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From: KP Singh <kpsingh@kernel.org>
To: Christian Brauner <brauner@kernel.org>
Cc: bpf@vger.kernel.org, linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Benjamin Tissoires <benjamin.tissoires@redhat.com>,
	Yosry Ahmed <yosryahmed@google.com>
Subject: Re: [PATCH v5 bpf-next 5/5] bpf/selftests: Add a selftest for bpf_getxattr
Date: Tue, 28 Jun 2022 19:52:22 +0200	[thread overview]
Message-ID: <CACYkzJ5ij9rth_v3KQrCVYsQr2STBEWq1EAzkDb5D06CoRRSjA@mail.gmail.com> (raw)
In-Reply-To: <20220628173344.h7ihvyl6vuky5xus@wittgenstein>

On Tue, Jun 28, 2022 at 7:33 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote:
> > A simple test that adds an xattr on a copied /bin/ls and reads it back
> > when the copied ls is executed.
> >
> > Signed-off-by: KP Singh <kpsingh@kernel.org>
> > ---
> >  .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

[...]

> > +SEC("lsm.s/bprm_committed_creds")
> > +void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
> > +{
> > +     struct task_struct *current = bpf_get_current_task_btf();
> > +     char dir_xattr_value[64] = {0};
> > +     int xattr_sz = 0;
> > +
> > +     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
> > +                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
> > +                             dir_xattr_value, 64);
>
> Yeah, this isn't right. You're not accounting for the caller's userns
> nor for the idmapped mount. If this is supposed to work you will need a
> variant of vfs_getxattr() that takes the mount's idmapping into account
> afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

So, If I understand correctly, we don't need xattr_permission (and
other checks in
vfs_getxattr) here as the BPF programs run as CAP_SYS_ADMIN.

but...

So, Is this bit what's missing then?

error = vfs_getxattr(mnt_userns, d, kname, ctx->kvalue, ctx->size);
if (error > 0) {
    if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
        posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
            ctx->kvalue, error);
    if (ctx->size && copy_to_user(ctx->value, ctx->kvalue, error))
        error = -EFAULT;
}
else if (error == -ERANGE && ctx->size >= XATTR_SIZE_MAX) {
    /* The file system tried to returned a value bigger
than XATTR_SIZE_MAX bytes. Not possible. */
    error = -E2BIG;
}

  reply	other threads:[~2022-06-28 17:53 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-28 16:19 [PATCH v5 bpf-next 0/5] Add bpf_getxattr KP Singh
2022-06-28 16:19 ` [PATCH v5 bpf-next 1/5] btf: Add a new kfunc set which allows to mark a function to be sleepable KP Singh
2022-06-28 16:19 ` [PATCH v5 bpf-next 2/5] bpf: kfunc support for ARG_PTR_TO_CONST_STR KP Singh
2022-06-28 16:19 ` [PATCH v5 bpf-next 3/5] bpf: Allow kfuncs to be used in LSM programs KP Singh
2022-06-28 16:19 ` [PATCH v5 bpf-next 4/5] bpf: Add a bpf_getxattr kfunc KP Singh
2022-06-28 17:22   ` Christian Brauner
2022-06-28 17:23   ` Al Viro
2022-06-28 17:29     ` KP Singh
2022-06-28 16:19 ` [PATCH v5 bpf-next 5/5] bpf/selftests: Add a selftest for bpf_getxattr KP Singh
2022-06-28 17:33   ` Christian Brauner
2022-06-28 17:52     ` KP Singh [this message]
2022-06-28 22:28       ` Alexei Starovoitov
2022-06-29  8:11         ` Christian Brauner
2022-06-29  9:55           ` Christian Brauner
2022-06-30  3:02             ` Alexei Starovoitov
2022-06-30 11:45               ` Christian Brauner
2022-06-30 12:21                 ` KP Singh
2022-06-30 12:23                   ` KP Singh
2022-06-30 13:26                   ` Christian Brauner
2022-06-30 13:29                     ` KP Singh
2022-06-30 13:47                       ` Christian Brauner
2022-06-30 14:37                         ` Christian Brauner
2022-06-30 16:10                         ` Casey Schaufler
2022-06-30 22:23                           ` KP Singh
2022-06-30 23:23                             ` Casey Schaufler
2022-07-01  8:32                               ` Amir Goldstein
2022-07-01  8:58                                 ` Christian Brauner
2022-07-01  9:24                                   ` Amir Goldstein
2022-06-30 16:28                   ` Amir Goldstein
2022-06-30 22:25                     ` KP Singh
2022-06-28 17:13 ` [PATCH v5 bpf-next 0/5] Add bpf_getxattr Christian Brauner
2022-06-28 17:20   ` KP Singh
2022-06-28 17:21     ` KP Singh
2022-06-29  1:36       ` Dave Chinner
2022-06-29  2:00         ` KP Singh
2022-06-29  2:05           ` KP Singh

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