Re: [PATCH 1/2] ext4: fix warning about stack corruption

[Kees Cook]

On Tue, Aug 1, 2017 at 5:04 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> After commit 62d1034f53e3 ("fortify: use WARN instead of BUG for now"),
> we get a warning about possible stack overflow from a memcpy that
> was not strictly bounded to the size of the local variable:
>
>     inlined from 'ext4_mb_seq_groups_show' at fs/ext4/mballoc.c:2322:2:
> include/linux/string.h:309:9: error: '__builtin_memcpy': writing between 161 and 1116 bytes into a region of size 160 overflows the destination [-Werror=stringop-overflow=]
>
> We actually had a bug here that would have been found by the warning,
> but it was already fixed last year in commit 30a9d7afe70e ("ext4: fix
> stack memory corruption with 64k block size").
>
> This replaces the fixed-length structure on the stack with a variable-length
> structure, using the correct upper bound that tells the compiler that
> everything is really fine here. I also change the loop count to check
> for the same upper bound for consistency, but the existing code is
> already correct here.
>
> Note that while clang won't allow certain kinds of variable-length arrays
> in structures, this particular instance is fine, as the array is at the
> end of the structure, and the size is strictly bounded.
>
> There is one remaining issue with the function that I'm not addressing
> here: With s_blocksize_bits==16, we don't actually print the last two
> members of the array, as we loop though just the first 14 members.
> This could be easily addressed by adding two extra columns in the output,
> but that could in theory break parsers in user space, and should be
> a separate patch if we decide to modify it.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  fs/ext4/mballoc.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 581e357e8406..803cab1939fe 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2295,9 +2295,12 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
>         int err, buddy_loaded = 0;
>         struct ext4_buddy e4b;
>         struct ext4_group_info *grinfo;
> +       unsigned char blocksize_bits = min_t(unsigned char,
> +                                            sb->s_blocksize_bits,
> +                                            EXT4_MAX_BLOCK_LOG_SIZE);
>         struct sg {
>                 struct ext4_group_info info;
> -               ext4_grpblk_t counters[EXT4_MAX_BLOCK_LOG_SIZE + 2];
> +               ext4_grpblk_t counters[blocksize_bits + 2];
>         } sg;
>
>         group--;
> @@ -2306,8 +2309,6 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
>                               " 2^0   2^1   2^2   2^3   2^4   2^5   2^6  "
>                               " 2^7   2^8   2^9   2^10  2^11  2^12  2^13  ]\n");
>
> -       i = (sb->s_blocksize_bits + 2) * sizeof(sg.info.bb_counters[0]) +
> -               sizeof(struct ext4_group_info);
>         grinfo = ext4_get_group_info(sb, group);
>         /* Load the group info in memory only if not already loaded. */
>         if (unlikely(EXT4_MB_GRP_NEED_INIT(grinfo))) {
> @@ -2319,7 +2320,7 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
>                 buddy_loaded = 1;
>         }
>
> -       memcpy(&sg, ext4_get_group_info(sb, group), i);
> +       memcpy(&sg, ext4_get_group_info(sb, group), sizeof(sg));
>
>         if (buddy_loaded)
>                 ext4_mb_unload_buddy(&e4b);
> @@ -2327,7 +2328,7 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
>         seq_printf(seq, "#%-5u: %-5u %-5u %-5u [", group, sg.info.bb_free,
>                         sg.info.bb_fragments, sg.info.bb_first_free);
>         for (i = 0; i <= 13; i++)
> -               seq_printf(seq, " %-5u", i <= sb->s_blocksize_bits + 1 ?
> +               seq_printf(seq, " %-5u", i <= blocksize_bits + 1 ?
>                                 sg.info.bb_counters[i] : 0);
>         seq_printf(seq, " ]\n");
>
> --
> 2.9.0
>



-- 
Kees Cook
Pixel Security