Re: [PATCH v2] kprobes/x86: Use copy_from_kernel_nofault() to read from unsafe address

[Jinghao Jia]

On 3/16/24 08:46, Masami Hiramatsu (Google) wrote:
> On Thu, 14 Mar 2024 18:56:35 -0500
> Jinghao Jia <jinghao7@xxxxxxxxxxxx> wrote:
> 
>> On 3/14/24 10:17, Masami Hiramatsu (Google) wrote:
>>> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>>>
>>> Read from an unsafe address with copy_from_kernel_nofault() in
>>> arch_adjust_kprobe_addr() because this function is used before checking
>>> the address is in text or not. Syzcaller bot found a bug and reported
>>> the case if user specifies inaccessible data area,
>>> arch_adjust_kprobe_addr() will cause a kernel panic.
>>
>> IMHO there is a check on the address in kallsyms_lookup_size_offset to see
>> if it is a kernel text address before arch_adjust_kprobe_addr is invoked.
> 
> Yeah, kallsyms does not ensure the page (especially data) exists.
> 
>>
>> The call chain is:
>>
>> register_kprobe()
>>   _kprobe_addr()
>>     kallsyms_lookup_size_offset() <- check on addr is here
>>     arch_adjust_kprobe_addr()
>>
>> I wonder why this check was not able to capture the problem in this bug
>> report (I cannot reproduce it locally).
> 
> I could reproduce it locally, it tried to access 'Y' data.
> (I attached my .config) And I ensured that this fixed the problem.
> 
> The reproduce test actually tried to access initdata area
> 
> ffffffff82fb5450 d __alt_reloc_selftest_addr
> ffffffff82fb5460 d int3_exception_nb.1
> ffffffff82fb5478 d tsc_early_khz
> ffffffff82fb547c d io_delay_override
> ffffffff82fb5480 d fxregs.0
> ffffffff82fb5680 d y                    <--- access this
> ffffffff82fb5688 d x
> ffffffff82fb56a0 d xsave_cpuid_features
> ffffffff82fb56c8 d l1d_flush_mitigation
> 
> `y` is too generic, so check `io_delay_override` which is on the
> same page.
> 
> $ git grep io_delay_override
> arch/x86/kernel/io_delay.c:static int __initdata io_delay_override;
> 
> As you can see, it is marked as `__initdata`, and the initdata has been
> freed before starting /init.
> 
> ----
> [    2.679161] Freeing unused kernel image (initmem) memory: 2888K
> [    2.688731] Write protecting the kernel read-only data: 24576k
> [    2.691802] Freeing unused kernel image (rodata/data gap) memory: 1436K
> [    2.746994] x86/mm: Checked W+X mappings: passed, no W+X pages found.
> [    2.748022] x86/mm: Checking user space page tables
> [    2.789520] x86/mm: Checked W+X mappings: passed, no W+X pages found.
> [    2.790527] Run /init as init process
> ----
> 
> So this has been caused because accessing freed initdata.

Thanks a lot for the explanation! I have confirmed the bug and tested the
patch with CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT=y (which explicitly marks
the init pages as not-present after boot).

Tested-by: Jinghao Jia <jinghao7@xxxxxxxxxxxx>

--Jinghao

> 
> Thank you,
> 
>>
>> Thanks,
>> --Jinghao
>>
>>>
>>> Reported-by: Qiang Zhang <zzqq0103.hey@xxxxxxxxx>
>>> Closes: https://urldefense.com/v3/__https://lore.kernel.org/all/CAKHoSAs2rof6vQVBw_Lg_j3QNku0CANZR2qmy4eT7R5Lo8MFbg@xxxxxxxxxxxxxx/__;!!DZ3fjg!_C9Dn6-GBlkyS2z34bDUBsEXkTkgWp45MNrd4Rl5I5slz2A3SrurXOxKupsxLMqxC2BMiySCTfB2-5fPhkLP1g$ 
>>> Fixes: cc66bb914578 ("x86/ibt,kprobes: Cure sym+0 equals fentry woes")
>>> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>>> ---
>>>  Changes in v2:
>>>   - Fix to return NULL if failed to access the address.
>>> ---
>>>  arch/x86/kernel/kprobes/core.c |   11 ++++++++++-
>>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
>>> index a0ce46c0a2d8..95e4ebe5d514 100644
>>> --- a/arch/x86/kernel/kprobes/core.c
>>> +++ b/arch/x86/kernel/kprobes/core.c
>>> @@ -335,7 +335,16 @@ static int can_probe(unsigned long paddr)
>>>  kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
>>>  					 bool *on_func_entry)
>>>  {
>>> -	if (is_endbr(*(u32 *)addr)) {
>>> +	u32 insn;
>>> +
>>> +	/*
>>> +	 * Since addr is not guaranteed to be safely accessed yet, use
>>> +	 * copy_from_kernel_nofault() to get the instruction.
>>> +	 */
>>> +	if (copy_from_kernel_nofault(&insn, (void *)addr, sizeof(u32)))
>>> +		return NULL;
>>> +
>>> +	if (is_endbr(insn)) {
>>>  		*on_func_entry = !offset || offset == 4;
>>>  		if (*on_func_entry)
>>>  			offset = 4;
>>>
> 
>

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