[PATCH v9] fs: clear file privilege bits when mmap writing

From: Kees Cook
Date: Thu Jan 14 2016 - 16:22:12 EST

Normally, when a user can modify a file that has setuid or setgid bits,
those bits are cleared when they are not the file owner or a member
of the group. This is enforced when using write and truncate but not
when writing to a shared mmap on the file. This could allow the file
writer to gain privileges by changing a binary without losing the
setuid/setgid/caps bits.

Changing the bits requires holding inode->i_mutex, so it cannot be done
during the page fault (due to mmap_sem being held during the fault).
Instead, clear the bits if PROT_WRITE is being used at mmap open time,
or added at mprotect time.

Since we can't do the check in the right place inside mmap (due to
holding mmap_sem), we have to do it before holding mmap_sem, which
means duplicating some checks, which have to be available to the non-MMU
builds too.

When walking VMAs during mprotect, we need to drop mmap_sem (while
holding a file reference) and restart the walk after clearing privileges.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v9:
- use file_needs_remove_privs, jack & koct9i
v8:
- use mmap/mprotect method, with mprotect walk restart, thanks to koct9i
v7:
- document and avoid arch-specific O_* values, viro
v6:
- clarify ETXTBSY situation in comments, luto
v5:
- add to f_flags instead, viro
- add i_mutex during __fput, jack
v4:
- delay removal instead of still needing mmap_sem for mprotect, yalin
v3:
- move outside of mmap_sem for real now, fengguang
- check return code of file_remove_privs, akpm
v2:
- move to mmap from fault handler, jack
---
include/linux/mm.h | 1 +
mm/mmap.c | 20 ++++----------------
mm/mprotect.c | 24 ++++++++++++++++++++++++
mm/util.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 79 insertions(+), 16 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 00bad7793788..b264c8be7114 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1912,6 +1912,7 @@ extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned lo

extern unsigned long mmap_region(struct file *file, unsigned long addr,
unsigned long len, vm_flags_t vm_flags, unsigned long pgoff);
+extern int do_mmap_shared_checks(struct file *file, unsigned long prot);
extern unsigned long do_mmap(struct file *file, unsigned long addr,
unsigned long len, unsigned long prot, unsigned long flags,
vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate);
diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a649f6b..b3424db0a29e 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1320,25 +1320,13 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
return -EAGAIN;

if (file) {
- struct inode *inode = file_inode(file);
+ int err;

switch (flags & MAP_TYPE) {
case MAP_SHARED:
- if ((prot&PROT_WRITE) && !(file->f_mode&FMODE_WRITE))
- return -EACCES;
-
- /*
- * Make sure we don't allow writing to an append-only
- * file..
- */
- if (IS_APPEND(inode) && (file->f_mode & FMODE_WRITE))
- return -EACCES;
-
- /*
- * Make sure there are no mandatory locks on the file.
- */
- if (locks_verify_locked(file))
- return -EAGAIN;
+ err = do_mmap_shared_checks(file, prot);
+ if (err)
+ return err;

vm_flags |= VM_SHARED | VM_MAYSHARE;
if (!(file->f_mode & FMODE_WRITE))
diff --git a/mm/mprotect.c b/mm/mprotect.c
index ef5be8eaab00..57cb81c11668 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -12,6 +12,7 @@
#include <linux/hugetlb.h>
#include <linux/shm.h>
#include <linux/mman.h>
+#include <linux/file.h>
#include <linux/fs.h>
#include <linux/highmem.h>
#include <linux/security.h>
@@ -375,6 +376,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,

vm_flags = calc_vm_prot_bits(prot);

+restart:
down_write(&current->mm->mmap_sem);

vma = find_vma(current->mm, start);
@@ -416,6 +418,28 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
goto out;
}

+ /*
+ * If we're adding write permissions to a shared file,
+ * we must clear privileges (like done at mmap time),
+ * but we have to juggle the locks to avoid holding
+ * mmap_sem while holding i_mutex.
+ */
+ if ((vma->vm_flags & VM_SHARED) && vma->vm_file &&
+ (newflags & VM_WRITE) && !(vma->vm_flags & VM_WRITE) &&
+ file_needs_remove_privs(vma->vm_file)) {
+ struct file *file = get_file(vma->vm_file);
+
+ start = vma->vm_start;
+ up_write(&current->mm->mmap_sem);
+ mutex_lock(&file_inode(file)->i_mutex);
+ error = file_remove_privs(file);
+ mutex_unlock(&file_inode(file)->i_mutex);
+ fput(file);
+ if (error)
+ return error;
+ goto restart;
+ }
+
error = security_file_mprotect(vma, reqprot, prot);
if (error)
goto out;
diff --git a/mm/util.c b/mm/util.c
index 9af1c12b310c..1882eaf33a37 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -283,6 +283,29 @@ int __weak get_user_pages_fast(unsigned long start,
}
EXPORT_SYMBOL_GPL(get_user_pages_fast);

+int do_mmap_shared_checks(struct file *file, unsigned long prot)
+{
+ struct inode *inode = file_inode(file);
+
+ if ((prot & PROT_WRITE) && !(file->f_mode & FMODE_WRITE))
+ return -EACCES;
+
+ /*
+ * Make sure we don't allow writing to an append-only
+ * file..
+ */
+ if (IS_APPEND(inode) && (file->f_mode & FMODE_WRITE))
+ return -EACCES;
+
+ /*
+ * Make sure there are no mandatory locks on the file.
+ */
+ if (locks_verify_locked(file))
+ return -EAGAIN;
+
+ return 0;
+}
+
unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
unsigned long len, unsigned long prot,
unsigned long flag, unsigned long pgoff)
@@ -291,6 +314,33 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
struct mm_struct *mm = current->mm;
unsigned long populate;

+ /*
+ * If we must remove privs, we do it here since doing it during
+ * page fault may be expensive and cannot hold inode->i_mutex,
+ * since mm->mmap_sem is already held.
+ */
+ if (file && (flag & MAP_TYPE) == MAP_SHARED && (prot & PROT_WRITE)) {
+ struct inode *inode = file_inode(file);
+ int err;
+
+ if (!IS_NOSEC(inode)) {
+ /*
+ * Make sure we can't strip privs from a file that
+ * wouldn't otherwise be allowed to be mmapped.
+ */
+ err = do_mmap_shared_checks(file, prot);
+ if (err)
+ return err;
+
+ mutex_lock(&inode->i_mutex);
+ err = file_remove_privs(file);
+ mutex_unlock(&inode->i_mutex);
+
+ if (err)
+ return err;
+ }
+ }
+
ret = security_mmap_file(file, prot, flag);
if (!ret) {
down_write(&mm->mmap_sem);
--
2.6.3


--
Kees Cook
Chrome OS & Brillo Security