Re: [PATCH] Revert "drivers: char: mem: Check {read,write}_kmem() addresses"

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 03, 2017 at 09:50:52PM +0100, Jason A. Donenfeld wrote:
> This reverts commit 148a1bc84398039e2b96ff78678c4d9a67f81452.
> 
> This commit was intended to fix a problem, but it did not do so
> correctly, resulting in /dev/kmem being entirely broken. So, revert this
> commit until a different solution is found. The following PoC shows
> breakage bisecting to this commit. On working systems, it returns 0 and
> prints the last message. On broken systems containing this commit, it
> exits returning EIO.
> 
>   #define _FILE_OFFSET_BITS 64
>   #include <unistd.h>
>   #include <fcntl.h>
>   #include <stdio.h>
>   #include <string.h>
>   #include <stdlib.h>
>   #include <errno.h>
>   #include <sys/types.h>
>   #include <sys/socket.h>
>   #include <netinet/in.h>
> 
>   int main(int argc, char *argv[])
>   {
>   	int fd, sock;
>   	FILE *f;
>   	char line[256], *ptr;
>   	unsigned long addr = 0;
>   	struct sockaddr_in saddr = { .sin_family = AF_INET, .sin_port = 55193 };
> 
>   	sock = socket(AF_INET, SOCK_DGRAM, 0);
>   	if (sock < 0) {
>   		perror("socket");
>   		return errno;
>   	}
>   	if (bind(sock, (struct sockaddr *)&saddr, sizeof(saddr)) < 0) {
>   		perror("bind");
>   		return errno;
>   	}
>   	f = fopen("/proc/net/udp", "r");
>   	if (!f) {
>   		perror("fopen(/proc/net/udp)");
>   		return errno;
>   	}
>   	if (!fgets(line, 256, f) || !fgets(line, 256, f)) {
>   		perror("fgets");
>   		return errno;
>   	}
>   	fclose(f);
>   	ptr = line + strlen(line) - 1;
>   	while (*--ptr == ' ');
>   	while (*--ptr != ' ');
>   	while (*(--ptr - 1) != ' ');
>   	addr = strtoul(ptr, NULL, 16);
> 
>   	printf("Attempting to read from skbuff at 0x%lx\n", addr);
> 
>   	fd = open("/dev/kmem", O_RDONLY);
>   	if (fd < 0) {
>   		perror("open(/dev/kmem)");
>   		return errno;
>   	}
>   	if (lseek(fd, addr, SEEK_SET) == (off_t)-1) {
>   		perror("lseek");
>   		return errno;
>   	}
>   	if (read(fd, &addr, sizeof(addr)) != sizeof(addr)) {
>   		perror("read");
>   		return errno;
>   	}
> 
>   	printf("It worked, reading the value %lx!\n", addr);
>   	close(fd);
>   	close(sock);
>   	return 0;
>   }
> 
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  drivers/char/mem.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 5bb1985ec484..a33163dbb913 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -381,9 +381,6 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
>  	char *kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
>  	int err = 0;
>  
> -	if (!pfn_valid(PFN_DOWN(p)))
> -		return -EIO;
> -
>  	read = 0;
>  	if (p < (unsigned long) high_memory) {
>  		low_count = count;
> @@ -512,9 +509,6 @@ static ssize_t write_kmem(struct file *file, const char __user *buf,
>  	char *kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
>  	int err = 0;
>  
> -	if (!pfn_valid(PFN_DOWN(p)))
> -		return -EIO;
> -
>  	if (p < (unsigned long) high_memory) {
>  		unsigned long to_write = min_t(unsigned long, count,
>  					       (unsigned long)high_memory - p);

Ick, I hate /dev/kmem...

Robin and Kefeng, any comments?  Reverting seems correct at this point
in time, any objection?

thanks,

greg k-h