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From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: gregkh@linuxfoundation.org, LKML <linux-kernel@vger.kernel.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
Subject: [PATCH] Revert "drivers: char: mem: Check {read,write}_kmem() addresses"
Date: Tue,  3 Jan 2017 21:50:52 +0100
Message-Id: <20170103205052.30517-1-Jason@zx2c4.com>
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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