Avoid a heap buffer overrun triggered by an integer overflow of the userspace
controlled "count" variable.
If userspace passes in a "count" of (size_t)-1l, the kmalloc size will overflow
to ((size_t)-1l + 2) = 1, so only one byte will be allocated. However, copy_from_user()
will attempt to copy 0xFFFFFFFF (or 0xFFFFFFFFFFFFFFFF on 64bit) bytes to the buffer.
A possible testcase could look like this:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char **argv)
{
int fd;
char c;
if (argc != 2) {
printf("Usage: %s /proc/acpi/ibm/filename\n", argv[0]);
return 1;
}
fd = open(argv[1], O_RDWR);
if (fd < 0) {
printf("Could not open proc file\n");
return 1;
}
write(fd, &c, (size_t)-1l);
}
We avoid the integer overrun by putting an arbitrary limit on the count.
PAGE_SIZE sounds like a sane limit.
Signed-off-by: Michael Buesch <mb@xxxxxxxxx>
---
This patch is completely untested due to lack of supported device.
The proc file is only writeable by root, so it's probably not exploitable as-is.
---
drivers/platform/x86/thinkpad_acpi.c | 2 ++
1 file changed, 2 insertions(+)
--- linux-2.6.orig/drivers/platform/x86/thinkpad_acpi.c
+++ linux-2.6/drivers/platform/x86/thinkpad_acpi.c
@@ -777,20 +777,22 @@ static int dispatch_procfs_read(char *pa
static int dispatch_procfs_write(struct file *file,
const char __user *userbuf,
unsigned long count, void *data)
{
struct ibm_struct *ibm = data;
char *kernbuf;
int ret;
if (!ibm || !ibm->write)
return -EINVAL;
+ if (count > PAGE_SIZE - 2)
+ return -EINVAL;
kernbuf = kmalloc(count + 2, GFP_KERNEL);
if (!kernbuf)
return -ENOMEM;
if (copy_from_user(kernbuf, userbuf, count)) {
kfree(kernbuf);
return -EFAULT;
}
--
Greetings, Michael.
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