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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> 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> X-Stat-Signature: r4as66qcshne5igftnhg6yu71sq47nst X-Rspamd-Queue-Id: 88D8A40011 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1787338845-930881 X-HE-Meta: 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 5P9R4wut BDd2x7dAr3JXwJDcINzhfDX5LdVB20EKtKc6s52NLdk93Xxik8PZjhgZZG5J1S91bxpIrJtUKQ1jUBZglodkg/GRkwGKah90vyZ2H70po14H+LuqAGMzHUgThPjP/G7S3Og3pYZTt1XiIe8siZw2BjmUyClWrQVodb2JsOLA1H2PmGcFB2BYIQE8bZbzyWm0n6OkJmRtUEC4yIH05Ep1IHsIOpYJiLn6p4xNLg2Ad+Kd07iloZ2aXpvDoFEGJK3KHaTc/0B7cqHMqHUtaQQo3Ig0xvWI53juzoxs45VKEIBH+KA5oyoiiZ/LfUWNnYAxIcG764cmL00DfkY67atolVGFmYkKCe22eGafl5djncI9akDj+7VDB4WLc7VYs4SR1IAvH/j/aqNinmgujPFp2Ycl/Gg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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