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From:   Sasha Levin <Alexander.Levin@microsoft.com>
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        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Kees Cook <keescook@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.14 30/67] /dev/mem: Add bounce buffer for
 copy-out
Thread-Topic: [PATCH AUTOSEL for 4.14 30/67] /dev/mem: Add bounce buffer for
 copy-out
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Date:   Thu, 8 Mar 2018 04:57:44 +0000
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From: Kees Cook <keescook@chromium.org>

[ Upstream commit 22ec1a2aea73b9dfe340dff7945bd85af4cc6280 ]

As done for /proc/kcore in

  commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")

this adds a bounce buffer when reading memory via /dev/mem. This
is needed to allow kernel text memory to be read out when built with
CONFIG_HARDENED_USERCOPY (which refuses to read out kernel text) and
without CONFIG_STRICT_DEVMEM (which would have refused to read any RAM
contents at all).

Since this build configuration isn't common (most systems with
CONFIG_HARDENED_USERCOPY also have CONFIG_STRICT_DEVMEM), this also tries
to inform Kconfig about the recommended settings.

This patch is modified from Brad Spengler/PaX Team's changes to /dev/mem
code in the last public patch of grsecurity/PaX based on my understanding
of the code. Changes or omissions from the original code are mine and
don't reflect the original grsecurity/PaX code.

Reported-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 drivers/char/mem.c | 27 ++++++++++++++++++++++-----
 security/Kconfig   |  1 +
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 970e1242a282..3a70dba2c645 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -107,6 +107,8 @@ static ssize_t read_mem(struct file *file, char __user =
*buf,
 	phys_addr_t p =3D *ppos;
 	ssize_t read, sz;
 	void *ptr;
+	char *bounce;
+	int err;
=20
 	if (p !=3D *ppos)
 		return 0;
@@ -129,15 +131,22 @@ static ssize_t read_mem(struct file *file, char __use=
r *buf,
 	}
 #endif
=20
+	bounce =3D kmalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!bounce)
+		return -ENOMEM;
+
 	while (count > 0) {
 		unsigned long remaining;
 		int allowed;
=20
 		sz =3D size_inside_page(p, count);
=20
+		err =3D -EPERM;
 		allowed =3D page_is_allowed(p >> PAGE_SHIFT);
 		if (!allowed)
-			return -EPERM;
+			goto failed;
+
+		err =3D -EFAULT;
 		if (allowed =3D=3D 2) {
 			/* Show zeros for restricted memory. */
 			remaining =3D clear_user(buf, sz);
@@ -149,24 +158,32 @@ static ssize_t read_mem(struct file *file, char __use=
r *buf,
 			 */
 			ptr =3D xlate_dev_mem_ptr(p);
 			if (!ptr)
-				return -EFAULT;
-
-			remaining =3D copy_to_user(buf, ptr, sz);
+				goto failed;
=20
+			err =3D probe_kernel_read(bounce, ptr, sz);
 			unxlate_dev_mem_ptr(p, ptr);
+			if (err)
+				goto failed;
+
+			remaining =3D copy_to_user(buf, bounce, sz);
 		}
=20
 		if (remaining)
-			return -EFAULT;
+			goto failed;
=20
 		buf +=3D sz;
 		p +=3D sz;
 		count -=3D sz;
 		read +=3D sz;
 	}
+	kfree(bounce);
=20
 	*ppos +=3D read;
 	return read;
+
+failed:
+	kfree(bounce);
+	return err;
 }
=20
 static ssize_t write_mem(struct file *file, const char __user *buf,
diff --git a/security/Kconfig b/security/Kconfig
index b5c2b5d0c6c0..87f2a6f842fd 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -154,6 +154,7 @@ config HARDENED_USERCOPY
 	bool "Harden memory copies between kernel and userspace"
 	depends on HAVE_HARDENED_USERCOPY_ALLOCATOR
 	select BUG
+	imply STRICT_DEVMEM
 	help
 	  This option checks for obviously wrong memory regions when
 	  copying memory to/from the kernel (via copy_to_user() and
--=20
2.14.1