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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> 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> X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 4AA9E14000A X-Stat-Signature: rcdx5tcxwbay7r1bk669amc41augrfaa X-Rspam-User: X-HE-Tag: 1789491848-807694 X-HE-Meta: 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 qheSrBUe bmJgu2lvA1MwP+tmcBxcmZOajGc+ltxsgY2pEfFHiI41DIP7EtfK+75rY8AAPZUUJPl1aY6Gskjiisj6+WpJedWo6J3OOx+HdUP9UNKqSFV87HE0p7X1uaBy7hqSJ9K5OrnfNKxfcOQsr3rb1/XCn/quwmRHfDYh5OWPGq8bm0TJTncdYRjvVb+GVpnvQq5v06iW4UNCxmQpArYHz0lL6NrAZ/Xv73shCdzpJzysnRo3rKx5HlOZURC2pmTWRuzKgZmaU0H+vp8dgI7ag2vcxCzbbUCjEV1ELarNja4rvSjDfGjHJAhZPKCvSe7vTMfCMbkDZUvY6njA2yEr8ise9cfiTLJs+Ru/M2hh2i7EW95jOl2xK2EsndJNy9VmDqhzrrdxifAZzj7hIOmODbEOSs8AvNyGvjkrxjsoH9A8dqx2SSUA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >