Re: [PATCH] /dev/mem: Add a new parameter strict_devmem to bypass strict devmem

[David Hildenbrand]

On 20.11.24 13:28, Yafang Shao wrote:
> When CONFIG_STRICT_DEVMEM is enabled, writing to /dev/mem to override
> kernel data for debugging purposes is prohibited. This configuration is
> always enabled on our production servers. However, there are times when we
> need to use the crash utility to modify kernel data to analyze complex
> issues.
>
> As suggested by Ingo, we can add a boot time knob of soft-enabling it.
> Therefore, a new parameter "strict_devmem=" is added. The reuslt are as
> follows,
>
> - Before this change
>    crash> wr panic_on_oops 0
>    wr: cannot write to /proc/kcore      <<<< failed
>
> - After this change
>    - default
>      crash> wr panic_on_oops 0
>      wr: cannot write to /proc/kcore    <<<< failed
>
>    - strict_devmem=off
>      crash> p panic_on_oops
>      panic_on_oops = $1 = 1
>      crash> wr panic_on_oops 0
>      crash> p panic_on_oops
>      panic_on_oops = $2 = 0            <<<< succeeded
>
>    - strict_devmem=invalid
>      [    0.230052] Invalid option string for strict_devmem: 'invalid'
>      crash> wr panic_on_oops 0
>      wr: cannot write to /proc/kcore  <<<< failed
>
> Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> ---
>   .../admin-guide/kernel-parameters.txt         | 16 ++++++++++++++
>   drivers/char/mem.c                            | 21 +++++++++++++++++++
>   2 files changed, 37 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1518343bbe22..7fe0f66d0dfb 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6563,6 +6563,22 @@
>   			them frequently to increase the rate of SLB faults
>   			on kernel addresses.
>   
> +	strict_devmem=
> +			[KNL] Under CONFIG_STRICT_DEVMEM, whether strict devmem
> +			is enabled for this boot. Strict devmem checking is used
> +			to protect the userspace (root) access to all of memory,
> +			including kernel and userspace memory. Accidental access
> +			to this is obviously disastrous, but specific access can
> +			be used by people debugging the kernel. Note that with
> +			PAT support enabled, even in this case there are
> +			restrictions on /dev/mem use due to the cache aliasing
> +			requirements.
> +		on	If IO_STRICT_DEVMEM=n, the /dev/mem file only allows
> +			userspace access to PCI space and the BIOS code and data
> +			regions. This is sufficient for dosemu and X and all
> +			common users of /dev/mem. (default)
> +		off	Disable strict devmem checks.
> +
>   	sunrpc.min_resvport=
>   	sunrpc.max_resvport=
>   			[NFS,SUNRPC]

This will allow to violate EXCLUSIVE_SYSTEM_RAM, and I am afraid I don't 
enjoy seeing devmem handling+config getting more complicated.

--
Cheers,

David / dhildenb