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.
But we can't do the check in the right place inside mmap, so 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.
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v4:
- fixed email to actually deliver again, sorry for any dups!
v3:
- move outside of mmap_sem for real now, fengguang
- check return code of file_remove_privs, akpm
v2:
- move check from page fault to mmap open, jack
---
include/linux/mm.h | 1 +
mm/mmap.c | 19 ++++---------------
mm/util.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 55 insertions(+), 15 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..bcbe592a2c49 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1321,24 +1321,13 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
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/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);
--
1.9.1
--
Kees Cook
Chrome OS & Brillo Security
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