Bug 231221

Created attachment 196939 [details]
Patch to reject inappropriate ioctls with EINVAL

Overview
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The drm_ioctl function (drivers/gpu/drm/drm_ioctl.c#783) uses DRM_IOCTL_NR(cmd) to determine which ioctl should be called, but it does not check if cmd is actually a valid ioctl for the driver.

Steps to Reproduce
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I was trying to determine if a given file descriptor belongs to a drm or an input device, for which I was using two ioctl requests: IOCGVERSION and DRM_IOCTL_VERSION. I expected requesting IOCGVERSION from a drm device would return EINVAL, but it returns 0. This simple PoC demonstrates this behavior:

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <dev/evdev/input.h>

int main(int argc, char** argv) {
        int fd = open("/dev/drm/0", O_RDONLY);
        int dummy = -1;
        int ret = ioctl(fd, EVIOCGVERSION, &dummy);
        printf("ret=%d, err=%s, dummy=%d\n", ret, strerror(errno), dummy);
}

Actual Results
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The above program shows that requesting EVIOCGVERSION from a drm device succeeds:

[0x00 ~]$ cc test2.c -o test2
[0x00 ~]$ ./test2
ret=0, err=No error: 0, dummy=0

Expected Results
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The expected result would be a failed ioctl call:

[0x00 ~]$ ./test2
ret=-1, err=Invalid argument, dummy=-1

Patch
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This problem exists because DRM_IOCTL_NR(EVIOCGVERSION) = 1, therefore drm_ioctl will use DRM_IOCTL_GET_UNIQUE (because DRM_IOCTL_NR(DRM_IOCTL_GET_UNIQUE) is also 1). The drm_ioctl.c in base compares IOCGROUP(cmd) with DRM_IOCTL_BASE, and returns -EINVAL if they differ. I have copied that comparison to the patch attached.