Re: [PATCH bpf 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

[Sohil Mehta]

On 1/18/2024 11:30 PM, Hou Tao wrote:
> From: Hou Tao <houtao1@xxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxx>

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