From: Julius Werner <firstname.lastname@example.org> To: email@example.com Cc: Julius Werner <firstname.lastname@example.org>, Greg Kroah-Hartman <email@example.com>, LKML <firstname.lastname@example.org>, Wei-Ning Huang <email@example.com>, Brian Norris <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive access Date: Thu, 9 Aug 2018 16:37:20 -0700 Message-ID: <CAODwPW84FYK3k5=n4gQ4EPJExhQ59LCEi31ZYi2Bq2y9FH_E_A@mail.gmail.com> (raw) In-Reply-To: <firstname.lastname@example.org> > Furthermore, I see that my system RAM excludes this coreboot table so it > doesn't fall into the bucket that CONFIG_STRICT_DEVMEM would find. Yes, that is intentional. We don't want the kernel to try to use that memory for anything else (since we want those tables to survive), so we mark them as reserved in the e820 map. > > (I guess an alternative would be to rewrite 'cbmem' to use > > /sys/bus/coreboot/devices if available to get its coreboot table > > information. But we'd still need to maintain the old path for > > backwards compatibility anyway, so that would really just make it more > > complicated.) > > This sounds like a good idea. Userspace reaching into /dev/mem is not > good from a kernel hardening perspective. That's why those strict devmem > configs exist. Can cbmem be updated to query information from device > drivers instead, so that we can enable CONFIG_IO_STRICT_DEVMEM as well? Well... problem is that cbmem doesn't just access the coreboot tables, it accesses more stuff. There is actually a larger memory region called CBMEM (that's what the utility is named after) which contains all sorts of random memory allocations that coreboot wanted to survive for the lifetime of the system. The coreboot table is one section in there, and it sort of serves as a directory for some of the others (although there's also just a general CBMEM directory... there's some redundancy there). But cbmem can also print some of the other CBMEM sections which it finds by querying the coreboot table, such as the firmware log or the boot timestamps. So the question is how we can get to that content if /dev/mem isn't available anymore. One option would be to just write separate kernel drivers to completely replace the cbmem utility (we already have one for the log, for example), but I think Linux generally doesn't want to have too much logic and parsing and stuff in kernel drivers. Another option is to add a driver that just exposes a sysfs file through which you could read (we don't need to write) the CBMEM area... but then we'd essentially want that to take absolute addresses because that's what the coreboot table pointers contain, so we would've just built /dev/mem by another name (for a restricted range). The nicest thing, really, would be if there was a way for a kernel driver to mark specific regions as "allowed" by /dev/mem. I don't suppose we'd be willing to introduce a mechanism like that?
next prev parent reply index Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-08-09 17:17 [PATCH v3 0/7] firmware: coreboot: Fix probe and simplify code Stephen Boyd 2018-08-09 17:17 ` [PATCH v3 1/7] firmware: coreboot: Let OF core populate platform device Stephen Boyd 2018-08-09 17:31 ` Brian Norris 2018-08-09 17:17 ` [PATCH v3 2/7] firmware: coreboot: Unmap ioregion on failure Stephen Boyd 2018-08-09 17:49 ` Brian Norris 2018-08-09 19:40 ` Stephen Boyd 2018-08-09 19:52 ` Brian Norris 2018-08-09 23:25 ` Stephen Boyd 2018-08-09 17:17 ` [PATCH v3 3/7] firmware: coreboot: Make bus registration symmetric Stephen Boyd 2018-08-09 18:10 ` Julius Werner 2018-08-09 23:30 ` Stephen Boyd 2018-08-09 17:17 ` [PATCH v3 4/7] firmware: coreboot: Collapse platform drivers into bus core Stephen Boyd 2018-08-09 17:17 ` [PATCH v3 5/7] firmware: coreboot: Remap RAM with memremap() instead of ioremap() Stephen Boyd 2018-08-09 18:24 ` Julius Werner 2018-08-09 22:07 ` Stephen Boyd 2018-08-09 17:17 ` [PATCH v3 6/7] firmware: coreboot: Only populate devices in coreboot_table_init() Stephen Boyd 2018-08-09 21:02 ` Julius Werner 2018-08-09 23:43 ` Stephen Boyd 2018-08-09 17:17 ` [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive access Stephen Boyd 2018-08-09 21:07 ` Julius Werner 2018-08-09 23:03 ` Stephen Boyd 2018-08-09 23:37 ` Julius Werner [this message] 2018-08-09 23:44 ` Julius Werner 2018-08-10 2:54 ` Stephen Boyd 2018-08-10 23:24 ` Stephen Boyd 2018-08-09 18:03 ` [PATCH v3 0/7] firmware: coreboot: Fix probe and simplify code Brian Norris
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