Re: [PATCH 2/2] mm: make copy_to_kernel_nofault() not fault on user addresses

[Omar Sandoval <osandov@osandov.com>]

On Wed, Sep 04, 2024 at 09:50:56AM +0200, Christophe Leroy wrote:
> Hi,
> 
> Le 02/09/2024 à 07:31, Omar Sandoval a écrit :
> > [Vous ne recevez pas souvent de courriers de osandov@osandov.com. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
> > 
> > From: Omar Sandoval <osandov@fb.com>
> > 
> > I found that on x86, copy_to_kernel_nofault() still faults on addresses
> > outside of the kernel address range (including NULL):
> > 
> >    # echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
> >    # echo g > /proc/sysrq-trigger
> >    ...
> >    [15]kdb> mm 0 1234
> >    [   94.652476] BUG: kernel NULL pointer dereference, address: 0000000000000000
> ...
> > 
> > Note that copy_to_kernel_nofault() uses pagefault_disable(), but it
> > still faults. This is because with Supervisor Mode Access Prevention
> > (SMAP) enabled, do_user_addr_fault() Oopses on a fault for a user
> > address from kernel space _before_ checking faulthandler_disabled().
> > 
> > copy_from_kernel_nofault() avoids this by checking that the address is
> > in the kernel before doing the actual memory access. Do the same in
> > copy_to_kernel_nofault() so that we get an error as expected:
> > 
> >    # echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
> >    # echo g > /proc/sysrq-trigger
> >    ...
> >    [17]kdb> mm 0 1234
> >    kdb_putarea_size: Bad address 0x0
> >    diag: -21: Invalid address
> > 
> > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > ---
> >   mm/maccess.c | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/maccess.c b/mm/maccess.c
> > index 72e9c03ea37f..d67dee51a1cc 100644
> > --- a/mm/maccess.c
> > +++ b/mm/maccess.c
> > @@ -61,6 +61,9 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
> >          if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
> >                  align = (unsigned long)dst | (unsigned long)src;
> > 
> > +       if (!copy_kernel_nofault_allowed(dst, size))
> > +               return -ERANGE;
> > +
> >          pagefault_disable();
> >          if (!(align & 7))
> >                  copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
> > --
> > 2.46.0
> > 
> 
> This patch leads to the following errors on ppc64le_defconfig:
> 
> [    2.423930][    T1] Running code patching self-tests ...
> [    2.428912][    T1] code-patching: test failed at line 395
> [    2.429085][    T1] code-patching: test failed at line 398
> [    2.429561][    T1] code-patching: test failed at line 432
> [    2.429679][    T1] code-patching: test failed at line 435
> 
> This seems to be linked to commit c28c15b6d28a ("powerpc/code-patching: Use
> temporary mm for Radix MMU"), copy_from_kernel_nofault_allowed() returns
> false for the patching area.

Thanks for testing. This patch isn't worth the trouble, so we can drop
it.

Thanks,
Omar