Re: [PATCH bpf v3 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

[Sohil Mehta]

On 2/2/2024 2:39 AM, 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 is triggered when:
> 
> 1) A bpf program uses bpf_probe_read_kernel() to read from the vsyscall
> page and invokes copy_from_kernel_nofault() which in turn calls
> __get_user_asm().
> 
> 2) Because the vsyscall page address is not readable from kernel space,
> a page fault exception is triggered accordingly.
> 
> 3) handle_page_fault() considers the vsyscall page address as a user
> space address instead of a kernel space address. This results in the
> fix-up setup by bpf not being applied and a page_fault_oops() is invoked
> due to SMAP.
> 
> Considering handle_page_fault() has already considered the vsyscall page
> address as a userspace address, fix the problem by disallowing vsyscall
> page read for copy_from_kernel_nofault().
> 
> Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Reported-by: syzbot+72aa0161922eba61b50e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@xxxxxxxxxxxxxx
> Reported-by: xingwei lee <xrivendell7@xxxxxxxxx>
> Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=w33LvScw@xxxxxxxxxxxxxx
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> ---
>  arch/x86/mm/maccess.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 

Reviewed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>