[PATCH 4.14 45/45] proc: do not access cmdline nor environ from file-backed areas

Greg Kroah-Hartman
2018-05-18T08:16:02Z

From: Willy Tarreau <w@1wt.eu>

commit 7f7ccc2ccc2e70c6054685f5e3522efa81556830 upstream.

proc_pid_cmdline_read() and environ_read() directly access the target
process' VM to retrieve the command line and environment. If this
process remaps these areas onto a file via mmap(), the requesting
process may experience various issues such as extra delays if the
underlying device is slow to respond.

Let's simply refuse to access file-backed areas in these functions.
For this we add a new FOLL_ANON gup flag that is passed to all calls
to access_remote_vm(). The code already takes care of such failures
(including unmapped areas). Accesses via /proc/pid/mem were not
changed though.

This was assigned CVE-2018-1120.

Note for stable backports: the patch may apply to kernels prior to 4.11
but silently miss one location; it must be checked that no call to
access_remote_vm() keeps zero as the last argument.

Reported-by: Qualys Security Advisory <qsa@qualys.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/proc/base.c | 8 ++++----
include/linux/mm.h | 1 +
mm/gup.c | 3 +++
3 files changed, 8 insertions(+), 4 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -263,7 +263,7 @@ static ssize_t proc_pid_cmdline_read(str
* Inherently racy -- command line shares address space
* with code and data.
*/
- rv = access_remote_vm(mm, arg_end - 1, &c, 1, 0);
+ rv = access_remote_vm(mm, arg_end - 1, &c, 1, FOLL_ANON);
if (rv <= 0)
goto out_free_page;

@@ -281,7 +281,7 @@ static ssize_t proc_pid_cmdline_read(str
int nr_read;

_count = min3(count, len, PAGE_SIZE);
- nr_read = access_remote_vm(mm, p, page, _count, 0);
+ nr_read = access_remote_vm(mm, p, page, _count, FOLL_ANON);
if (nr_read < 0)
rv = nr_read;
if (nr_read <= 0)
@@ -327,7 +327,7 @@ static ssize_t proc_pid_cmdline_read(str
bool final;

_count = min3(count, len, PAGE_SIZE);
- nr_read = access_remote_vm(mm, p, page, _count, 0);
+ nr_read = access_remote_vm(mm, p, page, _count, FOLL_ANON);
if (nr_read < 0)
rv = nr_read;
if (nr_read <= 0)
@@ -946,7 +946,7 @@ static ssize_t environ_read(struct file
max_len = min_t(size_t, PAGE_SIZE, count);
this_len = min(max_len, this_len);

- retval = access_remote_vm(mm, (env_start + src), page, this_len, 0);
+ retval = access_remote_vm(mm, (env_start + src), page, this_len, FOLL_ANON);

if (retval <= 0) {
ret = retval;
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2383,6 +2383,7 @@ static inline struct page *follow_page(s
#define FOLL_MLOCK 0x1000 /* lock present pages */
#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
#define FOLL_COW 0x4000 /* internal GUP flag */
+#define FOLL_ANON 0x8000 /* don't do file mappings */

static inline int vm_fault_to_errno(int vm_fault, int foll_flags)
{
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -544,6 +544,9 @@ static int check_vma_flags(struct vm_are
if (vm_flags & (VM_IO | VM_PFNMAP))
return -EFAULT;

+ if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
+ return -EFAULT;
+
if (write) {
if (!(vm_flags & VM_WRITE)) {
if (!(gup_flags & FOLL_FORCE))