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

[Jason A. Donenfeld]

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 <ja...@zx2c4.com>
Cc: Robin Murphy <robin.mur...@arm.com>
Cc: Kefeng Wang <wangkefeng.w...@huawei.com>
Cc: Greg Kroah-Hartman <gre...@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