On Sat, 3 Jun 2023, Demi Marie Obenour wrote:
> Otherwise subsequent code will dereference a misaligned
> `struct dm_target_spec *`, which is undefined behavior.
>
> Signed-off-by: Demi Marie Obenour <d...@invisiblethingslab.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: sta...@vger.kernel.org
> ---
> drivers/md/dm-ioctl.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index
> cc77cf3d410921432eb0c62cdede7d55b9aa674a..34fa74c6a70db8aa67aaba3f6a2fc4f38ef736bc
> 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1394,6 +1394,13 @@ static inline fmode_t get_mode(struct dm_ioctl *param)
> static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
> struct dm_target_spec **spec, char **target_params)
> {
> + static_assert(_Alignof(struct dm_target_spec) <= 8,
> + "struct dm_target_spec has excessive alignment
> requirements");
> + if (next % 8) {
> + DMERR("Next target spec (offset %u) is not 8-byte aligned",
> next);
> + return -EINVAL;
> + }
> +
> *spec = (struct dm_target_spec *) ((unsigned char *) last + next);
> *target_params = (char *) (*spec + 1);
>
> --
> Sincerely,
> Demi Marie Obenour (she/her/hers)
> Invisible Things Lab
Hi
Some architectures (such as 32-bit x86) specify that the alignment of
64-bit integers is only 4-byte. This could in theory break old userspace
code that only uses 4-byte alignment. I would change "next % 8" to "next %
__alignof__(struct dm_target_spec)".
I think that there is no need to backport this patch series to the stable
kernels because the bugs that it fixes may only be exploited by the user
with CAP_SYS_ADMIN privilege. So, there is no security or reliability
problem being fixed.
Mikulas
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