linux-kernel - Re: Linux 6.2-rc1

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Date:   Tue, 27 Dec 2022 19:40:30 -0800
From:   Kees Cook <kees@...nel.org>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: Linux 6.2-rc1

On December 26, 2022 9:52:12 PM PST, Guenter Roeck <linux@...ck-us.net> wrote:
>On Mon, Dec 26, 2022 at 05:32:28PM -0800, Kees Cook wrote:
>> On December 26, 2022 4:29:41 PM PST, Guenter Roeck <linux@...ck-us.net> wrote:
>> >On Mon, Dec 26, 2022 at 01:03:59PM -0800, Kees Cook wrote:
>> >> On December 26, 2022 12:56:29 PM PST, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> >> >On Mon, Dec 26, 2022 at 11:52 AM Guenter Roeck <linux@...ck-us.net> wrote:
>> >> >>
>> >> >> fs/f2fs/inline.c: In function 'f2fs_move_inline_dirents':
>> >> >> include/linux/fortify-string.h:59:33: error: '__builtin_memset' pointer overflow between offset [28, 898293814] and size [-898293787, -1] [-Werror=array-bounds]
>> >> >> fs/f2fs/inline.c:430:9: note: in expansion of macro 'memset'
>> >> >>   430 |         memset(dst.bitmap + src.nr_bitmap, 0, dst.nr_bitmap - src.nr_bitmap);
>> >> >>       |         ^~~~~~
>> >> >
>> >> >Well, that's unfortunate.
>> >> 
>> >> I'll look into this.
>> >> 
>> >
>> >I did some more testing. The problem is seen with gcc 11.3.0, but not with
>> >gcc 12.2.0 nor with gcc 10.3.0.
>> 
>> That's what I'd expect: 10 didn't have variable range tracking wired up to -Warray-bounds, 11 does, and we disable -Warray-bounds on 12 because of 3 separate 12-only GCC bugs.
>> 
>> > gcc bug ? Should I switch to gcc 12.2.0 for
>> >powerpc when build testing the latest kernel ?
>> 
>> Sure? But that'll just hide it. I suspect GCC has found a way for dst.nr_bitmap to be compile-time 27, so the size is always negative.
>> 
>dst.nr_bitmap is initialized with SIZE_OF_DENTRY_BITMAP,
>which is defined as:
>
>#define NR_DENTRY_IN_BLOCK      214     /* the number of dentry in a block */
>#define SIZE_OF_DIR_ENTRY       11      /* by byte */
>#define SIZE_OF_DENTRY_BITMAP   ((NR_DENTRY_IN_BLOCK + BITS_PER_BYTE - 1) / \
>                                        BITS_PER_BYTE)
>
>((214 + 8 - 1) / 8 = 27, so dst.nr_bitmap is indeed compile-time 27.
>
>Not sure how would know that src.nr_bitmap can be > 27, though.
>Am I missing something ?

I think it's saying it can't rule out it being larger? I.e. there is no obvious bounds checking for it. Perhaps:

if (src.nr_bitmap > dst.nr_bitmap) {
    err = -EFSCORRUPTED;
		goto out;
}


-Kees


-- 
Kees Cook