Re: [PATCH] lkdtm: fix memory copy size for WRITE_KERN

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Jan 26, 2021 at 6:13 AM Candle Sun <candlesea@gmail.com> wrote:
>
> On Mon, Jan 25, 2021 at 6:37 PM David Laight <David.Laight@aculab.com> wrote:
> >
> > From: Candle Sun
> > > Sent: 25 January 2021 08:56
> > >
> > > From: Candle Sun <candle.sun@unisoc.com>
> > >
> > > Though do_overwritten() follows do_nothing() in source code, the final
> > > memory address order is determined by compiler. We can't always assume
> > > address of do_overwritten() is bigger than do_nothing(). At least the
> > > Clang we are using places do_overwritten() before do_nothing() in the
> > > object. This causes the copy size in lkdtm_WRITE_KERN() is *really*
> > > big and WRITE_KERN test on ARM32 arch will fail.
> > >
> > > Compare the address order before doing the subtraction.
> >
> > It isn't clear that helps.
> > Compile with -ffunction-sections and/or do LTO an there
> > is no reason at all why the functions should be together.
> >
> > Even without that lkdtm_WRITE_KERN() could easily be between them.
> >
> > You need to get the size of the 'empty function' from the
> > symbol table.
> >
> >         David
>
> Thanks David.
>
> I think using abs() by Nick's advice would be better. But could you
> point out which kernel function can get function size?

The Elf symbol table should contain this info, IIUC.

Given a string literal of a symbol (such as a function identifier),
kallsyms_lookup_name() can be used to return its address.

From there we'd want to fetch the Elf_Sym for the address which should
contain a st_size field which I think corresponds to the size in bytes
of the function.  (At least, from playing with `llvm-readelf -s`)
Probably would want to validate it's an STT_FUNC symbol type, too.  We
basically want something like kexec_purgatory_find_symbol(), but that
knows about the entire kernel image, and not the purgatory image used
during kexec.  I don't see any such function currently in the
kernel...but it's a large codebase to search through.

>
> Regards,
> Candle
>
>
> >
> > >
> > > Signed-off-by: Candle Sun <candle.sun@unisoc.com>
> > > ---
> > >  drivers/misc/lkdtm/perms.c | 19 +++++++++----------
> > >  1 file changed, 9 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > > index 2dede2ef658f..fbfbdf89d668 100644
> > > --- a/drivers/misc/lkdtm/perms.c
> > > +++ b/drivers/misc/lkdtm/perms.c
> > > @@ -31,13 +31,13 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
> > >   * This just returns to the caller. It is designed to be copied into
> > >   * non-executable memory regions.
> > >   */
> > > -static void do_nothing(void)
> > > +static noinline void do_nothing(void)
> > >  {
> > >       return;
> > >  }
> > >
> > >  /* Must immediately follow do_nothing for size calculuations to work out. */
> > > -static void do_overwritten(void)
> > > +static noinline void do_overwritten(void)
> > >  {
> > >       pr_info("do_overwritten wasn't overwritten!\n");
> > >       return;
> > > @@ -110,15 +110,14 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)
> > >
> > >  void lkdtm_WRITE_KERN(void)
> > >  {
> > > -     size_t size;
> > > -     volatile unsigned char *ptr;
> > > +     unsigned long value_dow = (unsigned long)do_overwritten;
> > > +     unsigned long value_do =  (unsigned long)do_nothing;
> > > +     size_t size = (size_t)(value_dow > value_do ?
> > > +                     value_dow - value_do : value_do - value_dow);
> > >
> > > -     size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
> > > -     ptr = (unsigned char *)do_overwritten;
> > > -
> > > -     pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> > > -     memcpy((void *)ptr, (unsigned char *)do_nothing, size);
> > > -     flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
> > > +     pr_info("attempting bad %zu byte write at %px\n", size, do_overwritten);
> > > +     memcpy((void *)value_dow, (void *)value_do, size);
> > > +     flush_icache_range(value_dow, value_dow + (unsigned long)size);
> > >       pr_err("FAIL: survived bad write\n");
> > >
> > >       do_overwritten();
> > > --
> > > 2.17.0
> >
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)
> >



-- 
Thanks,
~Nick Desaulniers