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

[Robin Murphy <robin.murphy@arm.com>]

On 04/01/17 09:22, Greg KH wrote:
> 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?

Having spotted the (embarassingly obvious) problem, the real fix is an
even simpler two-liner. Patch incoming...

Robin.

> thanks,
> 
> greg k-h
>