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

Jason A. Donenfeld
2017-01-03T20:51:19Z

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);
--
2.11.0