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Subject[PATCH v2] fs: clear file privilege bits when mmap writing
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 time.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
---
v2:
- move check from page fault to mmap open
---
mm/mmap.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a649f6b..a27735aabc73 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
if (locks_verify_locked(file))
return -EAGAIN;

+ /*
+ * If we must remove privs, we do it here since
+ * doing it during page COW is expensive and
+ * cannot hold inode->i_mutex.
+ */
+ if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
+ mutex_lock(&inode->i_mutex);
+ file_remove_privs(file);
+ mutex_unlock(&inode->i_mutex);
+ }
+
vm_flags |= VM_SHARED | VM_MAYSHARE;
if (!(file->f_mode & FMODE_WRITE))
vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
--
1.9.1

--
Kees Cook
Chrome OS & Brillo Security


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