Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D2A81C38142 for ; Tue, 24 Jan 2023 07:30:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232865AbjAXHai (ORCPT ); Tue, 24 Jan 2023 02:30:38 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54494 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229791AbjAXHah (ORCPT ); Tue, 24 Jan 2023 02:30:37 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6DFDE1CF74 for ; Mon, 23 Jan 2023 23:29:54 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1674545393; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Qx1RbAHT5qrB3ss9onflU476htAeuefB/V7/7eAr+Dw=; b=aJ2oqcH59hum1opDYuQEu3qMzS8omjpL14r/pQhXJvOGeMskefIbAirr9G56K+qYho56tn ruph0jAxIRR/06DV2iLNE6V3EFq1kxbf4K19l1sqXovPV9LJ/8N6vNijyrETYVXs/T0/yO 0xn+2xB94pUGGUo5IWR7ZO54UH9xX/E= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-453-Dcjf4pi4NpCkoL9xQYXb6Q-1; Tue, 24 Jan 2023 02:29:52 -0500 X-MC-Unique: Dcjf4pi4NpCkoL9xQYXb6Q-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id D0D9E380406A; Tue, 24 Jan 2023 07:29:51 +0000 (UTC) Received: from localhost (unknown [10.39.193.37]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 41074492C3C; Tue, 24 Jan 2023 07:29:51 +0000 (UTC) From: Giuseppe Scrivano To: Aleksa Sarai Cc: linux-kernel@vger.kernel.org, keescook@chromium.org, bristot@redhat.com, ebiederm@xmission.com, brauner@kernel.org, viro@zeniv.linux.org.uk, alexl@redhat.com, peterz@infradead.org, bmasney@redhat.com Subject: Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl In-Reply-To: <20230124015348.6rvic5g6ymsfvj4e@senku> (Aleksa Sarai's message of "Tue, 24 Jan 2023 12:53:48 +1100") References: <20230120102512.3195094-1-gscrivan@redhat.com> <20230124015348.6rvic5g6ymsfvj4e@senku> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.1 (gnu/linux) Date: Tue, 24 Jan 2023 08:29:49 +0100 Message-ID: <87h6wgcrv6.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Aleksa Sarai writes: > On 2023-01-20, Giuseppe Scrivano wrote: >> This patch adds a new prctl called PR_HIDE_SELF_EXE which allows >> processes to hide their own /proc/*/exe file. When this prctl is >> used, every access to /proc/*/exe for the calling process will >> fail with ENOENT. >> >> This is useful for preventing issues like CVE-2019-5736, where an >> attacker can gain host root access by overwriting the binary >> in OCI runtimes through file-descriptor mishandling in containers. >> >> The current fix for CVE-2019-5736 is to create a read-only copy or >> a bind-mount of the current executable, and then re-exec the current >> process. With the new prctl, the read-only copy or bind-mount copy is >> not needed anymore. >> >> While map_files/ also might contain symlinks to files in host, >> proc_map_files_get_link() permissions checks are already sufficient. > > I suspect this doesn't protect against the execve("/proc/self/exe") > tactic (because it clears the bit on execve), so I'm not sure this is > much safer than PR_SET_DUMPABLE (yeah, it stops root in the source > userns from accessing /proc/$pid/exe but the above attack makes that no > longer that important). it protects against that attack too. It clears the bit _after_ the execve() syscall is done. If you attempt execve("/proc/self/exe") you still get ENOENT: ``` #include #include #include #include int main(void) { int ret; ret = prctl(65, 1, 0, 0, 0); if (ret != 0) exit(1); execl("/proc/self/exe", "foo", NULL); exit(2); } ``` # strace -e prctl,execve ./hide-self-exe execve("./hide-self-exe", ["./hide-self-exe"], 0x7fff975a3690 /* 39 vars */) = 0 prctl(0x41 /* PR_??? */, 0x1, 0, 0, 0) = 0 execve("/proc/self/exe", ["foo"], 0x7ffcf51868b8 /* 39 vars */) = -1 ENOENT (No such file or directory) +++ exited with 2 +++ I've also tried execv'ing with a script that uses "#!/proc/self/exe" and I get the same ENOENT. > > I think the only way to fix this properly is by blocking re-opens of > magic links that have more permissions than they originally did. I just > got back from vacation, but I'm working on fixing up [1] so it's ready > to be an RFC so we can close this hole once and for all. so that relies on the fact opening /proc/self/exe with O_WRONLY fails with ETXTBSY? > [1]: https://github.com/cyphar/linux/tree/magiclink/open_how-reopen > >> >> Signed-off-by: Giuseppe Scrivano >> --- >> v2: https://lkml.org/lkml/2023/1/19/849 >> >> Differences from v2: >> >> - fixed the test to check PR_SET_HIDE_SELF_EXE after fork >> >> v1: https://lkml.org/lkml/2023/1/4/334 >> >> Differences from v1: >> >> - amended more information in the commit message wrt map_files not >> requiring the same protection. >> - changed the test to verify PR_HIDE_SELF_EXE cannot be unset after >> a fork. >> >> fs/exec.c | 1 + >> fs/proc/base.c | 8 +++++--- >> include/linux/sched.h | 5 +++++ >> include/uapi/linux/prctl.h | 3 +++ >> kernel/sys.c | 9 +++++++++ >> tools/include/uapi/linux/prctl.h | 3 +++ >> 6 files changed, 26 insertions(+), 3 deletions(-) >> >> diff --git a/fs/exec.c b/fs/exec.c >> index ab913243a367..5a5dd964c3a3 100644 >> --- a/fs/exec.c >> +++ b/fs/exec.c >> @@ -1855,6 +1855,7 @@ static int bprm_execve(struct linux_binprm *bprm, >> /* execve succeeded */ >> current->fs->in_exec = 0; >> current->in_execve = 0; >> + task_clear_hide_self_exe(current); >> rseq_execve(current); >> acct_update_integrals(current); >> task_numa_free(current, false); >> diff --git a/fs/proc/base.c b/fs/proc/base.c >> index 9e479d7d202b..959968e2da0d 100644 >> --- a/fs/proc/base.c >> +++ b/fs/proc/base.c >> @@ -1723,19 +1723,21 @@ static int proc_exe_link(struct dentry *dentry, struct path *exe_path) >> { >> struct task_struct *task; >> struct file *exe_file; >> + long hide_self_exe; >> >> task = get_proc_task(d_inode(dentry)); >> if (!task) >> return -ENOENT; >> exe_file = get_task_exe_file(task); >> + hide_self_exe = task_hide_self_exe(task); >> put_task_struct(task); >> - if (exe_file) { >> + if (exe_file && !hide_self_exe) { >> *exe_path = exe_file->f_path; >> path_get(&exe_file->f_path); >> fput(exe_file); >> return 0; >> - } else >> - return -ENOENT; >> + } >> + return -ENOENT; >> } >> >> static const char *proc_pid_get_link(struct dentry *dentry, >> diff --git a/include/linux/sched.h b/include/linux/sched.h >> index 853d08f7562b..8db32d5fc285 100644 >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -1790,6 +1790,7 @@ static __always_inline bool is_percpu_thread(void) >> #define PFA_SPEC_IB_DISABLE 5 /* Indirect branch speculation restricted */ >> #define PFA_SPEC_IB_FORCE_DISABLE 6 /* Indirect branch speculation permanently restricted */ >> #define PFA_SPEC_SSB_NOEXEC 7 /* Speculative Store Bypass clear on execve() */ >> +#define PFA_HIDE_SELF_EXE 8 /* Hide /proc/self/exe for the process */ >> >> #define TASK_PFA_TEST(name, func) \ >> static inline bool task_##func(struct task_struct *p) \ >> @@ -1832,6 +1833,10 @@ TASK_PFA_CLEAR(SPEC_IB_DISABLE, spec_ib_disable) >> TASK_PFA_TEST(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable) >> TASK_PFA_SET(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable) >> >> +TASK_PFA_TEST(HIDE_SELF_EXE, hide_self_exe) >> +TASK_PFA_SET(HIDE_SELF_EXE, hide_self_exe) >> +TASK_PFA_CLEAR(HIDE_SELF_EXE, hide_self_exe) >> + >> static inline void >> current_restore_flags(unsigned long orig_flags, unsigned long flags) >> { >> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h >> index a5e06dcbba13..f12f3df12468 100644 >> --- a/include/uapi/linux/prctl.h >> +++ b/include/uapi/linux/prctl.h >> @@ -284,4 +284,7 @@ struct prctl_mm_map { >> #define PR_SET_VMA 0x53564d41 >> # define PR_SET_VMA_ANON_NAME 0 >> >> +#define PR_SET_HIDE_SELF_EXE 65 >> +#define PR_GET_HIDE_SELF_EXE 66 >> + >> #endif /* _LINUX_PRCTL_H */ >> diff --git a/kernel/sys.c b/kernel/sys.c >> index 5fd54bf0e886..e992f1b72973 100644 >> --- a/kernel/sys.c >> +++ b/kernel/sys.c >> @@ -2626,6 +2626,15 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, >> case PR_SET_VMA: >> error = prctl_set_vma(arg2, arg3, arg4, arg5); >> break; >> + case PR_SET_HIDE_SELF_EXE: >> + if (arg2 != 1 || arg3 || arg4 || arg5) >> + return -EINVAL; >> + task_set_hide_self_exe(current); >> + break; >> + case PR_GET_HIDE_SELF_EXE: >> + if (arg2 || arg3 || arg4 || arg5) >> + return -EINVAL; >> + return task_hide_self_exe(current) ? 1 : 0; >> default: >> error = -EINVAL; >> break; >> diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h >> index a5e06dcbba13..f12f3df12468 100644 >> --- a/tools/include/uapi/linux/prctl.h >> +++ b/tools/include/uapi/linux/prctl.h >> @@ -284,4 +284,7 @@ struct prctl_mm_map { >> #define PR_SET_VMA 0x53564d41 >> # define PR_SET_VMA_ANON_NAME 0 >> >> +#define PR_SET_HIDE_SELF_EXE 65 >> +#define PR_GET_HIDE_SELF_EXE 66 >> + >> #endif /* _LINUX_PRCTL_H */ >> -- >> 2.38.1 >>