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Date: Tue, 08 Sep 2020 11:24:25 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Mickaël Salaün <mic@...ikod.net>,
linux-kernel@...r.kernel.org
Cc: 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>,
Miklos Szeredi <mszeredi@...hat.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,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Mickaël Salaün <mic@...ux.microsoft.com>,
Stephen
Smalley <stephen.smalley.work@...il.com>,
John Johansen
<john.johansen@...onical.com>
Subject: Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for
faccessat2(2)
On Tue, 2020-09-08 at 14:43 +0200, Mickaël Salaün wrote:
> On 08/09/2020 14:28, Mimi Zohar wrote:
> > Hi Mickael,
> >
> > On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
> >> diff --git a/fs/open.c b/fs/open.c
> >> index 9af548fb841b..879bdfbdc6fa 100644
> >> --- a/fs/open.c
> >> +++ b/fs/open.c
> >> @@ -405,9 +405,13 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
> >> if (mode & ~S_IRWXO) /* where's F_OK, X_OK, W_OK, R_OK? */
> >> return -EINVAL;
> >>
> >> - if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
> >> + if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH |
> >> + AT_INTERPRETED))
> >> return -EINVAL;
> >>
> >> + /* Only allows X_OK with AT_INTERPRETED for now. */
> >> + if ((flags & AT_INTERPRETED) && !(mode & S_IXOTH))
> >> + return -EINVAL;
> >> if (flags & AT_SYMLINK_NOFOLLOW)
> >> lookup_flags &= ~LOOKUP_FOLLOW;
> >> if (flags & AT_EMPTY_PATH)
> >> @@ -426,7 +430,30 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
> >>
> >> inode = d_backing_inode(path.dentry);
> >>
> >> - if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
> >> + if ((flags & AT_INTERPRETED)) {
> >> + /*
> >> + * For compatibility reasons, without a defined security policy
> >> + * (via sysctl or LSM), using AT_INTERPRETED must map the
> >> + * execute permission to the read permission. Indeed, from
> >> + * user space point of view, being able to execute data (e.g.
> >> + * scripts) implies to be able to read this data.
> >> + *
> >> + * The MAY_INTERPRETED_EXEC bit is set to enable LSMs to add
> >> + * custom checks, while being compatible with current policies.
> >> + */
> >> + if ((mode & MAY_EXEC)) {
> >
> > Why is the ISREG() test being dropped? Without dropping it, there
> > would be no reason for making the existing test an "else" clause.
>
> The ISREG() is not dropped, it is just moved below with the rest of the
> original code. The corresponding code (with the path_noexec call) for
> AT_INTERPRETED is added with the next commit, and it relies on the
> sysctl configuration for compatibility reasons.
Dropping the S_ISREG() check here without an explanation is wrong and
probably unsafe, as it is only re-added in the subsequent patch and
only for the "sysctl_interpreted_access" case. Adding this new test
after the existing test is probably safer. If the original test fails,
it returns the same value as this test -EACCES.
Mimi
>
> >
> >> + mode |= MAY_INTERPRETED_EXEC;
> >> + /*
> >> + * For compatibility reasons, if the system-wide policy
> >> + * doesn't enforce file permission checks, then
> >> + * replaces the execute permission request with a read
> >> + * permission request.
> >> + */
> >> + mode &= ~MAY_EXEC;
> >> + /* To be executed *by* user space, files must be readable. */
> >> + mode |= MAY_READ;
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