| Message ID | 20200723171227.446711-3-mic@digikod.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Add support for O_MAYEXEC | expand |
Mickaël Salaün <mic@digikod.net> writes: > From: Kees Cook <keescook@chromium.org> > > The execve(2)/uselib(2) syscalls have always rejected non-regular > files. Recently, it was noticed that a deadlock was introduced when trying > to execute pipes, as the S_ISREG() test was happening too late. This was > fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files > during execve()"), but it was added after inode_permission() had already > run, which meant LSMs could see bogus attempts to execute non-regular > files. > > Move the test into the other inode type checks (which already look > for other pathological conditions[1]). Since there is no need to use > FMODE_EXEC while we still have access to "acc_mode", also switch the > test to MAY_EXEC. > > Also include a comment with the redundant S_ISREG() checks at the end of > execve(2)/uselib(2) to note that they are present to avoid any mistakes. The comment is: > + /* > + * may_open() has already checked for this, so it should be > + * impossible to trip now. But we need to be extra cautious > + * and check again at the very end too. > + */ Those comments scare me. Why do you need to be extra cautious? How can the file type possibly change between may_open and anywhere? The type of a file is immutable after it's creation. If the comment said check just in case something went wrong with code maintenance I could understand but that isn't what the comment says. Also the fallthrough change below really should be broken out into it's own change. > My notes on the call path, and related arguments, checks, etc: > > do_open_execat() > struct open_flags open_exec_flags = { > .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC, > .acc_mode = MAY_EXEC, > ... > do_filp_open(dfd, filename, open_flags) > path_openat(nameidata, open_flags, flags) > file = alloc_empty_file(open_flags, current_cred()); > do_open(nameidata, file, open_flags) > may_open(path, acc_mode, open_flag) > /* new location of MAY_EXEC vs S_ISREG() test */ > inode_permission(inode, MAY_OPEN | acc_mode) > security_inode_permission(inode, acc_mode) > vfs_open(path, file) > do_dentry_open(file, path->dentry->d_inode, open) > /* old location of FMODE_EXEC vs S_ISREG() test */ > security_file_open(f) > open() > > [1] https://lore.kernel.org/lkml/202006041910.9EF0C602@keescook/ > > Signed-off-by: Mickaël Salaün <mic@digikod.net> > Signed-off-by: Kees Cook <keescook@chromium.org> > Link: https://lore.kernel.org/r/20200605160013.3954297-3-keescook@chromium.org > --- > fs/exec.c | 14 ++++++++++++-- > fs/namei.c | 6 ++++-- > fs/open.c | 6 ------ > 3 files changed, 16 insertions(+), 10 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index d7c937044d10..bdc6a6eb5dce 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -141,8 +141,13 @@ SYSCALL_DEFINE1(uselib, const char __user *, library) > if (IS_ERR(file)) > goto out; > > + /* > + * may_open() has already checked for this, so it should be > + * impossible to trip now. But we need to be extra cautious > + * and check again at the very end too. > + */ > error = -EACCES; > - if (!S_ISREG(file_inode(file)->i_mode)) > + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode))) > goto exit; > > if (path_noexec(&file->f_path)) > @@ -886,8 +891,13 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags) > if (IS_ERR(file)) > goto out; > > + /* > + * may_open() has already checked for this, so it should be > + * impossible to trip now. But we need to be extra cautious > + * and check again at the very end too. > + */ > err = -EACCES; > - if (!S_ISREG(file_inode(file)->i_mode)) > + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode))) > goto exit; > > if (path_noexec(&file->f_path)) > diff --git a/fs/namei.c b/fs/namei.c > index 72d4219c93ac..a559ad943970 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -2849,16 +2849,18 @@ static int may_open(const struct path *path, int acc_mode, int flag) > case S_IFLNK: > return -ELOOP; > case S_IFDIR: > - if (acc_mode & MAY_WRITE) > + if (acc_mode & (MAY_WRITE | MAY_EXEC)) > return -EISDIR; > break; > case S_IFBLK: > case S_IFCHR: > if (!may_open_dev(path)) > return -EACCES; > - /*FALLTHRU*/ > + fallthrough; ^^^^^^^^^^^ That is an unrelated change and should be sent separately. > case S_IFIFO: > case S_IFSOCK: > + if (acc_mode & MAY_EXEC) > + return -EACCES; > flag &= ~O_TRUNC; > break; > } > diff --git a/fs/open.c b/fs/open.c > index 6cd48a61cda3..623b7506a6db 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -784,12 +784,6 @@ static int do_dentry_open(struct file *f, > return 0; > } > > - /* Any file opened for execve()/uselib() has to be a regular file. */ > - if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) { > - error = -EACCES; > - goto cleanup_file; > - } > - > if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) { > error = get_write_access(inode); > if (unlikely(error))
diff --git a/fs/exec.c b/fs/exec.c index d7c937044d10..bdc6a6eb5dce 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -141,8 +141,13 @@ SYSCALL_DEFINE1(uselib, const char __user *, library) if (IS_ERR(file)) goto out; + /* + * may_open() has already checked for this, so it should be + * impossible to trip now. But we need to be extra cautious + * and check again at the very end too. + */ error = -EACCES; - if (!S_ISREG(file_inode(file)->i_mode)) + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode))) goto exit; if (path_noexec(&file->f_path)) @@ -886,8 +891,13 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags) if (IS_ERR(file)) goto out; + /* + * may_open() has already checked for this, so it should be + * impossible to trip now. But we need to be extra cautious + * and check again at the very end too. + */ err = -EACCES; - if (!S_ISREG(file_inode(file)->i_mode)) + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode))) goto exit; if (path_noexec(&file->f_path)) diff --git a/fs/namei.c b/fs/namei.c index 72d4219c93ac..a559ad943970 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -2849,16 +2849,18 @@ static int may_open(const struct path *path, int acc_mode, int flag) case S_IFLNK: return -ELOOP; case S_IFDIR: - if (acc_mode & MAY_WRITE) + if (acc_mode & (MAY_WRITE | MAY_EXEC)) return -EISDIR; break; case S_IFBLK: case S_IFCHR: if (!may_open_dev(path)) return -EACCES; - /*FALLTHRU*/ + fallthrough; case S_IFIFO: case S_IFSOCK: + if (acc_mode & MAY_EXEC) + return -EACCES; flag &= ~O_TRUNC; break; } diff --git a/fs/open.c b/fs/open.c index 6cd48a61cda3..623b7506a6db 100644 --- a/fs/open.c +++ b/fs/open.c @@ -784,12 +784,6 @@ static int do_dentry_open(struct file *f, return 0; } - /* Any file opened for execve()/uselib() has to be a regular file. */ - if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) { - error = -EACCES; - goto cleanup_file; - } - if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) { error = get_write_access(inode); if (unlikely(error))