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Date: Tue, 7 Nov 2023 15:25:01 -0800
From: Kees Cook <keescook@...omium.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Alexander Potapenko <glider@...gle.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Yury Norov <yury.norov@...il.com>,
	Syed Nayyar Waris <syednwaris@...il.com>,
	kernel test robot <lkp@...el.com>, oe-kbuild-all@...ts.linux.dev,
	linux-hardening@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [alobakin:pfcp 11/19] include/linux/bitmap.h:642:17: warning:
 array subscript [1, 1024] is outside array bounds of 'long unsigned int[1]'

On Tue, Nov 07, 2023 at 05:44:00PM +0100, Alexander Lobakin wrote:
> From: Alexander Potapenko <glider@...gle.com>
> Date: Tue, 7 Nov 2023 17:33:56 +0100
> 
> > On Tue, Nov 7, 2023 at 2:23 PM Alexander Lobakin
> > <aleksander.lobakin@...el.com> wrote:
> >>
> >> From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> >> Date: Mon, 6 Nov 2023 20:23:52 +0200
> >>
> >>> On Mon, Nov 06, 2023 at 05:31:34PM +0100, Alexander Lobakin wrote:
> >>>
> >>>>> | Reported-by: kernel test robot <lkp@...el.com>
> >>>>> | Closes: https://lore.kernel.org/oe-kbuild-all/202310170708.fJzLlgDM-lkp@intel.com/
> >>>
> >>>> Not sure how to approach this :z It was also captured on the version you
> >>>> sent 2 weeks ago, so this could've been resolved already.
> >>>
> >>> Is it in the repository already? if so, we should revert it.
> >>> Otherwise you have time to think and fix.
> >>
> >> Nah, neither Alex' series nor mine. And I'd say this should rather be
> >> resolved in the functions Alex introduce.
> >>
> >> Thanks,
> >> Olek
> > 
> > Sorry, I couldn't reproduce the problem using the instructions at
> > https://download.01.org/0day-ci/archive/20231017/202310170708.fJzLlgDM-lkp@intel.com/reproduce
> > locally, maybe that's because I only have gcc-11 and higher.
> > 
> > But if I'm understanding correctly what's going on, then GCC will be
> > reporting the same issue in the following code:
> > 
> > =======================================================
> > #include <stddef.h>
> > #include <stdbool.h>
> > 
> > #define BITS_PER_LONG 64
> > #define unlikely(x) x
> > #define UL(x) (x##UL)
> > #define GENMASK(h, l) \
> >         (((~UL(0)) - (UL(1) << (l)) + 1) & \
> >          (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
> > 
> > #define BIT_WORD(nr)            ((nr) / BITS_PER_LONG)
> > #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
> > #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
> > 
> > inline void bitmap_write(unsigned long *map,
> >                                 unsigned long value,
> >                                 unsigned long start, unsigned long nbits)
> > {
> >         size_t index;
> >         unsigned long offset;
> >         unsigned long space;
> >         unsigned long mask;
> >         bool fit;
> > 
> >         if (unlikely(!nbits))
> >                 return;
> > 
> >         mask = BITMAP_LAST_WORD_MASK(nbits);
> >         value &= mask;
> >         offset = start % BITS_PER_LONG;
> >         space = BITS_PER_LONG - offset;
> >         fit = space >= nbits;
> >         index = BIT_WORD(start);
> > 
> >         map[index] &= (fit ? (~(mask << offset)) :
> > ~BITMAP_FIRST_WORD_MASK(start));
> >         map[index] |= value << offset;
> >         if (fit)
> >                 return;
> > 
> >         map[index + 1] &= BITMAP_FIRST_WORD_MASK(start + nbits);
> >         map[index + 1] |= (value >> space);
> > }
> > 
> > unsigned long foo(unsigned int n) {
> >     unsigned long bm[1] = {0};
> >     bitmap_write(bm, 1, n, 2);
> >     return bm[0];
> > }
> > =======================================================
> > (see also https://godbolt.org/z/GfGfYje53)
> > 
> > If so, the problem is not specific to GCC 9, trunk GCC also barks on this code:
> > 
> > =======================================================
> > In function 'bitmap_write',
> >     inlined from 'bitmap_write' at <source>:15:13,
> >     inlined from 'foo' at <source>:47:7:
> > <source>:40:12: warning: array subscript 1 is outside array bounds of
> > 'long unsigned int[1]' [-Warray-bounds=]
> >    40 |         map[index + 1] &= BITMAP_FIRST_WORD_MASK(start + nbits);
> >       |         ~~~^~~~~~~~~~~
> > =======================================================
> > 
> > If this is true for the code in drivers/gpio/gpio-pca953x.c,
> > suppressing the report for GCC 9 won't help for other versions.
> > Given that this report is isolated in a single file, we probably need
> 
> I tested it on GCC 9 using modified make.cross from lkp and it triggers
> on one more file:
> 
> drivers/thermal/intel/intel_soc_dts_iosf.c: In function 'sys_get_curr_temp':
> ./include/linux/bitmap.h:601:18: error: array subscript [1,
> 288230376151711744] is outside array bounds of 'long unsigned int[1]'
> [-Werror=array-bounds]
> 
> > to give the compiler some hints about the range of values passed to
> > bitmap_write() rather than suppressing the optimizations.
> 
> OPTIMIZER_HIDE_VAR() doesn't disable optimizations if I get it
> correctly, rather shuts up the compiler in cases like this one.
> 
> I've been thinking of using __member_size() from fortify-string.h, we
> could probably optimize the object code even a bit more while silencing
> this warning.
> Adding Kees, maybe he'd like to participate in sorting this out as well.

I'm trying to find all the pieces for this code, so I might be working
from the wrong version or something, but I think this is the change:
https://github.com/alobakin/linux/commit/66808fb20fed014a522b868322d54daef14a6bd8

and the induced warning is correctly analyzed in this thread (i.e. GCC
can't convince itself that it'll never reach the out of bounds access).
Does this work?

-	if (fit)
+	if (fit || index + 1 >= __member_size(map))
 		return;


-- 
Kees Cook

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