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 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.

Christophe