Re: Memory keys and io_uring.

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 2/12/21 8:45 PM, Jens Axboe wrote:
> On 2/11/21 11:59 PM, Aneesh Kumar K.V wrote:
>>
>> Hi,
>>
>> I am trying to estabilish the behaviour we should expect when passing a
>> buffer with memory keys attached to io_uring syscalls. As show  in the
>> blow test
>>
>> /*
>>   * gcc -Wall -O2 -D_GNU_SOURCE -o pkey_uring pkey_uring.c -luring
>>   */
>> #include <stdio.h>
>> #include <fcntl.h>
>> #include <string.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #include <sys/mman.h>
>> #include "liburing.h"
>>
>> #define PAGE_SIZE  (64 << 10)
>>
>> int main(int argc, char *argv[])
>> {
>> 	int fd, ret, pkey;
>> 	struct io_uring ring;
>> 	struct io_uring_sqe *sqe;
>> 	struct io_uring_cqe *cqe;
>> 	struct iovec iovec;
>> 	void *buf;
>>
>> 	if (argc < 2) {
>> 		printf("%s: file\n", argv[0]);
>> 		return 1;
>> 	}
>>
>> 	ret = io_uring_queue_init(1, &ring, IORING_SETUP_SQPOLL);
>> 	if (ret < 0) {
>> 		fprintf(stderr, "queue_init: %s\n", strerror(-ret));
>> 		return 1;
>> 	}
>>
>> 	fd = open(argv[1], O_RDONLY | O_DIRECT);
>> 	if (fd < 0) {
>> 		perror("open");
>> 		return 1;
>> 	}
>>
>> 	if (posix_memalign(&buf, PAGE_SIZE, PAGE_SIZE))
>> 		return 1;
>> 	iovec.iov_base = buf;
>> 	iovec.iov_len = PAGE_SIZE;
>>
>> 	//mprotect(buf, PAGE_SIZE, PROT_NONE);
>> 	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE);
>> 	pkey_mprotect(buf, PAGE_SIZE, PROT_READ | PROT_WRITE, pkey);
>>
>>
>> 	sqe = io_uring_get_sqe(&ring);
>> 	if (!sqe) {
>> 		perror("io_uring_get_sqe");
>> 		return 1;
>> 	}
>> 	io_uring_prep_readv(sqe, fd, &iovec, 1, 0);
>>
>> 	ret = io_uring_submit(&ring);
>> 	if (ret != 1) {
>> 		fprintf(stderr, "io_uring_submit: %s\n", strerror(-ret));
>> 		return 1;
>> 	}
>>
>> 	ret = io_uring_wait_cqe(&ring, &cqe);
>>
>> 	if (cqe->res < 0)
>> 		fprintf(stderr, "iouring submit failed %s\n", strerror(-cqe->res));
>> 	else
>> 		fprintf(stderr, "iouring submit success\n");
>>
>> 	io_uring_cqe_seen(&ring, cqe);
>>
>> 	/*
>> 	 * let's access this via a read syscall
>> 	 */
>> 	ret = read(fd, buf, PAGE_SIZE);
>> 	if (ret < 0)
>> 		fprintf(stderr, "read failed : %s\n", strerror(errno));
>>
>> 	close(fd);
>> 	io_uring_queue_exit(&ring);
>>
>> 	return 0;
>> }
>>
>> A read syscall do fail with EFAULT. But we allow read via io_uring
>> syscalls. Is that ok? Considering memory keys are thread-specific we
>> could debate that kernel thread can be considered to be the one that got all access
>> allowed via keys or we could update that access is denied via kernel
>> thread for any key value other than default key (key 0). Other option
>> is to inherit the memory key restrictions when doing
>> io_uring_submit() and use the same when accessing the userspace from
>> kernel thread.
>>
>> Any thoughts here with respect to what should be behaviour?
> 
> It this a powerpc thing? I get -EFAULT on x86 for both reads, io_uring
> and regular syscall. That includes SQPOLL, not using SQPOLL, or
> explicitly setting IOSQE_ASYNC on the sqe.
> 

Interesting, I didn't check x86 because i don't have hardware that 
supports memory keys. I am trying to make ppc64 behavior compatible with 
other archs here.

IIUC, in your test io_wqe/sqe kernel thread did hit access fault when 
touching the buffer on x86? That is different from what Dave explained 
earlier.

With the patch 8c511eff1827 ("powerpc/kuap: Allow kernel thread to 
access userspace after kthread_use_mm") I now have key 0 access  allowed 
but all other keys denied with ppc64. I was planning to change that to 
allow all key access based on reply from Dave.  I would be curious to 
understand what made x86 deny the access and how did kthread inherit the 
key details.



-aneesh