16:00 ` [PATCH 11/18] maccess: remove strncpy_from_unsafe Christoph Hellwig
2020-05-13 19:11   ` Linus Torvalds
2020-05-13 19:11     ` Linus Torvalds
2020-05-13 19:28     ` Christoph Hellwig
2020-05-13 22:36       ` Daniel Borkmann
2020-05-13 23:03         ` Linus Torvalds
2020-05-13 23:03           ` Linus Torvalds
2020-05-13 23:24           ` Daniel Borkmann
2020-05-13 23:20         ` Masami Hiramatsu [this message]
2020-05-13 23:59           ` Linus Torvalds
2020-05-13 23:59             ` Linus Torvalds
2020-05-14  1:00             ` Masami Hiramatsu
2020-05-14  2:43               ` Linus Torvalds
2020-05-14  2:43                 ` Linus Torvalds
2020-05-14  9:44                 ` Masami Hiramatsu
2020-05-14 10:27                   ` Daniel Borkmann
2020-05-13 23:28         ` Al Viro
2020-05-13 23:58           ` Daniel Borkmann
2020-05-14 10:01             ` David Laight
2020-05-14 10:21               ` Daniel Borkmann
2020-05-13 16:00 ` [PATCH 12/18] maccess: always use strict semantics for probe_kernel_read Christoph Hellwig
2020-05-13 16:00 ` [PATCH 13/18] maccess: move user access routines together Christoph Hellwig
2020-05-13 16:00 ` [PATCH 14/18] maccess: allow architectures to provide kernel probing directly Christoph Hellwig
2020-05-13 19:36   ` Linus Torvalds
2020-05-13 19:36     ` Linus Torvalds
2020-05-13 19:40     ` Christoph Hellwig
2020-05-13 19:48       ` Linus Torvalds
2020-05-13 19:48         ` Linus Torvalds
2020-05-13 19:54         ` Christoph Hellwig
2020-05-16  3:42   ` Masami Hiramatsu
2020-05-18 15:09     ` Christoph Hellwig
2020-05-13 16:00 ` [PATCH 15/18] x86: use non-set_fs based maccess routines Christoph Hellwig
2020-05-13 16:00 ` [PATCH 16/18] maccess: rename probe_kernel_{read,write} to copy_{from,to}_kernel_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 17/18] maccess: rename probe_user_{read,write} to copy_{from,to}_user_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 18/18] maccess: rename probe_kernel_address to get_kernel_nofault Christoph Hellwig
2020-05-13 19:37 ` clean up and streamline probe_kernel_* and friends v2 Linus Torvalds
2020-05-13 19:37   ` Linus Torvalds
2020-05-13 23:04 ` Daniel Borkmann
2020-05-13 23:20   ` Linus Torvalds
2020-05-13 23:20     ` Linus Torvalds
2020-05-19  5:50   ` Christoph Hellwig

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