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 0C6AA389104; Fri, 21 Aug 2026 19:00:44 +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=1787338846; cv=none; b=sUScS5hvhbIOS0mTb2kSnltMeD9m+y03videg8hemC821Wg0tST8iatoPncOhiSHXj66p3b83idpKAXviHP4lI72dgT4tU/CMDly2xABDlS9vOepw8/brfmWpr5vFo4SQ/JeqRw3O4SeZM9fkPBWmYXTFY5y4Xcx1rA+ZaGDNls= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787338846; c=relaxed/simple; bh=sJMt2+jcVX1k3HFVL2ohPOWYZ1ZC+u7VBGIFRLs38O4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=c1jfuMi6dSxy/q3kei78f+4sCHozes3ZrHVebuu581In9NQcSfD6c2L4Qv6TWzmLKWomoZM2+DmtF2YpE9uCCNSoYXaW+SGid0vpTT1sUgdHcMgfGL8dWM0cSM0eyJM5v3SLnG3XumH/FzuCLZ8cfHb0q6+S7JGWryv04kDbrWo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PvGzyhO/; 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="PvGzyhO/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 51E5C1F00A3D; Fri, 21 Aug 2026 19:00:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787338844; bh=IEagc8SPvaN9Oa8aVDhOR1OaWuRI/QhLc1rk2xR/OeI=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=PvGzyhO/TCg78i4TiCwfMnJRwrKx7JYZd3h0Y7oeoimxcZzu3dVhDc9n3IV3Hxx5x wRuCEK/MZio1uqv0KZr8DYt9EI6twnrrm0D71OOF9CgvLRk0NWVDoJHUvN0YkHXlJK sCRdrQhAcH9dogaUc3//75WN6R9lMm/InG/xkGpiiCmcAJqdwZGtTp0Ev80CmflA4q ofmSK00z5G2iiA1fcljuDrSbJcoFhQX2LhkmvOyofNUYlfXehJD/fqUl/CS+jeYcIl 4hzD/J0JgIZrP3oJdw68GrBwXNAgDrZzjs5+IkT9z4XX2Lb2SpRCgr39slcYaB0hpe hfx8qAZhxA2Lw== Date: Fri, 21 Aug 2026 20:00:36 +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 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS) Message-ID: References: <20260818-selinux-pokemem-v1-0-90cd2357ee05@google.com> <20260818-selinux-pokemem-v1-2-90cd2357ee05@google.com> Precedence: bulk X-Mailing-List: linux-security-module@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: <20260818-selinux-pokemem-v1-2-90cd2357ee05@google.com> On Tue, Aug 18, 2026 at 09:51:06PM +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 > --- > fs/proc/base.c | 6 ++++++ > include/linux/lsm_hook_defs.h | 1 + > include/linux/security.h | 6 ++++++ > security/security.c | 15 +++++++++++++++ > 4 files changed, 28 insertions(+) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index bec6197329dc..3dfaef49bb70 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -851,6 +851,8 @@ 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 bypassed due to introspection? */ > + bool introspection; > }; > > static int mem_open(struct inode *inode, struct file *file) > @@ -864,12 +866,14 @@ 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->introspection = 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 +890,8 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > } > return ptrace_active; > default: > + if (priv->introspection) > + return security_introspect_mem_foll_force(file->f_cred) == 0; As per 3/3 I wonder if you need an additional parameter to cover the fd -> some other process case? Like: if (priv->owned_by_owner) { const bool is_remote = current->mm != priv->mm; return !security_fd_from_owner_mem_foll_force(file->f_cred, is_remote); } (I'm not sure how LSM hooks are supposed to look :) > return true; > } > } > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h > index 65c9609ec207..67452f71bedf 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, introspect_mem_foll_force, 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..f8483be58bc8 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_introspect_mem_foll_force(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_introspect_mem_foll_force(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..d0f790a534eb 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -595,6 +595,21 @@ int security_ptrace_traceme(struct task_struct *parent) > return call_int_hook(ptrace_traceme, parent); > } > > +/** > + * security_introspect_mem_foll_force() - Check if introspective FOLL_FORCE is allowed > + * @subject: credentials of the process accessing its own memory > + * > + * Check if FOLL_FORCE is allowed for a process accessing its own memory, which > + * bypasses the security_ptrace_access_check() hook. This should be updated to also explicitly mention the fd case. As surely in that case this is not true? Unless I'm missing something. > + * This is only used when the system is configured with PROC_MEM_FORCE_ALWAYS. > + * > + * Return: Returns 0 if permission is granted. > + */ > +int security_introspect_mem_foll_force(const struct cred *subject) > +{ > + return call_int_hook(introspect_mem_foll_force, subject); > +} > + > /** > * security_capget() - Get the capability sets for a process > * @target: target process > > -- > 2.55.0.737.g08866a6d13-goog > -- Cheers, Lorenzo