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Subject[PATCH AUTOSEL for 4.15 35/78] /dev/mem: Add bounce buffer for copy-out
Date
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 6aefe5370e5b..052011bcf100 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 = *ppos;
ssize_t read, sz;
void *ptr;
+ char *bounce;
+ int err;

if (p != *ppos)
return 0;
@@ -129,15 +131,22 @@ static ssize_t read_mem(struct file *file, char __user *buf,
}
#endif

+ bounce = kmalloc(PAGE_SIZE, GFP_KERNEL);
+ if (!bounce)
+ return -ENOMEM;
+
while (count > 0) {
unsigned long remaining;
int allowed;

sz = size_inside_page(p, count);

+ err = -EPERM;
allowed = page_is_allowed(p >> PAGE_SHIFT);
if (!allowed)
- return -EPERM;
+ goto failed;
+
+ err = -EFAULT;
if (allowed == 2) {
/* Show zeros for restricted memory. */
remaining = clear_user(buf, sz);
@@ -149,24 +158,32 @@ static ssize_t read_mem(struct file *file, char __user *buf,
*/
ptr = xlate_dev_mem_ptr(p);
if (!ptr)
- return -EFAULT;
-
- remaining = copy_to_user(buf, ptr, sz);
+ goto failed;

+ err = probe_kernel_read(bounce, ptr, sz);
unxlate_dev_mem_ptr(p, ptr);
+ if (err)
+ goto failed;
+
+ remaining = copy_to_user(buf, bounce, sz);
}

if (remaining)
- return -EFAULT;
+ goto failed;

buf += sz;
p += sz;
count -= sz;
read += sz;
}
+ kfree(bounce);

*ppos += read;
return read;
+
+failed:
+ kfree(bounce);
+ return err;
}

static ssize_t write_mem(struct file *file, const char __user *buf,
diff --git a/security/Kconfig b/security/Kconfig
index b0cb9a5f9448..3709db95027f 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
--
2.14.1
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