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

[Erhard Furtner]

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%7Cd75d39f3c9b04d5a3aef08dd3a5ea6e9%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638730900391403538%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=unzkFDX%2BfA1H%2F%2BIvbuBqFRH9ZJVN6vuJJkOegIDtHlw%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

Regards,
Erhard
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