From patchwork Fri Aug 2 08:02:25 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Adrian Ratiu X-Patchwork-Id: 13751215 Received: from madrid.collaboradmins.com (madrid.collaboradmins.com [46.235.227.194]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D6FE43D6A; Fri, 2 Aug 2024 08:03:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=46.235.227.194 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722585813; cv=none; b=t/WJPq+FIzZMdPlnnD69uHuw0/aQBao9nC+hQrUlJfMT1zqMvvuHOwNmJiivrNNSBGvZKDOZclcXoBtPWA84OeqvKRhFmOum81H92XPl/oFLe/RfrIBvDzTn/nkPoI0kRBSxgykeINAZb1kauQHPqJJFlNMJQbUWJ+P3gPbWzr8= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722585813; c=relaxed/simple; bh=kaMFpxHzNkrDtihVBi54kX+DE52xC/pY5UehobLP9Qo=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=gPiz1cpFMG6jK0Nk+7pjaIHsjfokOpISkteO923/pE0sye0F0gwDk5aV57KvA1CtALN/WV0uajYJ/AxXsvOn2/nB74szfrZJVafUaxPfDzoqtgaTC1Bt8kmptfeISvYaSc2rfMsBPbksGEuke0NWQhAccS1OjKd9N073fpl1cd4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=collabora.com; spf=pass smtp.mailfrom=collabora.com; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b=PgtGf+SS; arc=none smtp.client-ip=46.235.227.194 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=collabora.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=collabora.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b="PgtGf+SS" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1722585810; bh=kaMFpxHzNkrDtihVBi54kX+DE52xC/pY5UehobLP9Qo=; h=From:To:Cc:Subject:Date:From; b=PgtGf+SS3vqXcgSlHZT/DCu/JHcsY70Bkl3xXMc5XTShbHXgqoAws5LlSaHPQry6u 4HixZbwpw2m4EScPp31nvs3sKqIn4uQiGRxLmQ1LMrdZCW/stHXq/44SrW9FCnkyUy FBpqCt3L+oxMol+cCcRxZo91gB9i1LjJXISTRouIu815zK0aUSaCTJ++383IsHJJ7j lhj2LLSs99xtPrG4Mzh1d+o7kwcKjx4YYSMIJMNKTjiV+nsurWbXlfRhjj+8/eR/0N +j1lrwoQjg8JHnvqIfNucpnrpLotE8+srObIdsarEoF71p0ErPdlRNOAdo0bz+6c8D a92XuB4kUiDfQ== Received: from gentoo.ratioveremundo.com (cola.collaboradmins.com [195.201.22.229]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: aratiu) by madrid.collaboradmins.com (Postfix) with ESMTPSA id 0D3FC3782212; Fri, 2 Aug 2024 08:03:28 +0000 (UTC) From: Adrian Ratiu To: linux-fsdevel@vger.kernel.org Cc: linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, kernel@collabora.com, gbiv@google.com, inglorion@google.com, ajordanr@google.com, Adrian Ratiu , Doug Anderson , Jeff Xu , Jann Horn , Kees Cook , Ard Biesheuvel , Christian Brauner , Linus Torvalds Subject: [PATCH v5] proc: add config & param to block forcing mem writes Date: Fri, 2 Aug 2024 11:02:25 +0300 Message-ID: <20240802080225.89408-1-adrian.ratiu@collabora.com> X-Mailer: git-send-email 2.44.2 Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 This adds a Kconfig option and boot param to allow removing the FOLL_FORCE flag from /proc/pid/mem write calls because it can be abused. The traditional forcing behavior is kept as default because it can break GDB and some other use cases. Previously we tried a more sophisticated approach allowing distributions to fine-tune /proc/pid/mem behavior, however that got NAK-ed by Linus [1], who prefers this simpler approach with semantics also easier to understand for users. Link: https://lore.kernel.org/lkml/CAHk-=wiGWLChxYmUA5HrT5aopZrB7_2VTa0NLZcxORgkUe5tEQ@mail.gmail.com/ [1] Cc: Doug Anderson Cc: Jeff Xu Cc: Jann Horn Cc: Kees Cook Cc: Ard Biesheuvel Cc: Christian Brauner Suggested-by: Linus Torvalds Signed-off-by: Linus Torvalds Signed-off-by: Adrian Ratiu --- Changes in v5: * Wrapped task fields accesses in READ_ONCE() * Added all values to proc_mem_force_table[] and changed the default of lookup_constant() to preserve the initial value set via Kconfig Changes in v4: * Fixed doc punctuation, used passive tense, improved wording consistency, fixed default value wording * Made struct constant_table a static const __initconst * Reworked proc_mem_foll_force() indentation and var declarations to make code clearer * Reworked enum + struct definition so lookup_constant() defaults to 'always'. Changes in v3: * Simplified code to use shorthand ifs and a lookup_constant() table Changes in v2: * Added bootparam on top of Linus' patch * Slightly reworded commit msg --- .../admin-guide/kernel-parameters.txt | 10 +++ fs/proc/base.c | 61 ++++++++++++++++++- security/Kconfig | 32 ++++++++++ 3 files changed, 102 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index f1384c7b59c9..8396e015aab3 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4788,6 +4788,16 @@ printk.time= Show timing data prefixed to each printk message line Format: (1/Y/y=enable, 0/N/n=disable) + proc_mem.force_override= [KNL] + Format: {always | ptrace | never} + Traditionally /proc/pid/mem allows memory permissions to be + overridden without restrictions. This option may be set to + restrict that. Can be one of: + - 'always': traditional behavior always allows mem overrides. + - 'ptrace': only allow mem overrides for active ptracers. + - 'never': never allow mem overrides. + If not specified, default is the CONFIG_PROC_MEM_* choice. + processor.max_cstate= [HW,ACPI] Limit processor to maximum C-state max_cstate=9 overrides any DMI blacklist limit. diff --git a/fs/proc/base.c b/fs/proc/base.c index 72a1acd03675..f389c69767fa 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -85,6 +85,7 @@ #include #include #include +#include #include #include #include @@ -117,6 +118,40 @@ static u8 nlink_tid __ro_after_init; static u8 nlink_tgid __ro_after_init; +enum proc_mem_force { + PROC_MEM_FORCE_ALWAYS, + PROC_MEM_FORCE_PTRACE, + PROC_MEM_FORCE_NEVER +}; + +static enum proc_mem_force proc_mem_force_override __ro_after_init = + IS_ENABLED(CONFIG_PROC_MEM_NO_FORCE) ? PROC_MEM_FORCE_NEVER : + IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE : + PROC_MEM_FORCE_ALWAYS; + +static const struct constant_table proc_mem_force_table[] __initconst = { + { "always", PROC_MEM_FORCE_ALWAYS }, + { "ptrace", PROC_MEM_FORCE_PTRACE }, + { "never", PROC_MEM_FORCE_NEVER }, + { } +}; + +static int __init early_proc_mem_force_override(char *buf) +{ + if (!buf) + return -EINVAL; + + /* + * lookup_constant() defaults to proc_mem_force_override to preseve + * the initial Kconfig choice in case an invalid param gets passed. + */ + proc_mem_force_override = lookup_constant(proc_mem_force_table, + buf, proc_mem_force_override); + + return 0; +} +early_param("proc_mem.force_override", early_proc_mem_force_override); + struct pid_entry { const char *name; unsigned int len; @@ -835,6 +870,28 @@ static int mem_open(struct inode *inode, struct file *file) return ret; } +static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) +{ + struct task_struct *task; + bool ptrace_active = false; + + switch (proc_mem_force_override) { + case PROC_MEM_FORCE_NEVER: + return false; + case PROC_MEM_FORCE_PTRACE: + task = get_proc_task(file_inode(file)); + if (task) { + ptrace_active = READ_ONCE(task->ptrace) && + READ_ONCE(task->mm) == mm && + READ_ONCE(task->parent) == current; + put_task_struct(task); + } + return ptrace_active; + default: + return true; + } +} + static ssize_t mem_rw(struct file *file, char __user *buf, size_t count, loff_t *ppos, int write) { @@ -855,7 +912,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf, if (!mmget_not_zero(mm)) goto free; - flags = FOLL_FORCE | (write ? FOLL_WRITE : 0); + flags = write ? FOLL_WRITE : 0; + if (proc_mem_foll_force(file, mm)) + flags |= FOLL_FORCE; while (count > 0) { size_t this_len = min_t(size_t, count, PAGE_SIZE); diff --git a/security/Kconfig b/security/Kconfig index 412e76f1575d..a93c1a9b7c28 100644 --- a/security/Kconfig +++ b/security/Kconfig @@ -19,6 +19,38 @@ config SECURITY_DMESG_RESTRICT If you are unsure how to answer this question, answer N. +choice + prompt "Allow /proc/pid/mem access override" + default PROC_MEM_ALWAYS_FORCE + help + Traditionally /proc/pid/mem allows users to override memory + permissions for users like ptrace, assuming they have ptrace + capability. + + This allows people to limit that - either never override, or + require actual active ptrace attachment. + + Defaults to the traditional behavior (for now) + +config PROC_MEM_ALWAYS_FORCE + bool "Traditional /proc/pid/mem behavior" + help + This allows /proc/pid/mem accesses to override memory mapping + permissions if you have ptrace access rights. + +config PROC_MEM_FORCE_PTRACE + bool "Require active ptrace() use for access override" + help + This allows /proc/pid/mem accesses to override memory mapping + permissions for active ptracers like gdb. + +config PROC_MEM_NO_FORCE + bool "Never" + help + Never override memory mapping permissions + +endchoice + config SECURITY bool "Enable different security models" depends on SYSFS