BUG: KASAN: vmalloc-out-of-bounds in copy_to_kernel_nofault+0xd8/0x1c8 (v6.13-rc6, PowerMac G4)

[Christophe Leroy]

Le 22/01/2025 à 16:32, Christophe Leroy a écrit :
> 
> 
> Le 22/01/2025 à 00:21, Erhard Furtner a écrit :
>> On Tue, 21 Jan 2025 23:07:25 +0100
>> Christophe Leroy <christophe.leroy at csgroup.eu> wrote:
>>
>>>> Meanwhile I bisected the bug. Offending commit is:
>>>>
>>>>    # git bisect good
>>>> 32913f348229c9f72dda45fc2c08c6d9dfcd3d6d is the first bad commit
>>>> commit 32913f348229c9f72dda45fc2c08c6d9dfcd3d6d
>>>> Author: Linus Torvalds <torvalds at linux-foundation.org>
>>>> Date:   Mon Dec 9 10:00:25 2024 -0800
>>>>
>>>>       futex: fix user access on powerpc
>>>>       The powerpc user access code is special, and unlike other 
>>>> architectures
>>>>       distinguishes between user access for reading and writing.
>>>>       And commit 43a43faf5376 ("futex: improve user space accesses") 
>>>> messed
>>>>       that up.  It went undetected elsewhere, but caused ppc32 to 
>>>> fail early
>>>>       during boot, because the user access had been started with
>>>>       user_read_access_begin(), but then finished off with just a plain
>>>>       "user_access_end()".
>>>>       Note that the address-masking user access helpers don't even 
>>>> have that
>>>>       read-vs-write distinction, so if powerpc ever wants to do address
>>>>       masking tricks, we'll have to do some extra work for it.
>>>>       [ Make sure to also do it for the EFAULT case, as pointed out by
>>>>         Christophe Leroy ]
>>>>       Reported-by: Andreas Schwab <schwab at linux-m68k.org>
>>>>       Cc: Christophe Leroy <christophe.leroy at csgroup.eu>
>>>>       Link: https://eur01.safelinks.protection.outlook.com/? 
>>>> url=https%3A%2F%2Flore.kernel.org%2Fall%2F87bjxl6b0i.fsf%40igel.home%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7Cb4c1dc7184f54a410a0e08dd3a7270b6%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638730985407902881%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=E5Yp9jopCPE1NFuBM8rs%2B1jXZ%2FXAaKvBGpcEP%2BaMyz0%3D&reserved=0
>>>>       Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
>>>>
>>>>    kernel/futex/futex.h | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>>
>>>> Indeed, reverting 32913f348229c9f72dda45fc2c08c6d9dfcd3d6d on top of 
>>>> v6.13 makes the KASAN hit disappear.
>>>
>>> That looks terribly odd.
>>>
>>> On G4, user_read_access_begin() and user_read_access_end() are no-op
>>> because book3s/32 can only protect user access by kernel against write.
>>> Read is always granted.
>>>
>>> So the bug must be an indirect side effect of what user_access_end()
>>> does. user_access_end() does a sync. Would the lack of sync (once
>>> replaced user_access_end() by user_read_access_end() ) lead to some odd
>>> re-ordering ? Or another possibility is that user_access_end() is called
>>> on some kernel address (I see in the description of commit 43a43faf5376
>>> ("futex: improve user space accesses") that the replaced __get_user()
>>> was expected to work on kernel adresses) ? Calling user_access_begin()
>>> and user_access_end() is unexpected and there is no guard so it could
>>> lead to strange segment settings which hides a KASAN hit. But once the
>>> fix the issue the KASAN resurfaces ? Could this be the problem ?
>>>
>>> Do you have a way to reproduce the bug on QEMU ? It would enable me to
>>> investigate it further.
>>
>> Attached v6.13 .config plays nicely with qemu ttyS0 (forgot to disable 
>> SERIAL_8250 and set SERIAL_PMACZILOG + SERIAL_PMACZILOG_CONSOLE 
>> instead as I prefer the PCI Serial card in my G4).
>>
>> The KASAN hit also shows up on qemu 8.2.7 via via:
>> qemu-system-ppc -machine mac99,via=pmu -cpu 7450 -m 2G -nographic - 
>> append console=ttyS0 -kernel vmlinux-6.13.0-PMacG4 -hda Debian-VM_g4.img
>>
> 
> I was able to reproduce it with v6.13 with QEMU when loading test_bpf 
> module.
> 
> On my side, the problem doesn't disappear when reverting of commit 
> 32913f348229 ("futex: fix user access on powerpc")
> 
> I bisected it to commit e4137f08816b ("mm, kasan, kmsan: instrument 
> copy_from/to_kernel_nofault"), which makes a lot more sense to me.
> 
> It might be a problem in the way patch_instruction() is implemented on 
> powerpc, to be investigated.

I think the problem is commit 37bc3e5fd764 ("powerpc/lib/code-patching: 
Use alternate map for patch_instruction()")

Can you try the change below:

diff --git a/arch/powerpc/lib/code-patching.c 
b/arch/powerpc/lib/code-patching.c
index af97fbb3c257..8a378fc19074 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -108,7 +108,7 @@ static int text_area_cpu_up(unsigned int cpu)
  	unsigned long addr;
  	int err;

-	area = get_vm_area(PAGE_SIZE, VM_ALLOC);
+	area = get_vm_area(PAGE_SIZE, 0);
  	if (!area) {
  		WARN_ONCE(1, "Failed to create text area for cpu %d\n",
  			cpu);