linux-kernel - Re: Memory keys and io

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Message-ID: <b6ed27dc-7dec-aab5-acfc-073a30e49422@kernel.dk>
Date:   Fri, 12 Feb 2021 08:15:10 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: Memory keys and io_uring.

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.

-- 
Jens Axboe