From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.hallyn.com (mail.hallyn.com [178.63.66.53]) (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 55F5243E48B; Tue, 15 Sep 2026 17:54:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=178.63.66.53 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789494873; cv=none; b=Kc6B/v3/QQ16b3Ir3sHO81Iauyq6LDq6A/n0Jg9pLgdh6Y0LOC5+GIFaA39h4JYRZq+d+c64WrjZQt2LMbIZ++JRsgwr47OAE5h53ng4Q3sQP58iYjzdrT82Z8UqQyv5QPs2Of+7dAGa5LQUgO4PuZYu9rjqlEkwbOpwoXBhcbE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789494873; c=relaxed/simple; bh=v63FY1fnrEkx7QXoR+GSQJrHmBwDUBI7qUvyhgXbjJs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=CLM3JaSuf92L3BJGp4iRYllH6psad6XOH7xwNB2xyDtJCY18HU+xKWW9AqXdpvKf3FVAdNy08jWIaEysqSkbIXoQJbQqfwJvzo0SdkZJzviaKGm9sG1OMNs0kMIPiJxsGRgByGcNxf1zypeST5Z0DfDgyX7mvYP4p6thqKclKgQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=hallyn.com; spf=unknown smtp.mailfrom=hallyn.com; dkim=pass (2048-bit key) header.d=hallyn.com header.i=@hallyn.com header.b=PmIGImVV; arc=none smtp.client-ip=178.63.66.53 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=hallyn.com Authentication-Results: smtp.subspace.kernel.org; spf=tempfail smtp.mailfrom=hallyn.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=hallyn.com header.i=@hallyn.com header.b="PmIGImVV" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=hallyn.com; s=mail; t=1789494867; bh=v63FY1fnrEkx7QXoR+GSQJrHmBwDUBI7qUvyhgXbjJs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=PmIGImVVPLzOWAumb6PW4gmlneVv+6xM1ssscap9T86yUWBljn+p2Oe4CIeuTNomy R4XRJJzbs/FHPhGufWGaj99v8F+0jNMEZR9RzNk06ts81sf+zoQMwDJY+zcgm8G6cG gABgIEWhRHDeodGimlEH6KvdGSxsw/csqozdOy8UBNUlPxB1dBOb4nzjGcuQT6kEJc UjwPqdVsqPdB/CKRGB4hODB73ZpqHpi+SpHqg2xATLd6qEeXP2LjMeqajlTf/Jzmzi fiL8M2Q3os1jyaxpNS+cBkHGFPtj4pzkCY7a0M5IfkU9ChBuWtkG8io1mgHQ6xOC81 O8SwtEMelSm0w== Received: by mail.hallyn.com (Postfix, from userid 1001) id 080F9D83; Tue, 15 Sep 2026 12:54:27 -0500 (CDT) Date: Tue, 15 Sep 2026 12:54:27 -0500 From: "Serge E. Hallyn" To: Paul Moore Cc: "Serge Hallyn (AMD)" , Jann Horn , James Morris , Stephen Smalley , Jeff Xu , =?iso-8859-1?Q?Thi=E9baud?= 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" , Lorenzo Stoakes , Vlastimil Babka , Pedro Falcato , David Hildenbrand , linux-mm@kvack.org Subject: Re: [PATCH v3 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS) Message-ID: References: <20260907-selinux-pokemem-v3-0-0bafbaeafe50@google.com> <20260907-selinux-pokemem-v3-2-0bafbaeafe50@google.com> Precedence: bulk X-Mailing-List: selinux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Sep 15, 2026 at 01:24:27PM -0400, Paul Moore wrote: > On Tue, Sep 15, 2026 at 1:04 PM Serge Hallyn (AMD) wrote: > > On Mon, Sep 07, 2026 at 11:00:17PM +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. > > > > > > Acked-by: Lorenzo Stoakes (ARM) > > > Acked-by: David Hildenbrand (Arm) > > > Signed-off-by: Jann Horn > > > --- > > > fs/proc/base.c | 14 ++++++++++++-- > > > include/linux/lsm_hook_defs.h | 1 + > > > include/linux/security.h | 7 +++++++ > > > security/security.c | 25 +++++++++++++++++++++++++ > > > 4 files changed, 45 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/proc/base.c b/fs/proc/base.c > > > index bec6197329dc..295b21203c7f 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; > > > > > > @@ -884,10 +891,13 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > > > READ_ONCE(task->parent) == current; > > > put_task_struct(task); > > > } > > > - return ptrace_active; > > > + if (!ptrace_active) > > > + return false; > > > + break; > > > default: > > > - return true; > > > + break; > > > } > > > + return security_mem_foll_force(file->f_cred, priv->opened_by_owner) == 0; > > > } > > > > > > static ssize_t mem_rw(struct file *file, char __user *buf, > > > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h > > > index 65c9609ec207..12f84a1e6fab 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, const struct cred *subject, bool opened_by_owner) > > > 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..e8bc2e644241 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(const struct cred *subject, bool opened_by_owner); > > > int security_capget(const struct task_struct *target, > > > kernel_cap_t *effective, > > > kernel_cap_t *inheritable, > > > @@ -676,6 +677,12 @@ static inline int security_ptrace_traceme(struct task_struct *parent) > > > return cap_ptrace_traceme(parent); > > > } > > > > > > +static inline int security_mem_foll_force(const struct cred *subject, > > > + bool opened_by_owner) > > > +{ > > > + 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..2cde1efdb7a6 100644 > > > --- a/security/security.c > > > +++ b/security/security.c > > > @@ -595,6 +595,31 @@ int security_ptrace_traceme(struct task_struct *parent) > > > return call_int_hook(ptrace_traceme, parent); > > > } > > > > > > +/** > > > + * security_mem_foll_force() - Check if FOLL_FORCE is allowed > > > + * @subject: credentials using which /proc/$pid/mem was opened > > > + * @opened_by_owner: whether checks on open() were bypassed because the opener > > > + * has the same MM as the target > > > + * > > > + * Check if FOLL_FORCE is allowed for accessing process memory through > > > + * /proc/$pid/mem. opened_by_owner signals whether the opener's MM was the same > > > + * as the target MM, meaning the security_ptrace_access_check() hook was > > > + * bypassed on open(). > > > + * (Current 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"), @opened_by_owner is still true.) > > > + * > > > + * Note that this hook is only designed to be useful in the opened_by_owner > > > + * case, where the subject credentials effectively also describe the object. > > > > Given this, would it make more sense to call the hook something > > like `security_mem_foll_force_self()` and only call it in the > > opened_by_owner==true case? > > Sometimes we can't avoid it, but in general I'd like to see us avoid > calling LSM hooks conditionally, I'd much prefer the hook > implementation (either at the LSM framework level or the individual > LSM) do the conditional check. See my comments on the previous > revision. Ah. Sorry, I had missed that. +1.