Re: [PATCH] x86_64: uninline TASK_SIZE

Andy Lutomirski
2019-04-23T17:22:14Z

On Tue, Apr 23, 2019 at 4:13 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Andy Lutomirski <luto@kernel.org> wrote:
>
> > > Saving 2KB on a defconfig is quite a lot.
> >
> > Saving 2kB of text by adding 8 bytes to thread_info seems rather
> > dubious to me. You only need 256 tasks before you lose. My
> > not-particularly-loaded laptop has 865 tasks right now.
>
> I was suggesting current->task_size or thread_info->task_size as a way to
> 100% avoid the function call overhead. Worth a tiny amount of RAM - even
> with 1 million tasks it's only 4MB of RAM. ;-)
>
> Some TASK_SIZE users are prominent syscalls: mmap(),
>
> > As a general principle, the mere existence of TIF_ADDR32 is a bug. The
> > value of that flag is *wrong* under the 32-bit variant of CRIU. How
> > about instead making some more progress toward getting rid of dubious
> > TASK_SIZE users? I'm working on a little series to get rid of most of
> > them. Meanwhile: it sure looks like a large fraction of the users are
> > confused as to whether TASK_SIZE is the highest user address or the
> > lowest non-user address.
>
> I really like that, replacing TASK_SIZE with *nothing* would be even
> faster.
>
> In fact instead of just reducing its usage I'd suggest removing TASK_SIZE
> altogether and renaming TASK_SIZE_MAX back to TASK_SIZE, or something
> like that - the confusion from the deceptively macro-ish naming of
> TASK_SIZE is real.
>
> The original commit of making TASK_SIZE dynamic on the task's compat flag
> was done in:
>
> 84929801e14d: [PATCH] x86_64: TASK_SIZE fixes for compatibility mode processes
>
> Here's the justification given:
>
> Appended patch will setup compatibility mode TASK_SIZE properly. This will
> fix atleast three known bugs that can be encountered while running
> compatibility mode apps.
>
> a) A malicious 32bit app can have an elf section at 0xffffe000. During
> exec of this app, we will have a memory leak as insert_vm_struct() is
> not checking for return value in syscall32_setup_pages() and thus not
> freeing the vma allocated for the vsyscall page. And instead of exec
> failing (as it has addresses > TASK_SIZE), we were allowing it to
> succeed previously.
>
> b) With a 32bit app, hugetlb_get_unmapped_area/arch_get_unmapped_area
> may return addresses beyond 32bits, ultimately causing corruption
> because of wrap-around and resulting in SEGFAULT, instead of returning
> ENOMEM.
>
> c) 32bit app doing this below mmap will now fail.
>
> mmap((void *)(0xFFFFE000UL), 0x10000UL, PROT_READ|PROT_WRITE,
> MAP_FIXED|MAP_PRIVATE|MAP_ANON, 0, 0);
>
> I believe a) is addressed by getting rid of the vsyscall page - but it
> might also not be a current problem because the vsyscall page has its own
> gate-vma now.
>

I suspect that whatever issue this is predates my involvement in Linux
:) The "gate" VMA, aka vsyscall, is at 0xffffffffff600000, and the
vDSO setup code shouldn't anywhere near that fragile. Also, if this
really a bug, we have it for the 64-bit case, too, and TASK_SIZE isn't
going to help.

> b) shouldn't be an issue if the mmap allocator correctly treats the
> compat bit - this doesn't require generic TASK_SIZE variations though, as
> the mmap allocator is already specific to arch/x86/.

This was mostly fixed by the CRIU folks a while back, I think.

>
> c) is a variant of a) I believe, which should be fixed by now.
>
> I just looked through some of the current TASK_SIZE users, and *ALL* of
> them seem dubious to me, with the exception of the mmap allocators. In

Even the munmap one seems to be a bug.

> fact some of them seem to be active bugs:
>
> kernel/:
>
> - PR_SET_MM_MAP, PR_SET_MM_MAP_SIZE, prctl_set_mm() et al. Ugh, what a
> nasty prctl()! But besides that, the TASK_SIZE restriction to the ABI
> looks questionable: if we offer this CRIU functionality then why
> should it be impossible for a 32-bit CRIU task to switch to 64-bit?
>
> - kernel/events/core.c: TASK_SIZE_MAX should be a fine filter here, in
> fact it's probably *wrong* to restrict the profiling data here just
> because the task happens to be in 32-bit compat mode.

Yep. I have a patch fo rthis.

>
> - kernel/rseq.c: is this TASK_SIZE restriction even required, wouldn't
> TASK_SIZE_MAX be sufficient?

Presumably.

> So I concur 100% that most TASK_SIZE uses are questionable. In fact think
> 84929801e14d was a mistake, and we should effectively revert it
> carefully, by:
>
> - First by moving almost all TASK_SIZE users over to TASK_SIZE_MAX,
> analyzing and justifying the impact case by case.
>
> - Then making the mmap allocators compat compatible (ha) without relying
> on TASK_SIZE.

I have a somewhat hacky patch for this right now.

>
> - Renaming TASK_SIZE back to TASK_SIZE_MAX and getting rid of the
> TASK_SIZE and TASK_SIZE_MAX differentiation.
>
> Or am I missing some complication?

Seems like a great idea to me.

BTW, what the heck is up with get_gate_page()? I'm struggling to
understand what it's even trying to do. If there's an architecture
that allows a user program to mremap() or otherwise position its gate
VMA between TASK_SIZE and TASK_SIZE_MAX, then that code is going to
explode horribly.

A whole bunch of work in this direction is here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/fixes

It's almost entirely untested.