proc: do not access cmdline nor environ from file-backed areas

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>

3 files changed, 8 insertions, 4 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 1b2ede6abcdf..1a76d751cf3c 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -261,7 +261,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
          * 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;
 
@@ -279,7 +279,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
                         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)
@@ -325,7 +325,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
                                 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 *file, char __user *buf,
                 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;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ac1f06a4be6..c080af584ddd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2493,6 +2493,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
 #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)
 {
diff --git a/mm/gup.c b/mm/gup.c
index 76af4cfeaf68..541904a7c60f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -544,6 +544,9 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
         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))