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Message-ID: <12453ce8-0b78-4c1c-9aca-de4cc366e3e1@illinois.edu>
Date: Sun, 17 Mar 2024 10:53:59 -0500
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v2] kprobes/x86: Use copy_from_kernel_nofault() to read
 from unsafe address
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Qiang Zhang <zzqq0103.hey@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H . Peter Anvin"
 <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org
References: <20240315000753.a448251fce0291e041f76c13@kernel.org>
 <171042945004.154897.2221804961882915806.stgit@devnote2>
 <fb6919c5-8acf-4ee3-8fd2-1d483b274867@illinois.edu>
 <20240316224630.01bd6b91938720f5083e0d07@kernel.org>
Content-Language: en-US
From: Jinghao Jia <jinghao7@illinois.edu>
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Content-Type: multipart/mixed; boundary="------------un0QyYflrBZMKRZTnFZ8EihV";
 protected-headers="v1"
From: Jinghao Jia <jinghao7@illinois.edu>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
 LKML <linux-kernel@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
 Qiang Zhang <zzqq0103.hey@gmail.com>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@linux.intel.com>, "H . Peter Anvin"
 <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>, x86@kernel.org
Message-ID: <12453ce8-0b78-4c1c-9aca-de4cc366e3e1@illinois.edu>
Subject: Re: [PATCH v2] kprobes/x86: Use copy_from_kernel_nofault() to read
 from unsafe address
References: <20240315000753.a448251fce0291e041f76c13@kernel.org>
 <171042945004.154897.2221804961882915806.stgit@devnote2>
 <fb6919c5-8acf-4ee3-8fd2-1d483b274867@illinois.edu>
 <20240316224630.01bd6b91938720f5083e0d07@kernel.org>
In-Reply-To: <20240316224630.01bd6b91938720f5083e0d07@kernel.org>

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On 3/16/24 08:46, Masami Hiramatsu (Google) wrote:
> On Thu, 14 Mar 2024 18:56:35 -0500
> Jinghao Jia <jinghao7@illinois.edu> wrote:
>=20
>> On 3/14/24 10:17, Masami Hiramatsu (Google) wrote:
>>> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>>>
>>> Read from an unsafe address with copy_from_kernel_nofault() in
>>> arch_adjust_kprobe_addr() because this function is used before checki=
ng
>>> 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 invok=
ed.
>=20
> Yeah, kallsyms does not ensure the page (especially data) exists.
>=20
>>
>> 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 bu=
g
>> report (I cannot reproduce it locally).
>=20
> I could reproduce it locally, it tried to access 'Y' data.
> (I attached my .config) And I ensured that this fixed the problem.
>=20
> The reproduce test actually tried to access initdata area
>=20
> 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
>=20
> `y` is too generic, so check `io_delay_override` which is on the
> same page.
>=20
> $ git grep io_delay_override
> arch/x86/kernel/io_delay.c:static int __initdata io_delay_override;
>=20
> As you can see, it is marked as `__initdata`, and the initdata has been=

> freed before starting /init.
>=20
> ----
> [    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: 14=
36K
> [    2.746994] x86/mm: Checked W+X mappings: passed, no W+X pages found=
=2E
> [    2.748022] x86/mm: Checking user space page tables
> [    2.789520] x86/mm: Checked W+X mappings: passed, no W+X pages found=
=2E
> [    2.790527] Run /init as init process
> ----
>=20
> 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=3Dy (which explicitly ma=
rks
the init pages as not-present after boot).

Tested-by: Jinghao Jia <jinghao7@illinois.edu>

--Jinghao

>=20
> Thank you,
>=20
>>
>> Thanks,
>> --Jinghao
>>
>>>
>>> Reported-by: Qiang Zhang <zzqq0103.hey@gmail.com>
>>> Closes: https://urldefense.com/v3/__https://lore.kernel.org/all/CAKHo=
SAs2rof6vQVBw_Lg_j3QNku0CANZR2qmy4eT7R5Lo8MFbg@mail.gmail.com/__;!!DZ3fjg=
!_C9Dn6-GBlkyS2z34bDUBsEXkTkgWp45MNrd4Rl5I5slz2A3SrurXOxKupsxLMqxC2BMiySC=
TfB2-5fPhkLP1g$=20
>>> Fixes: cc66bb914578 ("x86/ibt,kprobes: Cure sym+0 equals fentry woes"=
)
>>> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>>> ---
>>>  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, unsigne=
d 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 =3D !offset || offset =3D=3D 4;
>>>  		if (*on_func_entry)
>>>  			offset =3D 4;
>>>
>=20
>=20

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Lrc=
=zLH+
-----END PGP SIGNATURE-----

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