Re: [PATCH v2 0/5] implement lightweight guard pages

[David Hildenbrand <david@redhat.com>]

On 23.10.24 08:24, Dmitry Vyukov wrote:
> Hi Florian, Lorenzo,
> 
> This looks great!
> 
> What I am VERY interested in is if poisoned pages cause SIGSEGV even when
> the access happens in the kernel. Namely, the syscall still returns EFAULT,
> but also SIGSEGV is queued on return to user-space.
> 
> Catching bad accesses in system calls is currently the weak spot for
> all user-space bug detection tools (GWP-ASan, libefence, libefency, etc).
> It's almost possible with userfaultfd, but catching faults in the kernel
> requires admin capability, so not really an option for generic bug
> detection tools (+inconvinience of userfaultfd setup/handler).
> Intercepting all EFAULT from syscalls is not generally possible
> (w/o ptrace, usually not an option as well), and EFAULT does not always
> mean a bug.
> 
> Triggering SIGSEGV even in syscalls would be not just a performance
> optimization, but a new useful capability that would allow it to catch
> more bugs.

Right, we discussed that offline also as a possible extension to the 
userfaultfd SIGBUS mode.

I did not look into that yet, but I was wonder if there could be cases 
where a different process could trigger that SIGSEGV, and how to (and if 
to) handle that.

For example, ptrace (access_remote_vm()) -> GUP likely can trigger that. 
I think with userfaultfd() we will currently return -EFAULT, because we 
call get_user_page_vma_remote() that is not prepared for dropping the 
mmap lock. Possibly that is the right thing to do, but not sure :)

These "remote" faults set FOLL_REMOTE -> FAULT_FLAG_REMOTE, so we might 
be able to distinguish them and perform different handling.

-- 
Cheers,

David / dhildenb