Re: [PATCH v2 2/3] devmem: Block mmap access when read/write access is restricted

[Kees Cook]

On Thu, Apr 10, 2025 at 06:22:30PM -0700, Dan Williams wrote:
> Back in 2022 Kees noted that he is able to mmap System RAM below 1MB
> even with CONFIG_STRICT_DEVMEM=y [1]. That is allowed for x86 legacy
> compatibility reasons for userspace that wants to read BIOS data
> resident at that address. However, the expectation is that when
> devmem_is_allowed() returns 2 that the access is redirected to return
> zeroes.
> 
> That happens for the read()/write() case, but by code inspection for
> mmap(), there is no restriction.
> 
> Now, the confidential x86 VM (CVM) use case wants to depend on
> "devmem_is_allowed() == 2" guaranteeing that no mapping to potentially
> encrypted memory is established [2]. The options to enable that are
> teach mmap_mem() to meet the "zeroed buffer" implication of
> devmem_is_allowed() returning "2", or return -EPERM for that case.
> 
> Return -EPERM on the hope that userspace does not actually depend on the
> legacy behavior of being able to reliably map the first 1MB of memory on
> x86. I.e. that all legacy cases are using read()/write() to safely read
> zeroes. If that turns out not to be true then either a "map zeroes"
> scheme can be added, or the CVM case can return 3 from
> devmem_is_allowed() to hide the CVM restriction from legacy
> environments.
> 
> Link: http://lore.kernel.org/CAPcyv4iVt=peUAk1qx_EfKn7aGJM=XwRUpJftBhkUgQEti2bJA@xxxxxxxxxxxxxx [1]
> Link: http://lore.kernel.org/fd683daa-d953-48ca-8c5d-6f4688ad442c@xxxxxxxxx [2]
> Suggested-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  include/linux/io.h |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/io.h b/include/linux/io.h
> index 0642c7ee41db..564934f7e70d 100644
> --- a/include/linux/io.h
> +++ b/include/linux/io.h
> @@ -191,7 +191,12 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
>  	u64 cursor = from;
>  
>  	while (cursor < to) {
> -		if (!devmem_is_allowed(pfn))
> +		/*
> +		 * Any restricted access is treated as "no access", i.e.
> +		 * handle devmem_is_allowed() returning "2" to indicate
> +		 * restricted access.
> +		 */
> +		if (devmem_is_allowed(pfn) != 1)
>  			return 0;
>  		cursor += PAGE_SIZE;
>  		pfn++;

Looking through the 16 page of Debian Code Search results for
`open("/dev/mem")`, I find a LOT of mmap() use. Some random examples:

https://sources.debian.org/src/i810switch/0.6.5-7.1/i810switch.c/?hl=413#L402

https://sources.debian.org/src/radeontop/1.4-2/detect.c/?hl=91#L88

https://sources.debian.org/src/libdebian-installer/0.125/src/system/subarch-x86-linux.c/?hl=113#L93
Which includes this gem of a comment, implying that it uses mmap
_specifically to bypass the devmem restrictions_:
	/* Please note that we don't use mmap() for performance reasons here,
	 * but to workaround problems many people encountered when trying
	 * to read from /dev/mem using regular read() calls.
	 */

I don't think we can just fail the mmap. :(

-- 
Kees Cook