Messages in this thread Patch in this message | | | Date | Wed, 2 Dec 2015 16:03:42 -0800 | From | Kees Cook <> | Subject | [PATCH v2] fs: clear file privilege bits when mmap writing |
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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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