[PATCH] ARM: uaccess: avoid alignment faults in copy_[from|to]_kernel_nofault

[Ard Biesheuvel]

On Tue, 18 Jan 2022 at 09:38, Arnd Bergmann <arnd at arndb.de> wrote:
>
> On Tue, Jan 18, 2022 at 9:28 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> >
> > The helpers that are used to implement copy_from_kernel_nofault() and
> > copy_to_kernel_nofault() cast a void* to a pointer to a wider type,
> > which may result in alignment faults on ARM if the compiler decides to
> > use double-word or multiple-word load/store instructions.
> >
> > So use the unaligned accessors where needed: when the type's size > 1
> > and the input was not aligned already by the caller.
> >
> > Cc: <stable at vger.kernel.org>
> > Fixes: 2df4c9a741a0 ("ARM: 9112/1: uaccess: add __{get,put}_kernel_nofault")
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
>
> Reviewed-by: Arnd Bergmann <arnd at arndb.de>
>
> It took me a bit to see whythis works, maybe mention commit 2423de2e6f4d
> ("ARM: 9115/1: mm/maccess: fix unaligned copy_{from,to}_kernel_nofault")
> in the description for clarification.
>

Ack.

> Did you run into actual faults, or did you find this problem by
> reading the code?
>

I was seeing actual faults:

[    4.447003]  copy_from_kernel_nofault from prepend+0x3c/0xb4
[    4.453085]  prepend from prepend_path+0x118/0x34c
[    4.457930]  prepend_path from d_path+0x11c/0x184
[    4.462656]  d_path from proc_pid_readlink+0xbc/0x1d4
[    4.467928]  proc_pid_readlink from vfs_readlink+0xfc/0x110
[    4.473740]  vfs_readlink from do_readlinkat+0xb0/0x110
[    4.479024]  do_readlinkat from ret_fast_syscall+0x0/0x54