From: Sohil Mehta <sohil.mehta@intel.com>
To: Hou Tao <houtao@huaweicloud.com>, <x86@kernel.org>,
<bpf@vger.kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H . Peter Anvin" <hpa@zytor.com>, <linux-kernel@vger.kernel.org>,
xingwei lee <xrivendell7@gmail.com>,
"Jann Horn" <jannh@google.com>, <houtao1@huawei.com>
Subject: Re: [PATCH bpf 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()
Date: Mon, 22 Jan 2024 16:18:06 -0800 [thread overview]
Message-ID: <e83eb3e8-6d08-462b-9ffe-d843e439d7da@intel.com> (raw)
In-Reply-To: <20240119073019.1528573-3-houtao@huaweicloud.com>
On 1/18/2024 11:30 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
>
> When trying to use copy_from_kernel_nofault() to read vsyscall page
> through a bpf program, the following oops was reported:
>
> BUG: unable to handle page fault for address: ffffffffff600000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 3231067 P4D 3231067 PUD 3233067 PMD 3235067 PTE 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 1 PID: 20390 Comm: test_progs ...... 6.7.0+ #58
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
> RIP: 0010:copy_from_kernel_nofault+0x6f/0x110
> ......
> Call Trace:
> <TASK>
> ? copy_from_kernel_nofault+0x6f/0x110
> bpf_probe_read_kernel+0x1d/0x50
> bpf_prog_2061065e56845f08_do_probe_read+0x51/0x8d
> trace_call_bpf+0xc5/0x1c0
> perf_call_bpf_enter.isra.0+0x69/0xb0
> perf_syscall_enter+0x13e/0x200
> syscall_trace_enter+0x188/0x1c0
> do_syscall_64+0xb5/0xe0
> entry_SYSCALL_64_after_hwframe+0x6e/0x76
> </TASK>
> ......
> ---[ end trace 0000000000000000 ]---
>
> The oops happens as follows: A bpf program uses bpf_probe_read_kernel()
> to read from vsyscall page, bpf_probe_read_kernel() invokes
> copy_from_kernel_nofault() in turn and then invokes __get_user_asm(). A
> page fault exception is triggered accordingly, but handle_page_fault()
> considers the vsyscall page address as a userspace address instead of
> a kernel space address, so the fix-up set-up by bpf isn't applied.
This comment and the one in the code below seem contradictory and
confusing. Do we want the vsyscall page address to be considered as a
userspace address or not?
IIUC, the issue here is that the vsyscall page (in xonly mode) is not
really mapped and therefore running copy_from_kernel_nofault() on this
address is incorrect. This patch fixes this by making
copy_from_kernel_nofault() return an error for a vsyscall address.
> Because the exception happens in kernel space and page fault handling is
> disabled, page_fault_oops() is invoked and an oops happens.
>
> Fix it by disallowing vsyscall page read for copy_from_kernel_nofault().
>
[Maybe I have misunderstood the issue here and following questions are
not even relevant.]
But, what about vsyscall=emulate? In that mode the page is actually
mapped. Would we want the page read to go through then?
> Originally-from: Thomas Gleixner <tglx@linutronix.de>
Documentation/process/maintainer-tip.rst says to use "Originally-by:"
> diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
> index 6993f026adec9..bb454e0abbfcf 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 "mm_internal.h"
> +
> #ifdef CONFIG_X86_64
> bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
> {
> @@ -15,6 +17,10 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
> if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
> return false;
>
> + /* vsyscall page is also considered as userspace address. */
A bit more explanation about why this should happen might be useful.
> + if (is_vsyscall_vaddr(vaddr))
> + 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:[~2024-01-23 0:18 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-19 7:30 [PATCH bpf 0/3] Fix the read of vsyscall page through bpf Hou Tao
2024-01-19 7:30 ` [PATCH bpf 1/3] x86/mm: Move is_vsyscall_vaddr() into mm_internal.h Hou Tao
2024-01-20 0:35 ` Sohil Mehta
2024-01-20 1:29 ` Hou Tao
2024-01-19 7:30 ` [PATCH bpf 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault() Hou Tao
2024-01-23 0:18 ` Sohil Mehta [this message]
2024-01-25 7:18 ` Hou Tao
2024-01-19 7:30 ` [PATCH bpf 3/3] selftest/bpf: Test the read of vsyscall page under x86-64 Hou Tao
2024-01-22 6:30 ` Yonghong Song
2024-01-25 6:37 ` Hou Tao
2024-01-23 0:25 ` Sohil Mehta
2024-01-25 7:54 ` Hou Tao
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