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

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 <[email protected]>

Cc Robin Murphy <[email protected]>

Cc Kefeng Wang <[email protected]>

Cc Greg Kroah-Hartman <[email protected]>

---
 drivers/char/mem.c | 6 ------
 1 file changed, 6 deletions(-)

drivers/char/mem.c +0 -3

index 5bb1985ec484..a33163dbb913 100644
@@ -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;

Quoted 98 lines · 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 <[email protected]>

Cc Robin Murphy <[email protected]>

Cc Kefeng Wang <[email protected]>

Cc Greg Kroah-Hartman <[email protected]>

---
 drivers/char/mem.c | 6 ------
 1 file changed, 6 deletions(-)

drivers/char/mem.c +0 -3

index 5bb1985ec484..a33163dbb913 100644
@@ -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;

drivers/char/mem.c static ssize_t write_kmem(struct file *file, const char __user *buf, · line 514

Quoted 9 lines

drivers/char/mem.c +0 -3

@@ -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