From: Thomas Gleixner <tglx@linutronix.de>
To: Jann Horn <jannh@google.com>, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
John Fastabend <john.fastabend@gmail.com>,
bpf <bpf@vger.kernel.org>
Cc: syzbot <syzbot+72aa0161922eba61b50e@syzkaller.appspotmail.com>,
akpm@linux-foundation.org, bp@alien8.de, bp@suse.de,
dave.hansen@linux.intel.com, hpa@zytor.com,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
luto@kernel.org, mingo@redhat.com, netdev@vger.kernel.org,
peterz@infradead.org, syzkaller-bugs@googlegroups.com,
x86@kernel.org
Subject: Re: [syzbot] [mm?] BUG: unable to handle kernel paging request in copy_from_kernel_nofault
Date: Fri, 08 Dec 2023 22:01:16 +0100 [thread overview]
Message-ID: <87r0jwquhv.ffs@tglx> (raw)
In-Reply-To: <CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@mail.gmail.com>
On Fri, Dec 08 2023 at 15:11, Jann Horn wrote:
> On Tue, Nov 21, 2023 at 6:13 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>> > BUG: unable to handle page fault for address: ffffffffff600000
>>
>> This is VSYSCALL_ADDR.
>>
>> So the real question is why the BPF program tries to copy from the
>> VSYSCALL page, which is not mapped.
>
> The linked syz repro is:
>
> r0 = bpf$PROG_LOAD(0x5, &(0x7f00000000c0)={0x11, 0xb,
> &(0x7f0000000180)=@framed={{}, [@printk={@integer, {}, {}, {}, {},
> {0x7, 0x0, 0xb, 0x3, 0x0, 0x0, 0xff600000}, {0x85, 0x0, 0x0, 0x71}}]},
> &(0x7f0000000200)='GPL\x00', 0x0, 0x0, 0x0, 0x0, 0x0, '\x00', 0x0,
> 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
> 0x90)
> bpf$BPF_RAW_TRACEPOINT_OPEN(0x11,
> &(0x7f0000000540)={&(0x7f0000000000)='kfree\x00', r0}, 0x10)
>
> So syzkaller generated a BPF tracing program. 0x85 is BPF_JMP |
> BPF_CALL, which is used to invoke BPF helpers; 0x71 is 113, which is
> the number of the probe_read_kernel helper, which basically takes
> arbitrary values as input and casts them to kernel pointers, and then
> probe-reads them. And before that is some kinda ALU op with 0xff600000
> as immediate.
>
> So it looks like the answer to that question is "the BPF program tries
> to copy from the VSYSCALL page because syzkaller decided to write BPF
> code that does specifically that, and the BPF helper let it do that".
Indeed.
> copy_from_kernel_nofault() does check
> copy_from_kernel_nofault_allowed() to make sure the pointer really is
> a kernel pointer, and the X86 version of that rejects anything in the
> userspace part of the address space. But it does not know about the
> vsyscall area.
That's cureable. Untested fix below.
Thanks for the explanation!
tglx
---
diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
index 6993f026adec..8e846833aa37 100644
--- a/arch/x86/mm/maccess.c
+++ b/arch/x86/mm/maccess.c
@@ -3,6 +3,8 @@
#include <linux/uaccess.h>
#include <linux/kernel.h>
+#include <uapi/asm/vsyscall.h>
+
#ifdef CONFIG_X86_64
bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
{
@@ -15,6 +17,9 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
return false;
+ if ((vaddr & PAGE_MASK) == VSYSCALL_ADDR)
+ return false;
+
/*
* Allow everything during early boot before 'x86_virt_bits'
* is initialized. Needed for instruction decoding in early
next prev parent reply other threads:[~2023-12-08 21:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <000000000000c84343060a850bd0@google.com>
[not found] ` <87jzqb1133.ffs@tglx>
2023-12-08 14:11 ` [syzbot] [mm?] BUG: unable to handle kernel paging request in copy_from_kernel_nofault Jann Horn
2023-12-08 21:01 ` Thomas Gleixner [this message]
2023-12-21 11:59 ` Hou Tao
2024-02-22 14:29 ` Guilherme G. Piccoli
2024-02-22 16:04 ` Alexei Starovoitov
2024-02-23 1:09 ` Guilherme G. Piccoli
2024-02-23 1:11 ` Alexei Starovoitov
2024-02-25 19:03 ` Guilherme G. Piccoli
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87r0jwquhv.ffs@tglx \
--to=tglx@linutronix.de \
--cc=akpm@linux-foundation.org \
--cc=ast@kernel.org \
--cc=bp@alien8.de \
--cc=bp@suse.de \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=dave.hansen@linux.intel.com \
--cc=hpa@zytor.com \
--cc=jannh@google.com \
--cc=john.fastabend@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mingo@redhat.com \
--cc=netdev@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=syzbot+72aa0161922eba61b50e@syzkaller.appspotmail.com \
--cc=syzkaller-bugs@googlegroups.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).