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* [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access
@ 2019-11-18 21:09 Kees Cook
  2019-11-19 10:07 ` Borislav Petkov
  0 siblings, 1 reply; 3+ messages in thread
From: Kees Cook @ 2019-11-18 21:09 UTC (permalink / raw)
  To: Borislav Petkov
  Cc: Thomas Gleixner, Zhang Xiaoxu, linux-kernel, x86, tyhicks,
	colin.king, linux-security-module, Matthew Garrett

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 51d7b120418e
  ("/proc/iomem: only expose physical resource addresses to privileged
  users").

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

Reported-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/kernel/cpu/mtrr/if.c | 21 ++-------------------
 1 file changed, 2 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kernel/cpu/mtrr/if.c b/arch/x86/kernel/cpu/mtrr/if.c
index 4d36dcc1cf87..9c86aeae1b14 100644
--- a/arch/x86/kernel/cpu/mtrr/if.c
+++ b/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);
@@ -289,8 +276,6 @@ 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;
@@ -298,16 +283,12 @@ mtrr_ioctl(struct file *file, unsigned int cmd, unsigned long __arg)
 #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:
@@ -381,6 +362,8 @@ 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);
 }
 
-- 
2.17.1


-- 
Kees Cook

^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access
  2019-11-18 21:09 [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access Kees Cook
@ 2019-11-19 10:07 ` Borislav Petkov
  2019-11-20 20:24   ` James Morris
  0 siblings, 1 reply; 3+ messages in thread
From: Borislav Petkov @ 2019-11-19 10:07 UTC (permalink / raw)
  To: Kees Cook
  Cc: Thomas Gleixner, Zhang Xiaoxu, linux-kernel, x86, tyhicks,
	colin.king, linux-security-module, Matthew Garrett

On Mon, Nov 18, 2019 at 01:09:21PM -0800, Kees Cook wrote:
> 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 51d7b120418e
>   ("/proc/iomem: only expose physical resource addresses to privileged
>   users").
> 
> [1] https://lore.kernel.org/lkml/201911110934.AC5BA313@keescook/
> 
> Reported-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/kernel/cpu/mtrr/if.c | 21 ++-------------------
>  1 file changed, 2 insertions(+), 19 deletions(-)

Yap, LGTM, thanks!

Reviewed-by: Borislav Petkov <bp@suse.de>

However, as it has a user-visible impact and it is not an urgent thing
to have in the tree, I'd not queue this now but after the merge window
is done so that we have a maximum time of exposure in linux-next and we
can have ample time to addres fallout.

/me puts it on the list for after the merge window.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access
  2019-11-19 10:07 ` Borislav Petkov
@ 2019-11-20 20:24   ` James Morris
  0 siblings, 0 replies; 3+ messages in thread
From: James Morris @ 2019-11-20 20:24 UTC (permalink / raw)
  To: Borislav Petkov
  Cc: Kees Cook, Thomas Gleixner, Zhang Xiaoxu, linux-kernel, x86,
	tyhicks, colin.king, linux-security-module, Matthew Garrett

On Tue, 19 Nov 2019, Borislav Petkov wrote:

> >  arch/x86/kernel/cpu/mtrr/if.c | 21 ++-------------------
> >  1 file changed, 2 insertions(+), 19 deletions(-)
> 
> Yap, LGTM, thanks!
> 
> Reviewed-by: Borislav Petkov <bp@suse.de>


Acked-by: James Morris <jamorris@linux.microsoft.com>

-- 
James Morris
<jmorris@namei.org>


^ permalink raw reply	[flat|nested] 3+ messages in thread

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2019-11-18 21:09 [PATCH] x86/mtrr: Require CAP_SYS_ADMIN for all access Kees Cook
2019-11-19 10:07 ` Borislav Petkov
2019-11-20 20:24   ` James Morris

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