On Tue, Apr 4, 2017 at 3:55 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, Apr 4, 2017 at 3:37 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> >> For one of my systems, I see something like this: >> >> 00000000-00000fff : reserved >> 00001000-0008efff : System RAM >> 0008f000-0008ffff : reserved >> 00090000-0009f7ff : System RAM >> 0009f800-0009ffff : reserved > > That's fairly normal. > >> I note that there are two "System RAM" areas below 0x100000. > > Yes. Traditionally the area from about 4k to 640kB is RAM. With a > random smattering of BIOS areas. > >> * On x86, access has to be given to the first megabyte of ram because that area >> * contains BIOS code and data regions used by X and dosemu and similar apps. > > Rigth. Traditionally, dosemu did one big mmap of the 1MB area to just > get all the BIOS data in one go. > >> This means that it allows reads into even System RAM below 0x100000, >> but I think that's a mistake. > > What you think is a "mistake" is how /dev/mem has always worked. > > /dev/mem gave access to all the memory of the system. That's LITERALLY > the whole point of it. There was no "BIOS area" or anything else. It > was access to physical memory. > > We've added limits to it, but those limits came later, and they came > with the caveat that lots of programs used /dev/mem in various ways. > > Nobody was crazy enough to read /dev/mem one byte at a time trying to > follow BIOS tables. No, the traditional way was to just map (or read) > large chunks of it, and then follow the tables in the result. The > easiest way was to just do the whole low 1MB. > > There's no "mistake" here. The only thing that is mistaken is you > thinking that we can redefine reality and change history. I'm not trying to rewrite history. :) I'm try to understand the requirements for how the 1MB area was used, which you've explained the history of now. (Thank you!) > I already explained what the likely fix is: make devmem_is_allowed() > return a ternary value, so that those things that *do* read the BIOS > area can just continue to do so, but they see zeroes for the parts > that the kernel has taken over. Sounds good to me. I'll go work on that. -Kees -- Kees Cook Pixel Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
