> On 5 May 2017, at 15:55, Will Deacon <will.dea...@arm.com> wrote: > >> On Thu, May 04, 2017 at 07:40:50AM +0100, Ard Biesheuvel wrote: >>> On 3 May 2017 at 22:47, Goel, Sameer <sg...@codeaurora.org> wrote: >>> >>> >>>> On 5/3/2017 2:18 PM, Leif Lindholm wrote: >>>>> On Wed, May 03, 2017 at 11:07:45AM -0600, Goel, Sameer wrote: >>>>>> On 5/3/2017 5:26 AM, Will Deacon wrote: >>>>>> [adding some /dev/mem fans to cc] >>>>>> >>>>>>> On Tue, May 02, 2017 at 02:28:05PM -0600, Sameer Goel wrote: >>>>>>> Port architecture specific xlate and unxlate functions for /dev/mem >>>>>>> read/write. This sets up the mapping for a valid physical address if a >>>>>>> kernel direct mapping is not already present. >>>>>>> >>>>>>> This is a generic issue as a user space app should not be allowed to >>>>>>> crash >>>>>>> the kernel. >>>>>> >>>>>>> This issue was observed when systemd tried to access performance >>>>>>> pointer record from the FPDT table. >>>>>> >>>>>> Why is it doing that? Is there not a way to get this via /sys? >>>>> >>>>> There is no ACPI FPDT implementation in the kernel, so the userspace >>>>> systemd code is getting the FPDT table contents from /sys >>>>> and parsing the entries. The performance pointer record is a >>>>> reserved address populated by UEFI and the userspace code tries to >>>>> access it using /dev/mem. The physical address is valid, so cannot >>>>> push back on the user space code. >>>> >>>> OK, so then we need to add support for parsing this table in the >>>> kernel and exposing the referred-to regions in a controllable fashion. >>>> Maybe something that belongs under /sys/firmware/efi (adding Matt), or >>>> maybe something that deserves its own driver. >>>> >>>> The only two use-cases for /dev/mem on arm64 are: >>>> - Implementing interfaces in the kernel takes up-front effort. >>>> - Being able to accidentally panic the kernel from userland. >>>> >>> We will see this issue with any access using /dev/mem to a MEMBLOCK_NOMAP >>> marked >>> memblock. The kernel crash issue has to be fixed irrespective of ACPI FPDT >>> support. >>> >> >> I reported the same issue a couple of weeks ago [0]. So while we all >> agree that such accesses shouldn't oops the kernel, I think we may >> disagree on whether such accesses should be allowed in the first >> place, especially when using read/write on /dev/mem (as opposed to >> mmap()) > > Did you plan to respin those patches to address Alex's comments? I agree > that it would be good to close the oops, regardless of the rest of the > discussion here.
Agreed. I will look into this after my vacation (back on May 15th)
