Re: [PATCH bpf v2 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

[Hou Tao]

Hi,

On 1/30/2024 7:50 AM, Sohil Mehta wrote:
> Hi Hou Tao,
>
> I agree to your approach in this patch. Please see some comments below.
>
> On 1/26/2024 3:54 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:

[SNIP]
>> It seems the occurrence of oops depends on SMAP feature of CPU. It
>> happens as follow: 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().
>> Because the vsyscall page address is not readable for kernel space,
>> a page fault exception is triggered accordingly, 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. Because
>> the CPU has SMAP feature and the access happens in kernel mode, so
>> page_fault_oops() is invoked and an oops happens. If these is no SMAP
>> feature, the fix-up set-up by bpf will be applied and
>> copy_from_kernel_nofault() will return -EFAULT instead.
>>
> I find this paragraph to be a bit hard to follow. I think we can
> minimize the reference to SMAP here since it is only helping detect
> cross address space accesses.  How about something like the following:
>
> 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.

Thanks for the rephrasing. It is much better now.
>> 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().
>>
> I agree, following the same approach as handle_page_fault() seems
> reasonable.
>
>> 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 | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
>> index 6993f026adec9..d9272e1db5224 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 <asm/vsyscall.h>
>> +
>>  #ifdef CONFIG_X86_64
>>  bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
>>  {
>> @@ -15,6 +17,13 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
>>  	if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
>>  		return false;
>>  
>> +	/* Also consider the vsyscall page as userspace address. Otherwise,
>> +	 * reading the vsyscall page in copy_from_kernel_nofault() may
>> +	 * trigger an oops due to an unhandled page fault.
>> +	 */
> x86 prefers a slightly different style for multi-line comments. Please
> refer to https://docs.kernel.org/process/maintainer-tip.html#comment-style.

I see. Will update.
>
> How about rewording the above as:
>
> /*
>  * Reading from the vsyscall page may cause an unhandled fault in
>  * certain cases.  Though it is at an address above TASK_SIZE_MAX, it is
>  * usually considered as a user space address.
>  */

Thanks for the rewording. Will do in v3.
>
>> +	if (is_vsyscall_vaddr(vaddr))
>> +		return false;
>> +
> It would have been convenient if we had a common check for whether a
> particular address is a kernel address or not. fault_in_kernel_space()
> serves that purpose to an extent in other places.
>
> I thought we could rename fault_in_kernel_space() to
> vaddr_in_kernel_space() and use it here. But the check in
> copy_from_kernel_nofault_allowed() includes the user guard page as well.
> So the checks wouldn't exactly be the same.
>
> I am unsure of the implications if we get rid of that difference. Maybe
> we can leave it as-is for now unless someone else chimes in.

There is other difference between fault_in_kernel_space() and
copy_from_kernel_nofault_allowed(). fault_in_kernel_space() uses address
>= TASK_SIZE_MAX to check the kernel space address, but
copy_from_kernel_nofault_allowed() uses vaddr >= TASK_SIZE_MAX +
PAGE_SIZE to check the kernel space address, so I prefer to keep it as-is.
>
> Sohil