Re: [PATCH 08/14] taint: add taint for direct hardware access

Dan Williams
2021-02-08T22:01:48Z

[ add Jon Corbet as I'd expect him to be Cc'd on anything that
generically touches Documentation/ like this, and add Kees as the last
person who added a taint (tag you're it) ]

Jon, Kees, are either of you willing to ack this concept?

Top-posting to add more context for the below:

This taint is proposed because it has implications for
CONFIG_LOCK_DOWN_KERNEL among other things. These CXL devices
implement memory like DDR would, but unlike DDR there are
administrative / configuration commands that demand kernel
coordination before they can be sent. The posture taken with this
taint is "guilty until proven innocent" for commands that have yet to
be explicitly allowed by the driver. This is different than NVME for
example where an errant vendor-defined command could destroy data on
the device, but there is no wider threat to system integrity. The
taint allows a pressure release valve for any and all commands to be
sent, but flagged with WARN_TAINT_ONCE if the driver has not
explicitly enabled it on an allowed list of known-good / kernel
coordinated commands.

On Fri, Jan 29, 2021 at 4:25 PM Ben Widawsky <ben.widawsky@intel.com> wrote:
>
> For drivers that moderate access to the underlying hardware it is
> sometimes desirable to allow userspace to bypass restrictions. Once
> userspace has done this, the driver can no longer guarantee the sanctity
> of either the OS or the hardware. When in this state, it is helpful for
> kernel developers to be made aware (via this taint flag) of this fact
> for subsequent bug reports.
>
> Example usage:
> - Hardware xyzzy accepts 2 commands, waldo and fred.
> - The xyzzy driver provides an interface for using waldo, but not fred.
> - quux is convinced they really need the fred command.
> - xyzzy driver allows quux to frob hardware to initiate fred.
> - kernel gets tainted.
> - turns out fred command is borked, and scribbles over memory.
> - developers laugh while closing quux's subsequent bug report.
>
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> ---
> Documentation/admin-guide/sysctl/kernel.rst | 1 +
> Documentation/admin-guide/tainted-kernels.rst | 6 +++++-
> include/linux/kernel.h | 3 ++-
> kernel/panic.c | 1 +
> 4 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 1d56a6b73a4e..3e1eada53504 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -1352,6 +1352,7 @@ ORed together. The letters are seen in "Tainted" line of Oops reports.
> 32768 `(K)` kernel has been live patched
> 65536 `(X)` Auxiliary taint, defined and used by for distros
> 131072 `(T)` The kernel was built with the struct randomization plugin
> +262144 `(H)` The kernel has allowed vendor shenanigans
> ====== ===== ==============================================================
>
> See :doc:`/admin-guide/tainted-kernels` for more information.
> diff --git a/Documentation/admin-guide/tainted-kernels.rst b/Documentation/admin-guide/tainted-kernels.rst
> index ceeed7b0798d..ee2913316344 100644
> --- a/Documentation/admin-guide/tainted-kernels.rst
> +++ b/Documentation/admin-guide/tainted-kernels.rst
> @@ -74,7 +74,7 @@ a particular type of taint. It's best to leave that to the aforementioned
> script, but if you need something quick you can use this shell command to check
> which bits are set::
>
> - $ for i in $(seq 18); do echo $(($i-1)) $(($(cat /proc/sys/kernel/tainted)>>($i-1)&1));done
> + $ for i in $(seq 19); do echo $(($i-1)) $(($(cat /proc/sys/kernel/tainted)>>($i-1)&1));done
>
> Table for decoding tainted state
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> @@ -100,6 +100,7 @@ Bit Log Number Reason that got the kernel tainted
> 15 _/K 32768 kernel has been live patched
> 16 _/X 65536 auxiliary taint, defined for and used by distros
> 17 _/T 131072 kernel was built with the struct randomization plugin
> + 18 _/H 262144 kernel has allowed vendor shenanigans
> === === ====== ========================================================
>
> Note: The character ``_`` is representing a blank in this table to make reading
> @@ -175,3 +176,6 @@ More detailed explanation for tainting
> produce extremely unusual kernel structure layouts (even performance
> pathological ones), which is important to know when debugging. Set at
> build time.
> +
> + 18) ``H`` Kernel has allowed direct access to hardware and can no longer make
> + any guarantees about the stability of the device or driver.
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index f7902d8c1048..bc95486f817e 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -443,7 +443,8 @@ extern enum system_states {
> #define TAINT_LIVEPATCH 15
> #define TAINT_AUX 16
> #define TAINT_RANDSTRUCT 17
> -#define TAINT_FLAGS_COUNT 18
> +#define TAINT_RAW_PASSTHROUGH 18
> +#define TAINT_FLAGS_COUNT 19
> #define TAINT_FLAGS_MAX ((1UL << TAINT_FLAGS_COUNT) - 1)
>
> struct taint_flag {
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 332736a72a58..dff22bd80eaf 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -386,6 +386,7 @@ const struct taint_flag taint_flags[TAINT_FLAGS_COUNT] = {
> [ TAINT_LIVEPATCH ] = { 'K', ' ', true },
> [ TAINT_AUX ] = { 'X', ' ', true },
> [ TAINT_RANDSTRUCT ] = { 'T', ' ', true },
> + [ TAINT_RAW_PASSTHROUGH ] = { 'H', ' ', true },
> };
>
> /**
> --
> 2.30.0
>