Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

[Andy Lutomirski]

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. Itâs 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.
>> >
>> >   - Itâs 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