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 AA47642F712; Tue, 15 Sep 2026 17:04:08 +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=1789491851; cv=none; b=VnNhCzALSNrq+NNaxYePrnS+y8kEafOvgpCmflpnLK7HUb4AVnwTVcbUQgPzri3uVGTZ3DkRDpMWIO4mX3+hNpFGDhfTRVuJLf6ueYDSmwV/5NxRnJH95pqBole89sIgu4gFq7S5myjfpzVwMW+klNSJ1QlEg4SoO51GUAJFz+A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789491851; c=relaxed/simple; bh=AQ/9IN5OXYTHakGvdy0opHCSK4Z4C1mt79b6bh3UUIk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=n/cs+yzaGJ4bd2/aYvEi31wPhR5jzibMgkdmgV7FqkFP+cgaHhTLdcT2eTopFmkml7y8AwxPkWdcR4jwmUHsDeAWoxBzOl9WOUEXp6VoheDaUvGBJkvrqt42wttTYB6vwR0QgPwvgolgGsgG786oA5e9+CqWLT5K4FrkGlxvYpM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WRG+dve/; 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="WRG+dve/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 351091F000FF; Tue, 15 Sep 2026 17:04:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789491847; bh=PQzh3VGckL53Mndb/s9icBKbUyH8gPEYoAfCQcTHMv8=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=WRG+dve/iOrdQlsX6mrm2ROLt8HNIAmXP0TjZwot1akZghPwDQpk9LA7Cl2xzNmBN QGMi+zcjyBU46HINFulvtdJcFRqcc0K41hX0W0PFfUMn45aOEXZoQFNvjr9TLVECTi 7CZMo8dNZEMOYqZbh5KvglLerLodgm3zExj2AJQNmtCJ0OiqXNCuKl1p4ZFENZcxKr sSCtU5j2RXL273vrOWqtWPjLFye4dTJrQsRN41l8K3h7jpLNJXzGkNgwEVMFur2QlS jIqY6OV5Szw6LMjfqTMI96VMnfYSjVe9+siSqyFkosH2Y9SPBr7Euj9vrzCOf8ti44 sbXzIkrE+QEFQ== Date: Tue, 15 Sep 2026 12:04:03 -0500 From: "Serge Hallyn (AMD)" To: Jann Horn Cc: Paul Moore , James Morris , "Serge E. Hallyn" , 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: 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: <20260907-selinux-pokemem-v3-2-0bafbaeafe50@google.com> 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? I only suggest it because it seems to lower the cognitive load when looking at this code, so it might make it easier to maintain. > + * > + * Return: Returns 0 if permission is granted. > + */ > +int security_mem_foll_force(const struct cred *subject, > + bool opened_by_owner) > +{ > + return call_int_hook(mem_foll_force, subject, opened_by_owner); > +} > + > /** > * security_capget() - Get the capability sets for a process > * @target: target process > > -- > 2.55.0.1003.g10538fe699-goog >