On Mon, Dec 26, 2016 at 6:24 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > On Mon, Dec 26, 2016 at 06:06:01PM -0800, Andy Lutomirski wrote: >> On Mon, Dec 26, 2016 at 5:54 PM, Kirill A. Shutemov >> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: >> > This patch introduces new rlimit resource to manage maximum virtual >> > address available to userspace to map. >> > >> > On x86, 5-level paging enables 56-bit userspace virtual address space. >> > Not all user space is ready to handle wide addresses. It's known that >> > at least some JIT compilers use high bit in pointers to encode their >> > information. It collides with valid pointers with 5-level paging and >> > leads to crashes. >> > >> > The patch aims to address this compatibility issue. >> > >> > MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual >> > address available to map by userspace. >> > >> > The default hard limit will be RLIM_INFINITY, which basically means that >> > TASK_SIZE limits available address space. >> > >> > The soft limit will also be RLIM_INFINITY everywhere, but the machine >> > with 5-level paging enabled. In this case, soft limit would be >> > (1UL << 47) - PAGE_SIZE. It’s current x86-64 TASK_SIZE_MAX with 4-level >> > paging which known to be safe >> > >> > New rlimit resource would follow usual semantics with regards to >> > inheritance: preserved on fork(2) and exec(2). This has potential to >> > break application if limits set too wide or too narrow, but this is not >> > uncommon for other resources (consider RLIMIT_DATA or RLIMIT_AS). >> > >> > As with other resources you can set the limit lower than current usage. >> > It would affect only future virtual address space allocations. >> > >> > Use-cases for new rlimit: >> > >> > - Bumping the soft limit to RLIM_INFINITY, allows current process all >> > its children to use addresses above 47-bits. >> > >> > - Bumping the soft limit to RLIM_INFINITY after fork(2), but before >> > exec(2) allows the child to use addresses above 47-bits. >> > >> > - Lowering the hard limit to 47-bits would prevent current process all >> > its children to use addresses above 47-bits, unless a process has >> > CAP_SYS_RESOURCES. >> > >> > - It’s also can be handy to lower hard or soft limit to arbitrary >> > address. User-mode emulation in QEMU may lower the limit to 32-bit >> > to emulate 32-bit machine on 64-bit host. >> >> I tend to think that this should be a personality or an ELF flag, not >> an rlimit. > > My plan was to implement ELF flag on top. Basically, ELF flag would mean > that we bump soft limit to hard limit on exec. > >> That way setuid works right. > > Um.. I probably miss background here. > If a setuid program depends on the lower limit, then a malicious program shouldn't be able to cause it to run with the higher limit. The personality code should already get this case right because personalities are reset when setuid happens. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
