From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 84DF621CA13; Wed, 26 Aug 2026 13:07:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787749628; cv=none; b=QE8A9XyGGObDF8L+JeQq1LoURLCOt7HZ0NtepBCkdBrRLRt4HbWC5DBzZwZv8CQH010aNwkkVfqflZ/XI8FXRZezbOy/Snfcn6aUqiGl7XPSFwkivJ90uM5FZ/4fSpP3ZiW/DUcRX0Fo18mXYMosQemM5FmWefcZARLmqtLs+/s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787749628; c=relaxed/simple; bh=LkDPqDFg345DgqQ3m4DE3To0vpnoKah/CSfR9DfbKng=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=hOqT7w6ZxqXvBFTtHMKnTo1jGT43h9tMQnaKd2TJMnjvg0pWAogNvfbgsDd6zcv1vv2Tm4+FuMG++JpUWboPU0ck8BRd2ZbznEV3NTptvIHQNiEFcwI6oqAkOLO2C+iiUJppKl3u0zkOGlCMu8p4/gIifR/HC89fA9FMVuWo8oY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NxEG+kxn; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NxEG+kxn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 857CE1F000E9; Wed, 26 Aug 2026 13:07:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787749627; bh=N0uNhOoF0fsO+S9VbNFdNjCWH58LGVPdM6KbXTCpaA4=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=NxEG+kxnT6FOnlnSVrSWAVpAwTa7C5QXPrvPYzfYGHPS9FRNdKlXul8HG8zGK8elf 0mygCqOfR9eM3G+dQWdg/bMGnFWpcK2y8v4Yclu6jZMtaLboZWmhcv42qxKucUSKFP klimZ00U2FyUxGwskveMz1JD1boi8Z7yoZ0o18WqhCyQyYbxFq6O8mJq/8BuUCK+rw X28C16uva8QqUxBsalSioByWTAnep1PC9q0C9uJ+8+5S50+0X8O1fmOurQORIC978i 4NYcPfTIvqas2AY0e9QaKcmIFCW7ZeLQTXg6L2jmmIVYWqK/pIVylvBOqv8wMV0jF+ 9vSuVy515ZNHg== Date: Wed, 26 Aug 2026 14:06:59 +0100 From: "Lorenzo Stoakes (ARM)" To: Jann Horn Cc: Paul Moore , James Morris , "Serge E. Hallyn" , Stephen Smalley , Jeff Xu , =?utf-8?B?VGhpw6liYXVk?= Weksteen , Alexander Viro , Christian Brauner , Jan Kara , linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, Ondrej Mosnacek , selinux@vger.kernel.org, Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Pedro Falcato , David Hildenbrand , linux-mm@kvack.org Subject: Re: [PATCH v2 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS) Message-ID: References: <20260825-selinux-pokemem-v2-0-b46bc64916d8@google.com> <20260825-selinux-pokemem-v2-2-b46bc64916d8@google.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260825-selinux-pokemem-v2-2-b46bc64916d8@google.com> On Tue, Aug 25, 2026 at 08:39:18PM +0200, Jann Horn wrote: > If the system is running with PROC_MEM_FORCE_ALWAYS, LSMs currently have no > good opportunity to block a process from overwriting read-only code in its > own address space through FOLL_FORCE writes via /proc/self/mem. > The security_ptrace_access_check() LSM hook is bypassed when a process > opens /proc/self/mem because this is considered "introspection". > > This causes a hole in SELinux EXECMEM enforcement, which tries to ensure > that a process cannot create executable anonymous pages. > > PROC_MEM_FORCE_PTRACE prevents that and ensures that such FOLL_FORCE > accesses are only possible when the LSM allows ptrace() attachment; but it > is unclear how quickly PROC_MEM_FORCE_PTRACE can be deployed in > environments running lots of third-party code, such as Android. > > So, introduce a new LSM hook that can forbid FOLL_FORCE specifically for > such "introspective" accesses. > > Signed-off-by: Jann Horn Thanks for the name change! :) Nothing stands out so: Acked-by: Lorenzo Stoakes (ARM) > --- > fs/proc/base.c | 9 +++++++++ > include/linux/lsm_hook_defs.h | 1 + > include/linux/security.h | 6 ++++++ > security/security.c | 20 ++++++++++++++++++++ > 4 files changed, 36 insertions(+) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index bec6197329dc..dc6fdcb47b79 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -851,6 +851,11 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode) > /* private_data for proc_mem_operations */ > struct mem_private { > struct mm_struct *mm; > + /* > + * Was the ptrace access check on open bypassed because the opener used > + * the same MM (introspection)? > + */ > + bool opened_by_owner; > }; > > static int mem_open(struct inode *inode, struct file *file) > @@ -864,12 +869,14 @@ static int mem_open(struct inode *inode, struct file *file) > priv->mm = proc_mem_open(inode, PTRACE_MODE_ATTACH); > if (IS_ERR_OR_NULL(priv->mm)) > return priv->mm ? PTR_ERR(priv->mm) : -ESRCH; > + priv->opened_by_owner = priv->mm == current->mm; > file->private_data = no_free_ptr(priv); > return 0; > } > > static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > { > + struct mem_private *priv = file->private_data; > struct task_struct *task; > bool ptrace_active = false; > > @@ -886,6 +893,8 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > } > return ptrace_active; > default: > + if (priv->opened_by_owner) > + return security_mem_foll_force_opened_by_owner(file->f_cred) == 0; > return true; > } > } > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h > index 65c9609ec207..50e3f0abc676 100644 > --- a/include/linux/lsm_hook_defs.h > +++ b/include/linux/lsm_hook_defs.h > @@ -36,6 +36,7 @@ LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from, > LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child, > unsigned int mode) > LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent) > +LSM_HOOK(int, 0, mem_foll_force_opened_by_owner, const struct cred *subject) > LSM_HOOK(int, 0, capget, const struct task_struct *target, kernel_cap_t *effective, > kernel_cap_t *inheritable, kernel_cap_t *permitted) > LSM_HOOK(int, 0, capset, struct cred *new, const struct cred *old, > diff --git a/include/linux/security.h b/include/linux/security.h > index 153e9043058f..74eb876054b0 100644 > --- a/include/linux/security.h > +++ b/include/linux/security.h > @@ -338,6 +338,7 @@ int security_binder_transfer_file(const struct cred *from, > const struct cred *to, const struct file *file); > int security_ptrace_access_check(struct task_struct *child, unsigned int mode); > int security_ptrace_traceme(struct task_struct *parent); > +int security_mem_foll_force_opened_by_owner(const struct cred *subject); > int security_capget(const struct task_struct *target, > kernel_cap_t *effective, > kernel_cap_t *inheritable, > @@ -676,6 +677,11 @@ static inline int security_ptrace_traceme(struct task_struct *parent) > return cap_ptrace_traceme(parent); > } > > +static inline int security_mem_foll_force_opened_by_owner(const struct cred *subject) > +{ > + return 0; > +} > + > static inline int security_capget(const struct task_struct *target, > kernel_cap_t *effective, > kernel_cap_t *inheritable, > diff --git a/security/security.c b/security/security.c > index 71aea8fdf014..fff26ff65e07 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -595,6 +595,26 @@ int security_ptrace_traceme(struct task_struct *parent) > return call_int_hook(ptrace_traceme, parent); > } > > +/** > + * security_mem_foll_force_opened_by_owner() - Check if introspective FOLL_FORCE is allowed > + * @subject: credentials of the process accessing its own memory > + * > + * Check if FOLL_FORCE is allowed for accessing process memory through > + * /proc/$pid/mem in the case where the opener's MM was the same as the target > + * MM, meaning the security_ptrace_access_check() hook was bypassed on open(). > + * (current->mm does not matter for this; for example, if write() is called on > + * an FD that was received from another process which obtained it with > + * open("/proc/self/mem"), this hook still runs.) > + * > + * This is only used when the system is configured with PROC_MEM_FORCE_ALWAYS. > + * > + * Return: Returns 0 if permission is granted. > + */ > +int security_mem_foll_force_opened_by_owner(const struct cred *subject) > +{ > + return call_int_hook(mem_foll_force_opened_by_owner, subject); > +} > + > /** > * security_capget() - Get the capability sets for a process > * @target: target process > > -- > 2.55.0.860.g4b6b3295ed-goog > -- Cheers, Lorenzo