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

[Dan Williams]

Kees Cook wrote:
> 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_:

Oh.

> 	/* 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.
> 	 */

Looks like in that case it is trying to grab DMI platform information
and otherwise fallback to a "generic" system which I would expect wreaks
havoc for many people.

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

For the TVM case the havoc of failing mmap for DMI info is smaller and
the recommended fallback for /dev/mem being in accessible is
/sys/firmware/dmi/tables. So I feel ok making TVMs take the modern
replacement path which is what they would need to do anyway in the
lockdown_kernel case. Tom, Dave, what do you think?

...but yes, let's skip taking away the /dev/mem backdoor for the bare
metal case.