在特定物理地址处硬编码写入 RAM

Question

I want write something in userspace program, for example: write the value 3 in the physical address 0xF7F4900(260MB) from the start address 0x0 , while all kernel sits in the range 0-240MB. How can I do it? any idea please?

Best regards.

I thought to use mmap but I think it gets me the virtual address.

memfd = open("/dev/mem", O_RDWR);
map = mmap(0xF7F4900, sizeof(int), PROT_WRITE, MAP_SHARED, memfd, 0);
*map = 3;

Answer 1

Treat /dev/mem as a file and write to offset 0xF7F4900 in that file:

char value = 3;
int fd = open("/dev/mem", O_RDWR);
lseek(fd, 0xF7F4900, SEEK_SET);
write(fd, &value, 1);
close(fd);

If you want to mmap , the same thing applies. You should not try to map it to 0xF7F4900 in your process, you should instead map it to an arbitrary location and write to map[0xF7F4900] if you map from offset 0 or map[0x900] if you map from the page boundary 0xF7F4000 .

Note that arbitrary access to /dev/mem may require special kernel configuration, and is disabled entirely with UEFI Secure boot.

Answer 2

That's not possible in most operating systems as you could gain control over the system. Only one of the thousands vulnerabilitys