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Message-ID: <87a6z1m0u1.fsf@x220.int.ebiederm.org>
Date: Tue, 11 Aug 2020 14:36:38 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Mickaël Salaün <mic@...ikod.net>
Cc: linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>, Alexei
Starovoitov <ast@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, Andrew
Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>,
Christian Brauner <christian.brauner@...ntu.com>, Christian Heimes
<christian@...hon.org>, Daniel Borkmann <daniel@...earbox.net>, Deven
Bowers <deven.desai@...ux.microsoft.com>, Dmitry Vyukov
<dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>, Eric Chiang
<ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>, James
Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>, Jann Horn
<jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook
<keescook@...omium.org>, Lakshmi Ramasubramanian
<nramas@...ux.microsoft.com>, Matthew Garrett <mjg59@...gle.com>,
Matthew Wilcox <willy@...radead.org>, Michael Kerrisk
<mtk.manpages@...il.com>, Mimi Zohar <zohar@...ux.ibm.com>, Philippe
Trébuchet <philippe.trebuchet@....gouv.fr>, Scott Shell
<scottsh@...rosoft.com>, Sean Christopherson
<sean.j.christopherson@...el.com>, Shuah Khan <shuah@...nel.org>, Steve
Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>, Tetsuo
Handa <penguin-kernel@...ove.SAKURA.ne.jp>, Thibaut Sautereau
<thibaut.sautereau@...p-os.org>, Vincent Strubel
<vincent.strubel@....gouv.fr>, kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org, linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 3/7] exec: Move path_noexec() check earlier
Mickaël Salaün <mic@...ikod.net> writes:
> From: Kees Cook <keescook@...omium.org>
>
> The path_noexec() check, like the regular file check, was happening too
> late, letting LSMs see impossible execve()s. Check it earlier as well
> in may_open() and collect the redundant fs/exec.c path_noexec() test
> under the same robustness comment as the S_ISREG() check.
>
> My notes on the call path, and related arguments, checks, etc:
A big question arises, that I think someone already asked.
Why perform this test in may_open directly instead of moving
it into inode_permission. That way the code can be shared with
faccessat, and any other code path that wants it?
That would look to provide a more maintainable kernel.
Eric
> 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 path_noexec() 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)
> security_file_open(f)
> open()
> /* old location of path_noexec() test */
>
> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org
> ---
> fs/exec.c | 12 ++++--------
> fs/namei.c | 4 ++++
> 2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index bdc6a6eb5dce..4eea20c27b01 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
> * and check again at the very end too.
> */
> error = -EACCES;
> - if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> - goto exit;
> -
> - if (path_noexec(&file->f_path))
> + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> + path_noexec(&file->f_path)))
> goto exit;
>
> fsnotify_open(file);
> @@ -897,10 +895,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
> * and check again at the very end too.
> */
> err = -EACCES;
> - if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> - goto exit;
> -
> - if (path_noexec(&file->f_path))
> + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> + path_noexec(&file->f_path)))
> goto exit;
>
> err = deny_write_access(file);
> diff --git a/fs/namei.c b/fs/namei.c
> index a559ad943970..ddc9b25540fe 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> return -EACCES;
> flag &= ~O_TRUNC;
> break;
> + case S_IFREG:
> + if ((acc_mode & MAY_EXEC) && path_noexec(path))
> + return -EACCES;
> + break;
> }
>
> error = inode_permission(inode, MAY_OPEN | acc_mode);
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