x86/mtrr: Require CAP_SYS_ADMIN for all access

Zhang Xiaoxu noted that physical address locations for MTRR were visible
to non-root users, which could be considered an information leak.
In discussing[1] the options for solving this, it sounded like just
moving the capable check into open() was the first step.

If this breaks userspace, then we will have a test case for the more
conservative approaches discussed in the thread. In summary:

- MTRR should check capabilities at open time (or retain the
  checks on the opener's permissions for later checks).

- changing the DAC permissions might break something that expects to
  open mtrr when not uid 0.

- if we leave the DAC permissions alone and just move the capable check
  to the opener, we should get the desired protection. (i.e. check
  against CAP_SYS_ADMIN not just the wider uid 0.)

- if that still breaks things, as in userspace expects to be able to
  read other parts of the file as non-uid-0 and non-CAP_SYS_ADMIN, then
  we need to censor the contents using the opener's permissions. For
  example, as done in other /proc cases, like commit

  51d7b12 ("/proc/iomem: only expose physical resource addresses to privileged users").

[1] https://lore.kernel.org/lkml/201911110934.AC5BA313@keescook/

Reported-by: Zhang Xiaoxu <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Acked-by: James Morris <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Colin Ian King <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: [email protected]
Cc: Matthew Garrett <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tyler Hicks <[email protected]>
Cc: x86-ml <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/201911181308.63F06502A1@keescook

Author: kees

arch/x86/kernel/cpu/mtrr/if.c

@@ -101,9 +101,6 @@ mtrr_write(struct file *file, const char __user *buf, size_t len, loff_t * ppos)
 	int length;
 	size_t linelen;
 
-	if (!capable(CAP_SYS_ADMIN))
-		return -EPERM;
-
 	memset(line, 0, LINE_SIZE);
 
 	len = min_t(size_t, len, LINE_SIZE - 1);
@@ -226,8 +223,6 @@ mtrr_ioctl(struct file *file, unsigned int cmd, unsigned long __arg)
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_ADD_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err =
 		    mtrr_file_add(sentry.base, sentry.size, sentry.type, true,
 				  file, 0);
@@ -236,24 +231,18 @@ mtrr_ioctl(struct file *file, unsigned int cmd, unsigned long __arg)
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_SET_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err = mtrr_add(sentry.base, sentry.size, sentry.type, false);
 		break;
 	case MTRRIOC_DEL_ENTRY:
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_DEL_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err = mtrr_file_del(sentry.base, sentry.size, file, 0);
 		break;
 	case MTRRIOC_KILL_ENTRY:
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_KILL_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err = mtrr_del(-1, sentry.base, sentry.size);
 		break;
 	case MTRRIOC_GET_ENTRY:
@@ -279,8 +268,6 @@ mtrr_ioctl(struct file *file, unsigned int cmd, unsigned long __arg)
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_ADD_PAGE_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err =
 		    mtrr_file_add(sentry.base, sentry.size, sentry.type, true,
 				  file, 1);
@@ -289,25 +276,19 @@ mtrr_ioctl(struct file *file, unsigned int cmd, unsigned long __arg)
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_SET_PAGE_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err =
 		    mtrr_add_page(sentry.base, sentry.size, sentry.type, false);
 		break;
 	case MTRRIOC_DEL_PAGE_ENTRY:
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_DEL_PAGE_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err = mtrr_file_del(sentry.base, sentry.size, file, 1);
 		break;
 	case MTRRIOC_KILL_PAGE_ENTRY:
 #ifdef CONFIG_COMPAT
 	case MTRRIOC32_KILL_PAGE_ENTRY:
 #endif
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		err = mtrr_del_page(-1, sentry.base, sentry.size);
 		break;
 	case MTRRIOC_GET_PAGE_ENTRY:
@@ -410,6 +391,8 @@ static int mtrr_open(struct inode *inode, struct file *file)
 		return -EIO;
 	if (!mtrr_if->get)
 		return -ENXIO;
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
 	return single_open(file, mtrr_seq_show, NULL);
 }