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

[Google]

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.

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;
> > 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Attachment: .config
Description: Binary data